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Structural diversity in metal-organic nanoscale supramolecular architectures

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Structural diversity in metal-organic nanoscale supramolecular architectures
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Abourahma, Heba, 1974-
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University of South Florida
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Subjects / Keywords:
coordination polymers
isophthalic acid
SBU
paddlewheel
crystal engineering
Dissertations, Academic -- Chemistry -- Doctoral -- USF   ( lcsh )
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government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
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ABSTRACT: Supramolecular synthesis has gained much attention in recent years. Such an approach to synthesis represents an attractive alternative to traditional, multi-step synthesis, especially for making complex,nanoscopic structures. Of particular interest, in the context of this work, is the use of metal-organic interactions to direct the self-assembly of nanoscopic architectures. These interactions are highly directional, relatively "strong" (compared to other supramolecular interactions) and kinetically labile, which allows for "self-correction" and in turn the production, often in high yield, of defect-free products. This also means that a number of related, yet structurally diverse products (supramolecular isomers) could be isolated. The work presented herein demonstrates the supramolecular synthesis of related, yet structurally diverse family of metal-organic nanoscale supramolecular architectures that are based on the ubiquitous paddle-wheel dimetal tetracarboxylate secondary building unit (SBU) and angular dicarboxylate ligands. It also demonstrates that the SBU self-assembles into clusters of four (tetragonal) and three (trigonal) nanoscale secondary building units (nSBU), which further self-assemble into nanoscale structures that include discrete (0D) faceted polyhedra, tetragonal 2D sheets and another 2D sheet that conforms to the so-called Kagom lattice. In addition, the work herein demonstrates that synthesis under thermodynamic equilibrium conditions facilitates "self-correction" so that the most stable thermodynamic product is obtained. Synthesis, characterization and crystal structure analysis of these structures is presented herein.
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Thesis (Ph.D.)--University of South Florida, 2004.
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by Heba Abourahma.
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Structural Diversity in Metal-Organic Nanoscale Supramolecular Architectures by Heba Abourahma A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemistry College of Arts and Sciences University of South Florida Major Professor: Michael J. Zaworotko, Ph.D. Gregory Baker, Ph.D. Leonard MacGillivray, Ph.D. Edward Turos, Ph.D. Date of Approval: April 7, 2004 Keywords: crystal engineeri ng, coordination polymers, paddl ewheel, isophthalic acid, sbu Copyright 2004 Heba Abourahma

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To Lamees for her never ending encouragement and support and for her positive attitude in life

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Acknowledgements The author would like to sincerely tha nk her mentor and supervisor Professor Zaworotko for his support over the years a nd for all the opportunities he has made available for her professional growth and deve lopment. She is indebted to Arun Mondal, Jianjiang Lu, Brian Moulton and Rosa Bailey Walsh all of whom have made invaluable contributions to this project. She would like to particularly thank Dr. Victor Kravtsov who, in addition to making significant contri butions to the crystallography, has been a great friend. His support and encouragemen t have made a difference and are greatly appreciated. The author extends her appreciation to Dr. Gr egory Baker for all his help and advice, and to the chemistry staff at U SF who has been very helpful, friendly and accommodating. Finally, the author thanks her family for all their love and su pport over the years, without which none of this would have been possible

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i Table of Contents List of Tables v List of Figures vi List of Schemes x List of Abbreviations xi ABSTRACT xiii Chapter 1 1 INTRODUCTION 1 1.1 Supramolecular Chemistry 1 1.2 Crystal Engineering 2 1.3 Supramolecular Isomerism 4 1.3.1 Structural Isomerism 5 1.3.2 Conformational Isomerism 6 1.3.3 Catenane Isomerism 7 1.3.4 Optical Isomerism 7 1.4 Coordination Polymers 8 1.4.1 Examples of 3D Structures 9 1.4.2 Examples of 2D Structures 11 1.4.3 Examples of 1D Structures 13 1.4.4 Examples of 0D (Discrete) Structures 13

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ii 1.4.4.1 Polygons 16 1.4.4.2 Polyhedra 17 1.5 Nanochemistry and Nanotechnology 18 1.6 Characterization of Nanoscale Architectures 19 1.6 Scope and Focus 20 1.6.1 Scope 20 1.6.2 The Paddlewheel: a Secondary Building Unit (SBU) 23 1.6.3 Focus 25 Chapter 2 28 0D NANOSTRUCTURESNANOBALLS 28 2.1 Preface 28 2.2 Results and Discussion 30 2.3 Conclusions 48 2.4 Experimental 49 Chapter 3 52 TETRAGONAL 2D SHEETS 52 3.1 Preface 52 3.1.1 Square Grids 53 3.1.2 Calixarenes and Metallacalixarenes 54 3.2 Results and Discussion 56 3.3 Conclusions 69 3.4 Experimental 70 Chapter 4 78

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iii 2D KAGOM LATTICES: NANOSCALE MOLECULAR MAGNETS 78 4.1 Preface 78 4.1.1 Molecular Magnetism 79 4.1.2 Examples of Molecular Kagom Lattices 80 4.1.3 Modification of Molecular Kagom Lattice 82 4.2 Results and Discussion 84 4.3 Conclusions 103 4.4 Experimental 104 Chapter 5 108 ADDITIONAL RELATED CHROMOPHORES 108 5.1 Preface 108 5.1.1 Self-assembly Under Thermodynamic Equilibrium Conditions 108 5.1.2 CSD Survey of Cu(II) 109 5.1.3 Supramolecular Isomers 110 5.1.4 Isophthalic Acids 111 5.2 Results and Discussion 113 5.2.1 Compound 15 113 5.2.2 Compounds 16a and 16b 116 5.3 Conclusions 123 5.4 Experimental 124 Chapter 6 127 GENERAL CONCLUSIONS AND FUTURE DIRECTIONS 127 6.1 Summary 127

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iv 6.2 Conclusions 129 6.3 Future Directions 130 References 134 Appendixes 149 Appendix 1. Mass spectrum for compound 1 150 Appendix 2. Mass spectrum for compound 2 151 Appendix 3. Mass spectrum for compound 16b 152 ABOUT THE AUTHOR End Page

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v List of Tables Table 1.1: Bond distance da ta of surveyed paddlewheel moieties based on Cu(II) from the CSD 23 Table 2.1: Summary of characterisi tic features of of compounds 1-3* 45 Table 2.2: Crystallographi c data for compounds 1-3 51 Table 3.1: Crystallographic data for compounds 4a-9 76 Table 4.1: Crystallographi c data for compounds 10-14 107 Table 5.1: Crystallographic data for compounds 15, 16a, 16b 126

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vi List of Figures Figure 1.1: Platonic (regular) and Archimed ean (semi regular) solids 17 Figure 1.2: Paddlewheel dimetal tetracarboxylate is a 4-connected node (molecular square) and a linear spacer 20 Figure 1.3: Angular bdc links the SBUs at angle of 120 subtended by the carboxylate moieties 26 Figure 1.4: Possible distortion modes of the bdc moiety as it links the SBUs into extended structures 26 Figure 2.1: The nine faceted polyhedra 29 Figure 2.2: Crystal structure and sc hematic representation of compound 1* 32 Figure 2.3: Crystal structure packing a nd schematic representation for compound 1* 33 Figure 2.4: Crystal structure of compound 2* 35 Figure 2.5: Perspective view of the bcc packing in the crys tal structure of 2* 37 Figure 2.6: Microcrystals of 2 annealed for 24 hrs on mica 39 Figure 2.7: Crystal structure of compound 3* 39 Figure 2.8: Nanoball 3 encapsulates guest mo lecules in the triangular and square windows* 41 Figure 2.9: Host-guest interactions in the square windows of compound 3* 42 Figure 2.10: Host-guest in teractions in the triang ular windows of compound 3* 43

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vii Figure 2.11: Aromatic guest molecules occupy the interstitial space between nanoballs of compound 3* 44 Figure 2.12: Trigonal and tetragonal nSBU c onstituents of the nanoball structures 47 Figure 3.1: Tetragonal 2D sheets sustained by tetragonal nSBU building blocks 53 Figure 3.2: Atropisomers of a calix[4]arene 55 Figure 3.3: Angular aryldicar boxylates employed for making tetragonal 2D sheets 56 Figure 3.4: Crystal structure of compound 4a* 58 Figure 3.5: Crystal structure of 4b* 59 Figure 3.6: Nitrobenzene guest molecule s occupy the cavities in compound 4b 60 Figure 3.7: Crystal structure of compound 5* 61 Figure 3.8: Guest o -dichlorobenzene interacts with th e walls of the cavity via CH62 Figure 3.9: Atropismerism in nSBU constituents of tetra gonal 2D sheets based on bdc 62 Figure 3.10: Crystal structure of compound 6* 63 Figure 3.11: Crystal structure of compound 8* 66 Figure 3.12: Packing of squa re grids in 9 facilitates stacking interactions* 68 Figure 3.13: Tetragonal nSBU c onstituent of compounds 8 and 9 69 Figure 4.1: Trigonal nSBUs self-ass emble into trigonal 2D sheet 79 Figure 4.2: Four lattices that exhibit spin frustration 79 Figure 4.3: Crystal structure of a 2D stru cture that exhibits the Kagom lattice topology* 81 Figure 4.4: Schematic representation of the molecular Kagom lattice reported by Zaworotko et al 83 Figure 4.5: bdc ligands employe d for making Kagom lattices 84

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viii Figure 4.6: Crystal structure of compound 10* 85 Figure 4.7: Trigonal nSBU constituent of compound 10* 86 Figure 4.8: Crystal structure of compound 11* 87 Figure 4.9: Trigonal nSBU constituent of compound 11* 88 Figure 4.10: Crystal structure of compound 12* 90 Figure 4.11: Trigonal nSBU moiety in compound 12* 91 Figure 4.12: Bilayer packing of compound 12* 92 Figure 4.13: Supramolecular intera ctions in trigonal nSBU of 12* 93 Figure 4.14: Crystal structure of compound 13* 94 Figure 4.15: Trigonal nSBU constituent of compound 13* 95 Figure 4.16: Perspective view of a hexagon extracted from the crystal structure of 13* 97 Figure 4.17: Large channels in crystal structure of compound 13* 97 Figure 4.18: Crystal structure of compound 14* 98 Figure 4.19: Trigonal nSBU constituent of compound 14 100 Figure 4.20: Crystal structure of compound 14 reveals large channels* 101 Figure 5.1: Some of the po ssible chromophores of Cu(II) 109 Figure 5.2: Three supramolecular isomers po ssible from an angul ar node and a linear spacer 111 Figure 5.3: Crystal structure of compound 15 114 Figure 5.4: Stacking interactions in the crystal stru cture of compound 15* 115 Figure 5.5: Possible rearrangement in chromophore of compound 15* 116 Figure 5.6: Chromophore of compounds 16a and 16b* 117

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ix Figure 5.7: Crystal structur e of zigzag chain 16a* 118 Figure 5.8: Hydrogen bonding interac tions between chains of 16a 119 Figure 5.9: Crystal structure of compound 16b* 120 Figure 5.10: Crystal structure packing in 16b 121 Figure 5.11: Hydrogen bonding interactions between stack ing hexagons of 16b 121

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x List of Schemes Scheme 1.1: Schematic illustration of supramolecular superstructures obtained to date from the a T-shape node and a linear spacer 5 Scheme 1.2: Simple supramolecular archit ectures possible from the node and spacer strategy 9 Scheme 1.3: 2D supramolecular architectures possible from T-shape node and a linear spacer 12 Scheme 1.4: 1D structures based on a 90 angular node and a linear spacer 13 Scheme 1.5: Pie chart illustrating relative us e of transition metals in the paddlewheel cluster 21 Scheme 4.1: Schematic representation of the three packing styles observed in the Kagom structures 102

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xi List of Abbreviations 4-pic = 4-picoline 5-NO2-bdc = 5-Nitrobenzene-1,3-dicarboxylate 5-NO2-H2bdc = 5-Nitroisophthalic acid 5-OEt-bdc = 5-Ethoxybenzene-1,3-dicarboxylate 5-OEt-H2bdc = 5-Ethoxyiosphthalic acid 5-OH-bdc = 5-Hydroxybenzene-1,3-dicarboxylate 5-OH-H2bdc = 5-hydroxyisophthalic acid 5-OPr-bdc = 5-Propoxybenzene-1,3-dicarboxylate 5-OPr-H2bdc = 5-Propoxyisophthalic acid 5-ph-bdc = 5-Phenylbenzene-1,3-dicarboxylate 5-ph-H2bdc = 5-Phenylisophthalic acid AFM = Atomic force microscopy Bdc = Benzene-1,3-dicarboxylate CSD = Cambridge structural database DMF = Dimethylformamide DMSO = Dimethylsulfoxide ES FT-ICR = Electrospray ionization Four ier transform ion cy clotron resonance mass spectrometry

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xii ES-MS = Electrospray mass spectrometry Et = Ethyl EtOAc = Ethylacetate EtOH = Ethanol H2bdc = Isophthalic acid H3TMA = Trimesic acid MeCN = Acetonitrile MeOH = Methanol NMR = Nuclear magnetic resonance nSBU = Nanoscale secondary building unit pdc = 1-Methylpyrole-2,4-dicarboxylate Ph = Phenyl py = pyridine SBU = Secondary building unit TGA = Thermal gravitational analysis TG-MS = Thermal gravitational-mass spectrometry THF = Tetrahydrofuran TLC = Thin layer chromatography XPD = X-ray powder diffraction

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xiii Structural Diversity in Metal-Organic Nanoscale Supramolecular Architectures Heba Abourahma ABSTRACT Supramolecular synthesis has gained much attention in recent years. Such an approach to synthesis represents an attract ive alternative to traditional, multi-step synthesis, especially for making complex, nanos copic structures. Of particular interest, in the context of this work, is the use of metal-organic interactions to direct the selfassembly of nanoscopic architectures. Th ese interactions are highly directional, relatively “strong” (compared to other supramol ecular interactions) a nd kinetically labile, which allows for “self-correction” and in tu rn the production, often in high yield, of defect-free products. This also means that a number of related, yet structurally diverse products (supramolecular isom ers) could be isolated. The work presented herein demonstrates the supramolecular synthesis of related, yet structurally diverse family of metalorganic nanoscale supramolecular architectures that are based on the ubiquitous paddle-w heel dimetal tetracarboxylate secondary building unit (SBU) and angular dicarboxylate li gands. It also demonstrates that the SBU self-assembles into clusters of four (tetragonal) and three (trigonal) nanoscale secondary building units (nSBU), which further self-assem ble into nanoscale structures that include discrete (0D) faceted polyhedr a, tetragonal 2D sheets and anot her 2D sheet that conforms

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xiv to the so-called Kagom lattice. In addition, the work herein demonstrates that synthesis under thermodynamic equilibrium conditions facil itates “self-correction” so that the most stable thermodynamic product is obtained. Synthesis, characterization and crystal structure analysis of these st ructures is presented herein.

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1 Chapter 1 INTRODUCTION 1.1 Supramolecular Chemistry Supramolecular chemistry, which has its r oots in biological systems, was defined by Lehn as the chemistry beyond the molecule.1,2 The concepts of supramolecular chemistry are derived from biology and rely on the phenomena of mo lecular recognition and self-assembly: molecules recognize co mplementary sites (functionality, geometry, size, etc. ) on other molecules and associate into larger entities, supermolecules, via weaker, non-covalent interacti ons such as hydrogen bonding and stacking interactions. The phenomenon of molecular as sociation has been long recognized and the term “bermolekle” i.e. supermolecule was introduced as early as 1930s to describe highly organized assemblies that resu lt from the association of units.3 The application of the concepts of supramolecular chemistr y to synthesis, however, is a recent phenomenon.4,5 Such an approach to synthesis pr esents an attractive alternative to traditional, multi-step synthesis. The latt er, although very useful for the synthesis of relatively small organic molecules, is intrinsically limited, especially for the synthesis of complex nanoscale architectures. In additi on to being time consuming and overall low yielding, defected products are difficult to fi x because of the kinetic inertness of the

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2 covalent bond. Supramolecular synthesis, on the other hand, can be accomplished in a one-pot reaction.6 Upon combining the appropriate bu ilding blocks, they spontaneously self-assemble into a well-defined supr amolecular architecture under thermodynamic equilibration. In addition, since supramolecu lar interactions are specific and kinetically labile, defect-free products are obtained in a single step, often in high yield. Consequently, there has been a wide shift towards the use of this synthetic approach to building nanoscale architectures over the last decade as it offers the possibility of preparing compounds with complexity n earing those of biological systems.7 Various supramolecular interactions have be en exploited including hydrogen bonding,8,9 coordinate covalent bonds, elec trostatic and charge transfer interactions and aromatic stacking interactions. The di rectionality and reliability of the hydrogen bond and the coordinate covalent bond resulted in the two being perhaps the most widely used interactions in supramolecular synthesis.6,10 1.2 Crystal Engineering Crystal engineering10-15 is the application of the s upramolecular concepts of selfassembly to the design and synthesis of functi onal solids. The term “crystal engineering” was first introduced in 1955 by Pipensky.16 Schmidt’s work in topochemistry then showed that crystals can be thought of as supramolecular assemblies that result from a series of molecular recogniti on events and self-assembly.11 The work of Etter17 and Desiraju18 in the 1980s, which focused on using the Cambridge Structural Database19 (CSD) to analyze, interpre t and design non-covalent bonding patterns, specifically hydrogen bonding in organic solids, developed the field further. Desiraju later introduced

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3 the concept of “supr amolecular synthon”,15 which resulted in the wide spread of supramolecular synthesis via hydrogen bonding in teractions. Although the beginnings of crystal engineering were mainly concerned with organic molecules, the field today encompasses additionally organometallics20 and coordination compounds.6,7 Self-assembly via coordina tion, or “metal-directed”6 self-assembly provides a highly attractive supramolecular synthetic approach. The coordination bond is a relatively strong bond (10-30 kcal/mol) compared to other non-covalent interactions (0.710 kcal/mol) and in turn is kinetically stable Yet, it is thermodynamically labile, which facilitates “self-correction”.6,7 A bond that forms incorrectly can dissociate and reassociate correctly leading to a defect-free product. In addition, incorporation of transition metals into the supramolecular structure may have important implications for the chemical reactivity or physical properties of the superstructure such as color, Lewis acidity, magnetism, redox activity and luminescence.21 Furthermore, the wide range of available transition metals with various geometries can be “mixed-and-matched” with a variety of coordinating ligands to tailor-m ake products with specific shapes, sizes and properties. Since coordination compounds necessarily consist of at least two components, they are inherently modular.22 Modularity leads to st ructural diversity since the same type of network can be obtained fo r a number of different metals and organic ligands. Consequently, supramolecular synthe sis via the modular approach is powerful as it facilitates fine-tuning of structural and functional feat ures by careful selection the molecular building blocks. The concept of spontaneous self-assembly of molecular components into a desired superstructure is highly appealing; however, it must be acknowledged that a number of

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4 factors influence the self-assembly process and ultimately product formation. These include solvent effect, concen tration, pH, possible formation of kinetic products, and the presence of sterically or el ectronically demanding functi onal groups on the substrates.7 This can be viewed as an opportunity to ge nerate a number of related, yet structurally diverse networks from the same set of build ing blocks by controlling the factors that affect the reaction. Zaworotko el al. has coined the term “supramolecular isomerism” to describe structurally related supermolecules that result from different arrangement of the same molecular components in the solid state.10,12,23 1.3 Supramolecular Isomerism Just as atoms can combine in different ways on the molecular level and give rise to molecular isomerism, molecular compone nts of a modular system can combine in different ways to give rise to supramolecula r isomerism. Structural diversity, be it by varying the molecular component s or varying the architecture or topology (architectural isomerism)24 of the superstructure, results in divers ity in the bulk physical properties such as magnetism, optical activity, polarity, poros ity and robustness. For example, 3D structures would be expected to be zeo lite-like structures and have rigidity and porosity,25-27 whereas 2D structures are expected to behave as clay-mimics intercalating guest molecules between their layers.12,28 1D structures would be expected to have variability in close-packing in the context of how close the chains pack with respect to one another,29 whereas for 0D (discrete) structures one would expect monodispersity and solubility. Of course, the number of supr amolecular isomers possible from a set of building blocks is limited by the number of ra tional superstructures that can be generated

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5 (a)(b)(c) (d)(e)(f) (a)(b)(c) (d)(e)(f) Scheme 1.1 Schematic illustration of supramolecular superstructures obtained to date from the a T-shape node and a linear spacer (a) 1D ladder, (b) 2D brick wall, (c) 2D herringbone, (d) 3D Lincoln Logs, (e) a nother 3D framework, (f) 2D bilayer from the molecular components present in a network. We ca n classify supramolecular isomerism, just as one would for molecular isomerism, into the four following categories. 1.3.1 Structural Isomerism This type of isomerism arises when the molecular components of a system arrange in a number of differe nt ways in the solid state.23 Scheme 1.1 illustrates the diversity in the supramolecular superstructures obtained to date from the combination of a T-shape node, which could be a mer -substituted octahedral metal or a trisubstituted square planar metal moiety, a nd a linear spacer: (a) ladder,30-32 (b) brick wall,33 (c) herringbone,34-37 (d) 3D frame of “Lincoln Logs”,38,39 (e) another 3D frame40 and (f) 2D bilayer.41,42 Three of the isomers, the ladder,32 the bilayer,42 and the 3D frame40 have

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6 been observed for the asymmetr ic unit resulting from Co(NO3)2 as the metal and bipy as the linear ligand, whereas the ot her three have been seen in similar compounds that utilize bipy or an extended analogue as the linear spacer. It is apparent fr0m the schematic that these structures would have dramatically different chemical and physical properties. 1.3.2 Conformational Isomerism This type of isomerism is possible, when the organic ligand is flexible. Such isomerism is observed, for example, in the crystal structures obtained from 1,2bis(pyridyl)ethane (bipy-eta), wh ich can adopt two conformations: gauche or anti This can lead to subtle changes in the crysta l structure. For example, when Co(NO3)2 and bipy-eta are crystallized from chloroform, bipy-eta adopts the anti conformation and a ladder structure containing six chloro form molecules per cavity results.43 However, when a different crystallization solvent is used, such as MeCN23 or dioxane, a bilayer architecture is obtained where both conformations are observed: two antiand one gauchespacer ligand per metal. Conformati onal polymorphism is a closely related subject.44,45 5-Methyl-2-[(2-nitrophenylamino) ]-3-thiophenecarbonitrile is known to crystallize in at least six polymorphic pha ses that differ in the angle between the thiophene moiety and the o -nitroaniline fragment, which varies from 21.7 to 104.7.46 This is a dramatic illustration of how the conformational flexibility can influence crystal packing in organic compounds.

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7 1.3.3 Catenane Isomerism This type of isomerism is often observed in crystal structures of supermolecules that have large cavities. In the desire of the structure to close-pack and avoid open cavities, independent frameworks generally inte rpenetrate one another. In the context of coordination polymers, Batten and Robson have published a thorough review on the subject.47 It is becoming obvious that one can co ntrol the generation of either the open framework or the interpenetrated isomer by th e use of appropriate template molecules. For example, the diamondoid network based on Cd(CN)2 48 and square grids based on M(bipy)2X2 have been prepared as both catenated and open framework49 forms upon use of appropriate templates. In the contex t of organic molecules, trimesic acid, H3TMA, is perhaps the prototypical example. Pure H3TMA50 is known to self-assemble via Hbonding into 2D honeycomb grid with cavities of ca. 1.4 nm in diameter. Three phases of H3TMA have been characterized to date: two interpenetrated and one open framework. In one of the interpenetrated structur es, the honeycomb grid is puckered and interpenetrated by two other independen t networks (3-fold interpenetration).50,51 There still exist small cavities that host guest molecules. In the other interpenetrated form of H3TMA the sheets are flat and contain 1D ch annels. The open framework structure of H3TMA is obtained when it is crystallized in the presence of a long chain alkane.51,52 1.3.4 Optical Isomerism Chirality in crystals can result by one of at least four ways:10 (1) crystallization of achiral building blocks into a chiral space group, (2) building a chiral framework from

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8 achiral building blocks, (3) presence of chiral guest mo lecules in an achiral host framework built from achiral building blocks, (4) an achiral framework built from achiral building blocks and hosts achiral guest mo lecules. This type of supramolecular isomerism is highly relevant to spon taneous resolution of chiral solids.53-59 1.4 Coordination Polymers Coordination polymers represent an ideal example where the supramolecular concepts of self-assembly have been applie d to crystal engineeri ng. For the design and synthesis of coordination polymers, the “Wellsian” approach60 is widely practiced. Wells, whose work focused on inorganic networks,61 defined crystal structures in terms of their topology by reducing them to a seri es of points (nodes) of certain geometry (tetrahedral, trigonal planar, etc. ) that are connected to a fixe d number of other points. The resulting structures, which can also be calc ulated mathematically, can either be 1-, 2or 3-D periodic nets. Robson and co-workers62 later expanded Wells’ work on inorganic network structures into th e realm of metal-organic coordination polymers, which facilitated the rapid development of the fiel d. The “node and spacer” approach has been exceptionally successful in producing predicta ble network architectures. The “node” is typically a transition metal, whereas the “sp acer” is typically an organic ligand that propagates the node. Scheme 1.2 illustrates some of the simplest architectures that can be generated using commonly available metal mo ieties and linear organic spacer ligands.

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9 A number of features are a pparent from the scheme: th e structures are all modular22 consisting of at least two components; the dime nsionality of the resu lting superstructure is dependent on dimensionality of the node; the structures inherently contain voids which leads to porosity. In principle, these networ k structures can be rega rded as blueprint for the construction of networks from a diverse range of chemical components, provided the components are predisposed to self-assembly. Examples of coordination polymers are presented below. 1.4.1 Examples of 3D Structures The two most common types of 3D struct ures are the diamondoid and octahedral networks, both of which have precedence in nature. The former is generated from (a)(b)(c) (d)(e)(f) Scheme 1.2 Simple supramolecular architectures possi ble from the node and spacer strategy (a) 2D square grid (44 net), (b) 2D brick wall, (c) 2D honeycomb, (d) 3D diamondoid network, (e) 3D octahedral network, (f) 3D Lincoln Logs

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10 tetrahedral nodes, whereas the latter is generated from octa hedral nodes. The diamondoid networks reported by Robson in the 1990s,63-66 based on a tetrahedral metal node (Zn or Cd) and a linear spacer (CNligand), are the prototypal examples of metal-organic diamondoid networks. Both structures ar e 2-fold interpenetrated, but the Cd(CN)2 structure was also obtained as non-interpen etrated open framework in the presence of CCl4, which fits snugly inside the ca vity preventing interpenetration.67 The two Cd(CN)2 networks could be considered catenated and non-catenated supramolecular isomers of each other. Following Robson’s report, a num ber of modular diamondoid networks were reported with interpenetration (2-, 3-, 4-, 5, 7-, or 9-fold) being observed in all cases.68-71 Octahedral networks have been known and st udied for a long time and are exemplified by iron cyano compounds. X-ray studies72 of Berlin Green [FeIIIFeII(CN)6], Prussian blue, [KFeIIFeIII(CN)6] and Turnbull’s Blue, [K2FeII-FeII(CN)6] revealed that the metal acts as an octahedral node that is linked by the linear CNligand. These three networks are isostructural differing only in the amount of K+ included in the structure and the oxidation state of the iron ion. The first s ynthetic metal-organic octahedral networks were reported in 1995. Ionic73 and neutral74 examples of the octa hedral network were reported with different metal ions and diffe rent length organic spacer. Octahedral networks, which are less common relative to their diamondoid counterparts, have great potential as porous materials.25 This is highlighted by a r ecent report of an octahedral network, Zn4O(BDC)3,75 which has an unprecedented poros ity in a crystalline solid and remarkable stability to loss of guest even wh en heated to temperatures above 300 C. A number of other types of 3D networks that do not have precedence in nature have also been prepared. One strategy to obtain such 3D structures is by “pillaring” 2D structures.

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11 Effectively, in this synthetic strategy a vert ical linker is used to connect 2D sheets together resulting in 3D frameworks.24,76-80 Another strategy is to cross-link infinite 1D chains via metal-metal bonds into 3D “Lincoln Log” structures. Examples of the latter exhibit 3-fold interpenetration, yet they ar e open enough to facilitate ion exchange of loosely bound nitrate anions.38,39 1.4.2 Examples of 2D Structures The most common topology of 2D structures is the square grid. This topology has been reported for a variety of metals and linear bifunctional ligands. The first reported 2D grids were based on cyano ligands.81-84 Today, a wide range of spacer ligands with variable le ngths such as pyrazines,85-87 bipyridine32,74,88-92 and extended bipyridines90,93 have been used to generate square grids with different size cavities. Square grids are clay mimics and are capable of intercalating guest molecules between the grids, but additionally they can intercalate guest molecules in the square cavities present in the plane of the grid.10,94 Other possible 2D structures are illustrated in scheme 1.3, all of which are based on a T-shaped node: a) brick wall, b) herringbone, c) bilayer, d) long and short brick, and e) basket weave.

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12 It is interesting to note th at three of the topologies illu strated in scheme 1.3(a-c) have already been obtained synthetically and perhaps it is only a matter of time before the remaining two are obtained. The honeycomb topology is another example of a 2D net that is commonly observed in organic struct ures, but rarely obse rved in coordination polymers. This is because of the wide avai lability of trigonal organic nodes such as trimesic acid (benzene-1,3,5-tricar boxylic acid), and the rarity of trigonal and trigonal bipyramidal metal geometries. One example of a honeycomb coordination polymer is the crystal structure of [Cu(pyrazine)1.5]BF4 95 which is based on trigonal Cu(I). More honeycomb coordination polymers should be expect ed in the future as a large number of organic ligands with trigonal geometry ha ve been reported in recent literature.6,96-102 (a) (c)(d) (b) (d) Scheme 1.3 2D supramolecular architectures possible from T-shape node and a linear spacer (a) brick wall, (b) herringbone, (c) 2D bilayer, (d) long and short brick, (d) basket weave.

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13 1.4.3 Examples of 1D Structures These can be obtained from any meta l geometry, but require a ditopic coordinating ligand. Scheme 1.4 illustrates tw o possible examples of 1D coordination polymers, the zigzag chain and the helix. The zigzag chain is widely encountered, whereas the helix remains rare for coordina tion polymers. The helix is particularly interesting because of its inherent chirality. Another example of a 1D structure is the ladder motif (scheme 1.1(a)), which can be obtained from a T-shaped node and a linear spacer. The ladder is fundamentally different from the chain and the helix in that it contains cavities that can host guest molecules. 1.4.4 Examples of 0D (Discrete) Structures Although clearly not infinite polymers, it is appropriate to include 0D structures in this discussion because of their co nceptual relevance to the “Wellsian”60,103 approach. Recent years have witnessed increased number of reports of discrete 0D structures (a)(b) Scheme 1.4 1D structures based on a 90 an gular node and a linear spacer (a) zigzag chain, (b) helix. The two sructu res could be considered supramolecular isomers

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14 (metallocycles) with shapes th at approximate regular polygons such as triangles, squares, pentagons and hexagons and more complex pol yhedra (regular Platonic and semi-regular Archimedean). Polyhedral structures are know n in naturally occurring systems such as zeolites (Linde A is based on the edge-s keleton of fused truncated octahedral)104 and in biological self-assembled systems such as mammalian picornaoviruses105-109 and proteins.110 Three general design stra tegies have been outlined in the literature for the synthesis of 0D structures these in clude the “molecular library model”,96 the “symmetry interaction model” and the “weak-link model”.111 The “molecular library model” presents a reliable system for predetermining the shape of the resulting discre te polygon or polyhedron by geom etrical consideration of the molecular building blocks. For example, to construct a molecular triangle one could combine three linear spacers with three complementary, angular, ditopic ligand ( = 60). A molecular square could be assembled in a number of different ways: from the selfassembly of two complementary angular ditopic ligands ( = 90) in 1:1 ratio, or from the self-assembly of angular ditopic ligands ( = 90) with complementary linear spacers in 1:4 ratio. The same strategy could be app lied to making the more complex polyhedra, but obviously in this case tritopic ligands must be employed. For example, a dodecahedron (a Platonic solid) could be prep ared from the self-a ssembly of twenty angular tritopic ligands ( = 109.5) with thirty linear spacers. The geometry of the metal node is controlled by using chelating ligands or “directing ligands”, which block some sites on the metal and make available ones th at lead to the desired geometry. This approach was first applied by Verkade,112 then elaborated by Fujita,102 and finally systematized by Stang.96,113 It is a powerful approach because of its modularity, which

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15 facilitates the generation of the same architect ure from a variety of building blocks that have the appropriate geometries, in addition to facilitating the generation of a number of architectures from one building block upon co mbining it with different complementary ones. For example, the same angular ditopic ligand ( = 120), could be used to generate a rhomboid, a pentagon and a hexagon upon comb ining it with complementary angular ditopic ligands with angles of 60, 120 and 180, respectively. The “symmetry interaction model” empl oys multibranched chelating ligands and transition metals that are ligated by weakly coordinating ligands. The inherent symmetry of the coordination sites drives the formation of a desired supramolecular shape. This strategy is widely used by Saalfrank,114 Lehn115,116 and Raymond97,117 who reported a number of elegant structures including tetrahed ra, cylinders, prisms and helicates via this method. The “weak-link model”111 employs as starting materi als flexible ligands and transition metal complexes that are free of bl ocking ligands. The formation of the desired product is preceded by the formation of an in termediate metal complex which is ligated by hemilabile ligands. As the ligands leadi ng to the final product are introduced, they replace the weak hemilabile ligands resulti ng in the formation of the desired product. It is interesting to note that a number of discrete structures reported to date are highly complex and nanometric, i.e. have dimensions in the nanometer region. Such structures with high complexity would have not been readily feasible via traditional synthetic approaches. Few examples of such structures will be presented for illustrative purposes.

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16 1.4.4.1 Polygons The simplest of the polygons is the triangl e. There are relatively few examples of molecular triangles in the literature. Fujita et al .118 reported molecular triangles made with capped Pd corners and linear organic spac ers with variable lengths. Studies showed that this triangle is in equilibrium with the corresponding molecular square. Chan et al reported luminescent molecular triangles th at are obtained by combining wide angle molecular ligands ( = 150) with angular ditopic metal nodes ( = 90). Cotton et al .119 recently reported an example of a molecular triangle that is based on capped diruthenium tetracarboxylate building units. Molecular squares are the most widely studied molecular polygons. The groups of Fujita,118 Stang96,120 and Hupp121-123 have contributed consider ably in this area and a number of review articles have appeared c oncerning the subject. The molecular square is enthalpically favored as it is not strained and consequently some of the reported molecular triangles are found in equilibrium w ith their square count erparts. Molecular squares are generally prepared from a ditopic angular ligand ( = 90), typically a metal complex, and a linear spacer. It is therefore not surprising that the square planar Pd and Pt metals are the most exploited in making molecular squares. Cotton et al .119 also reported a molecular square isomer of the triangle based on diruthenium tetracarboxylates. Molecular squares could be considered supramolecular isomers of the infinite 1D zigzag chain and helix obtained from a 90 angle node and a linear spacer. Few examples of molecular pentagons124-126 and hexagons124,126-129 have been reported to date.

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17 1.4.4.2 Polyhedra The regular Platonic and se mi-regular Archimedean polyhedra have been widely studied in recent years and a number of review articles describing mo lecular versions of such polyhedra have appeared in the literature.6,96,97,130 Figure 1.1 illustrates the five Platonic and thirteen Archimedean polyhedr a. In both types of polyhedra, a convex structure results from molecular polygons shar ing edges. In Platonic solids, only one type of polygon exists in each polyhedron. For example, the cube results from the edgesharing of squares whereas the tetrahedron re sults from the edge-s haring of triangles. The Archimedean solids, on the other hand, re sult from more than one type of polygon sharing edges to yield the convex solid. All of the Archimedean so lids are derived from the Platonic solids by truncation or twisting. TetrahedronCubeOctahedronDodecahedronIcosahedron Truncated cubeTruncated dodecahedronTruncated icosahedronCubeoctahedron Truncated octahedron IcosidodecahedronSnub cubeSnub dodecahderonRhombicubeoctahedron Truncated octahedron Truncated tetrahedronRhombicosidodecahedron Truncated icosidodecahedron TetrahedronCubeOctahedronDodecahedronIcosahedron Truncated cubeTruncated dodecahedronTruncated icosahedronCubeoctahedron Truncated octahedron IcosidodecahedronSnub cubeSnub dodecahderonRhombicubeoctahedron Truncated octahedron Truncated tetrahedronRhombicosidodecahedron Truncated icosidodecahedron Figure 1.1 Platonic (regular) and Archimedean (semi regular) solids

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18 Molecular versions of such polyhedra ha ve been made via self-assembly of molecular components using non-covalent in teractions including hydrogen bonding and metal-organic covalent bonding. Atwood et al131 reported the synthesis and crystal structure of a neutral nanometr ic spheroid that conforms to a snub cube, an Archimedean solid, and results from the se lf-assembly of six calix[4]res orcinarenes and eight water molecules via 60 H-bonding interactions. Stang et al132 reported the synthesis of the wire-frame of a dodecahedron, a Platonic soli d, via 60 metal-ligand interactions of 50 components: 20 angular tritopic ligands of tr i(4’-pyridyl)methanol and 30 linear ditopic ligands of bis[4,4’-( trans -Pt(PR3)2OTf)]arene (R = Et, Ph and arene = benzene, biphenyl). The estimated diameters of the two derivatives of the dodecahedron are 5.5 and 7.5 nm, respectively. Fujita et al98 reported the self -assembly of 18 Pd(II) ions and 6 triangular ligands (1,3,5-tris(3,5 -pyrimidyl)benzene) into a nanoscale tetrahedral capsule all the faces of which are closed. A common feature to all these nanometric-sized, highly complex structures is that they result from the self-assembly of simple starting materials that are an order of magnitude smaller. 1.5 Nanochemistry and Nanotechnology Nanotechnology is a multidisciplinary field that encompasses chemistry, physics, biology and engineering. It is concerned w ith entities with dimensions in the 1-100 nm regions. This field has gained lots of attention in recent years for what it promises in revolutionizing many existing indust ries. Miniaturization is a key feature for this modern technology. A top-down approach that allows the manipulation of matter at a small level using methods such as photoand electron-beam lithography is used, largely, by

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19 physicists and engineers.133-135 However, this approach has its intrinsic limitations. An alternative and promising a pproach is the “bottom-up”136 approach utilized by chemists. The unique ability of chemists to manipulate molecules via synthesis puts them in an ideal position to develop bottom-up strate gies to building nanoscale molecules.137 This is a challenging task, but success has already been reported. A number of fascinating organic, metal-organic and i norganic structures, discrete6,101,115,116,119,131,132,138-161 and infinite,10,12,25,53,62,64,69,76,99,127,162-177 have been reported to date. 1.6 Characterization of Nanoscale Architectures With the advances made in the design and synthesis of nanoscale architectures, the formation of such structures has become more manageable. Th e biggest challenge, however, remains in the characte rization of such structures, pa rticularly the discrete selfassembled structures. Multinuclear NMR spectroscopy can provide useful data for monitoring the formation of the final pr oduct by monitoring the chemical shifts characteristic of metal-ligand interactions. Ho wever, it fails to provide adequate data in the case of highly symmetrical structures. In addition, if the superstr ucture is based on paramagnetic metals such as d9 Cu(II), this technique becomes obsolete. Mass spectrometry has become a widely used tool for the characterization of such systems. Electrospray mass spectrometry (ES-MS) in part icular, with its soft ionization conditions results in highly charged systems th at allow for the determination of m/z ratio with an isotopic distribution patterns. However, th e high dilution conditi ons typically required for ES-MS coupled with the lability of the metal-ligand bond can affect the thermodynamic equilibrium and result in the formation of entropically favored smaller

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20 assemblies. X-ray crystallography presents an ideal characterization technique; however, since most of the large self-assembled stru ctures are spheroids, they do not pack efficiently in the crystal lattice making it difficult to grow single crystals suitable for this technique and hindering taking full advantage of it. More over, the cavities in these structures and the interstitial space between th e spheroids are typically filled with solvent and/or counter ions, making the handling of th e crystals difficult and the refinement of the structure challenging because of disordered or non-located solvent and counterions. 1.6 Scope and Focus 1.6.1 Scope We have focused our research efforts on the paddlewheel dimetal tetracarboxylate cluster [M2(RCO2)4(L)2] illustrated in Figure 1.2. The cluster results from four 2bridging carboxylate moieties to two Cu(II) ions in the equatorial position. In addition, each Cu(II) ion is coordinated by one apical ligand. The chemical and structural features of this moiety have been widely studied and are well-established. Figure 1.2 Paddlewheel dimetal tetracarboxylate is a 4-connected node (mol ecular square) and a linear spacer

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21 A search using the Cambridge Structural Database (CSD)19 of deposited structures based on the paddlewheel up to January 2004 returned 1116 hits, involving 21 transition metals. Scheme 1.1 demonstrates th e relative use of d-block transition metals in the dimetal tetracarboxylate cluster. Cu(II) is the most widely used transition metal representing 43% (483 hits) of the structures in the CSD, followed by Rh representing 22% (240 hits), then Ru representing 10% (111 hits). 40% of the st ructures have oxygen coordinating ligands in the apical positi on whereas 36% have nitrogen coordinating ligands in the apical position. A survey of bond distances for all paddlewh eel structures of all transition metals returned a wide range of values. For the bond distances to be more meaningful and as the research in this dissertation will focus on Cu(II), a survey of bond distances of paddlewheel structures based on Cu(II) only was conducted. For structures with oxygen Scheme 1.5 Pie chart illustrating relative use of tran sition metals in the paddlewheel cluster Cu 43% Rh 22% Ru 10% Mo 9% Cr 4% Sc Ti V Cr Mn Fe Co Ni Cu Zn Y Zr Nb Mo Tc Ru Rh Pd Ag Cd La Hf Ta W Re Os Ir Pt Au Hg

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22 coordinating ligands in the apical position, the bond distance for the four basal Cu-O bonds ranged between 2.290 and 1.890 with an average of 1.968(29) . The apical Cu-O bond distance ranged between 2.283 and 1.932 with an average of 2.146(43) , whereas Cu-Cu bond distances ranged between 3.256 and 2.575 with an average of 2.640(82) . For structures with nitrogen coor dinating ligands in the apical position, the bond distance for the four basal Cu-O bonds ranged between 2.351 and 1.880 with an average of 1.974(38) . The apical Cu -N bond distance ranged between 2.360 and 2.005 with an average of 2.174(54) , whereas Cu-Cu bond distances ranged between 3.261 and 2.585 with an average of 2.702(128) . On average, bond distances in structures with nitrogen coordinating ligands in the apical position are longer, but within the standard deviation range fo r oxygen-coordinated structur es. Data of average bond distance of all structures (includes O -coordinating, N -coordinating and a mixture of O and N -coordinating ligands), which are summarized in Table 1.1., reveal that there is no statistical difference between the two types of st ructures. As can be seen from the table, bond distances of the four oxygen atoms of th e carboxylate functionalities coordinated in the basal plane range between 2.080 to 1.866 with an average of 1.968(15) . The copper distance to the apical ligand is longer with a ra nge of 2.570 to 2.092 and an average of 2.179(64) . This defines a square pyramidal geometry for the Cu(II) ions in the paddlewheel. The Cu-Cu separation ranges from 2.951 to 2.533 with an average of 2.646(50) .

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23Table 1.1 Bond distance data of surveyed paddlewheel moieties based on Cu(II) from the CSD Cu-O1 () Cu-O2 () Cu-O3 () Cu-O4 () Cu-L () Cu-Cu () Max 2.024 2.067 2.130 2.080 2.570 2.951 Min 1.902 1.866 1.912 1.909 2.092 2.553 Average 1.968(15) 1.968(17) 1.970(17) 1.970(18) 2.179(64) 2.646(50) 1.6.2 The Paddlewheel: a Secondary Building Unit (SBU) The use of the dimetal tetracarboxylate as a secondary build ing unit (SBU) for larger networks is a recent phenomenon. This is evident by the f act that 81% of the paddlewheel structures deposited in the database are discrete structur es. Furthermore, of small number of polymeric structures repor ted (18%), 86% were reported after 1980. The term “SBU” is derived from the no menclature of minerals and zeolites.104 SBUs are finite topological component units that comprise the zeolite framework. Yaghi et al introduced the term in the context of ex tended metal-organic frameworks as they used the paddlewheel to generate porous, zeolite-like structures.178 The term is widely used now to describe the di metal tetracarboxylate moiety. The appeal of the dimetal tetracarboxylate moiety as an SBU is due to a number of attractive features: 1) its chemical and structural featur es are widely studied and wellestablished; 2) it is known for a wide ra nge of metals including catalytically ( e.g. Rh2+) and magnetically ( e.g. Cu2+) active ones; 3) it is a large, rigid, well-defined structure, which means using it as a node with a rigid spacer will ensu re a rigid, stable superstructure, with potentially large pores;179 4) it is neutral, which eliminates counter ions that typically occupy space in the crys tal structure; 5) it is robust and readily accessible; 6) it is modular, which means it can be produced from different combination

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24 of carboxylate moieties and metal ions; 7) it is versatile as it can be utilized as a 4connected node, or as a linear spacer along the M-M bond (Fig 1.2). In this context, the use of polycarboxylate ligands has been shown to facilitate the utility of the SBU as a 4-connected node wh ere the carboxylate ligands coordinate in the equatorial positions and extend in four directions. A view down the four-fold axis of the SBU moiety reveals that it also resembles a molecular square and that binding of the carboxylates in the equatorial positions would result in linking “molecular squares” at the vertexes.144,163-166,180 The nature of the polycarboxylat e ligand, specifically the angle at which the carboxylate moieties are predisposed, controls the angle at which the molecular squares are connected. Linear dicarboxylat es, such as benzene-1,4-dicarboxylate, link the SBUs at angle of 180, whereas a ngular dicarboxylates, such as benzene-1,3dicarboxylate where the two car boxylates are predisposed at an angle of 120, link the SBUs at the same angle. The topology of th e resulting structure is directly influenced by the choice of the polycarboxylate ligand. The use of aromatic polycarboxylates affords structures with high degree of rigidity and stability, and in the case that the structure possesses large pores, exchange or removal of guest molecules can be facilitated without destroying the framework. This has been demonstrated in extended metal-organic framework (MOF) reported by Yaghi et al where a porous extended network was evacuated from coordinated and guest mol ecules without collapse of the network. Furthermore, due to the large size of the pores in the structure, it e xhibits high degree of porosity that exceeds that of known zeolites.181 Flexible polycarboxylates have also been shown to afford extended structures with a gr eat degree of structural diversity due to the rotational flexibility of the organic ligands.162

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25 The SBU could also be employed as a linea r spacer with an organic ligand acting as the node to generate extended networks. Structures generated via such strategy where the roles of the metal and organic ligand have been reversed compared to traditional coordination polymers have been termed “inverted” metal-organic frameworks.176,182 Using the SBU as a linear spacer is attractive, because not only does it provide a rigid, relatively long spacer, but it can also facilit ate decorating the walls of the resulting extended structure with the functionalities pr esent on the bridging carboxylate ligands. A number of examples have demonstrated the su ccessful use of the SBU as a linear spacer in infinite structures.176,183-188 1.6.3 Focus The focus of this dissertati on is on employing the dicopper (II) tetracarboxylate paddlewheel as a molecular square and connec ting it at the vertexes with angular aryl dicarboxylate ligands. We primarily focuse d on benzene-1,3-dicarboxylate (bdc) and its derivatives to link the SBUs. The angular bdc has the two carboxylate moieties rigidly predisposed at 120, which facilitates bridgi ng the SBUs at the same angle. Figure 1.3 illustrates the linking of SBUs at the angle subtended by bdc. Additionally, we employed the wider angle ligand 1-methylpyrrole -2,4-dicarboxylate (pdc) which has its carboxylates predisposed at an angle of 157. This ligand would link the SBUs in the same manner, but at the wider angle of the ligand.

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26 Confining the SBU into an extended stru cture is expected to have geometric consequences on the bridging angular dicar boxylate ligand. Figur e 1.4 illustrates two possible modes of distortion in the bridging ligand that woul d be expected in order to accommodate the geometry of the resulting structure. The carboxylate functionalit ies could twist out of pl ane with respect to one another by an angle or they could bend towards each other from the plane of the benzene ring by an angle while staying coplanar. Such distortion would help relieve the strain resulting from confining the paddlew heels into an extended structure. It is possible, of course, to have a combinati on of the two distortions. A published CSD (a) No distortion(c) bending (b) Twisting (a) No distortion(c) bending (b) Twisting Figure 1.4 Possible distortion modes of the bdc moiety as it links the SBUs into extended structures Figure 1.3 Angular bdc links the SBUs at angle of 120 subtended by the carboxylate moieties

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27 search of 42 discrete dicopper(II) tetraarylcarboxylate revealed that the phenyl ring twists from the plane of the Cu2O4 on average by an angle of 18(20) with a range of 0 to 88.96 and bends out of plane by an angle on average by 5(4) with a range of 0.18 to 17.98.189 Although the size of the sa mple of structures is small, it can serve as a reference point to determine the relative distortion of the bridging ligands in the structures presented in this work. Herein we will describe the synthesis and structural analysis of a diverse family of related structures, discrete and infinite that result from li nking molecular square dicopper tetracarboxylates at the vertexes by angular aryl dicarboxylates. We will demonstrate that it is possible to build nanoscale, comp lex structures, with drastically different and interesting properties from simple, commercia lly available starting materials in single step reactions. We will also demonstrate that the modularity of thes e systems is powerful as large diversity can be obtained by simple modification of reaction conditions. Finally, we will demonstrate that reactions co nducted under thermodynamic equilibrium conditions can result in a number of other chromophores, which would not be possible under solvothermal conditions that yield interesting nanoscale structures.

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28 Chapter 2 0D NANOSTRUCTURESNANOBALLS 2.1 Preface Polyhedral structur es are known in naturally occurring systems such as zeolites (Linde A is based on the edge-skele ton of fused truncated octahedra104) and in biological self-assembled systems such as mammalian picornaoviruses105-109 and proteins.110 The regular Platonic and semi-regular Archimedean polyhedra have been widely studied in recent years and a number of seminal papers describing molecular versions of such polyhedra have appeared in the literature.130,132,190 The design principles behind the development and isolation of these classes of compounds are based upon the concepts of self-assembly of geometrically suitable mol ecular components. In the context of metalorganic compounds, two general design principl es have been implemented to generate molecular versions of these solids. One st rategy involves the use of linear binfunctional rod-like ligands to connect molecular vertices with the appropriate geometry to generate the desired polyhedral shape.6,96 For example, a molecular version of the dodecahedron has been prepared from the reaction of a linear spacer with tetrahedral node.132 This design strategy, which has been widely used by Stang, generates the wire-frame or the edge-skeleton of the targeted polyhedron and in turn all the faces of the convex structure

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29 are open windows. The second design strate gy, which has been extensively used by Fujita, is “molecular paneling”.191 This approach is ba sed on connecting molecular moieties that have the shape of regular polygons at their ed ges. This design approach results in convex structures, all th e faces of which are closed. Other examples of uniform polyhedra such as prisms and antiprisms, polyhedra with star faces and vertices, and polyhedra with both concave and convex faces, have remained unexplored synthetically until recently.144,180,192 The latter group (one containing concave and c onvex faces) is known as faceted polyhedra. There are nine uniform faceted polyhedra that are closel y related to the Platonic and Archimedean solids but differ in the fact that their convex faces are constructed by connecting regular polygons at the vertices. This result s in both open (concave) and closed (convex) faces, hence the name faceted Figure 2.1 depicts all nine faceted (h) Small cubicuboctahedron (e) Small rhombihexahedron (b) Octahemioctahedron (h) Small cubicuboctahedron (e) Small rhombihexahedron (b) Octahemioctahedron (i) Small dodecicosidodecahedron (f) Small rhombidodecahedron (c) Small icosihemidodecahedron (i) Small dodecicosidodecahedron (f) Small rhombidodecahedron (c) Small icosihemidodecahedron (a) Tetrahemioctahedron (d) Cubohemioctahedron (g) Small dodecahemidodecahedron (a) Tetrahemioctahedron (d) Cubohemioctahedron (g) Small dodecahemidodecahedron Figure 2.1 The nine faceted polyhedra

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30 polyhedra, which can be divided into four groups based on their constituent polygons: triangles only (a-c), square s only (d-f), pentagons only (g ), mixed polygons (squares and triangles (h) and pentagons and triangles (i)). Since we targeted the molecular square SBU as a building block, the square-only f aceted polyhedra are relevant. These three polyhedra differ by the angle at which the squares are connected: the cubohemioctahedron (d) has the squares connected at 90 angle, the small rhombihexahedron (e) has the squares connected at 120 angle, and the small rhombidodecahedron (f) has the squares connected at 144 angle. Since our dicarboxylate linker, bdc, has its carboxylate moieties predis posed at an angle of 120, the generation of the small rhombihexahedron should be feasible. Herein we discuss the synthesis, character ization and structural analysis of three examples of the small rhombihexahedron obtained from the reaction of bdc and its derivatives with Cu(NO3)2. 2.2 Results and Discussion The slow diffusion of a methanolic solution of Cu(NO3)2 2.5H2O into a methanolic solution of H2bdc and pyridine containing nitrobenzene template molecules yielded blue-green crystals, compound 1 within hours. Single crystal X-ray data revealed that 1 is the spherical structure show n in Figure 2.2. There are two crystallographically independent copper ions in the crys tal structure of 1 that sit on a 4fold rotation axis; i.e. all coordinated oxygen atoms of the bridging bdc ligands are equivalent (dCu1…O1 = 1.9592(53) ; dCu2…O2 = 1.9522(53) ). The apically coordinated ligand is highly disordered and has been m odeled as a single oxygen atom, but could be

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31 methanol, water or pyridine (dCu1…O11 = 2.1644(105) ; dCu2…O21 = 2.1646(129) ). This defines a square pyramidal geometry around the copper ions. The two copper ions are separated by 2.6039(25) . All bond distances are in agreement w ith those calculated from the CSD search (section 1.7.1). Examin ing the bdc ligand to determine if it is distorted as it confines the SBUs into this spherical structure reveals that the twist angle by which the two carboxylates twist out of plane with respect to each other is 9.65 and the bend angle by which the two carboxylates bend towards each other, is 5.34. This is comparable to and observed in discrete SBUs ( avg= 18(20), avg= 5(4)).189 The structure consists of 24 bdc moieties, 12 Cu2 dimers, and 24 apical ligands coordinated to the Cu(II) ions; i.e. the structure results from the self-assembly of 72 components in a single step in to the spherical entity depicted in Figure 2.2. Representing the structure schematically by replacing each SB U with a square reveals that the squares connect at the vertices into a spherical st ructure that conforms to the shape of the small rhombihexahedron The structure has 12 square faces, 6 square windows, 8 triangular windows and a hollow cavity. Each squa re face contains in its center a Cu2 dimer with one Cu(II) ion lying on the exterior of the sphere a nd one on the interior of the sphere. In turn, two types of apical ligands exist, exterior and interior.

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32 Compound 1 has an effective exterior diameter of 2.89 nm, as measured from opposite H-atoms of the 5-position of oppos ite bdc ligands, and a molecular volume including the solven t sphere of 10,615 3. The diameter of the internal cavity of 1 measured from opposite copper ions is 16.02 . To calculate the effective volume available for guest molecules, however, one mu st take into account the volume occupied by the internal axial coordina ting ligands. Due to the high disorder of the coordinating apical ligands (over two positions and/or by rotation), location of all atoms of the coordinated ligand was problematic. As a re sult, the internal diameter could only be calculated based on located and refined c oordinated oxygen atom of the ligand. The effective diameter measured from opposite coordinated oxygen atoms is 8.89 , which results in a cavity volume of ca. 367 3. To put this in context, a C60 molecule has a diameter of 7 and an effective volume of ca. 326 3. In other words, the cavity in compound 1 is big enough to encapsulate a C60 molecule. The calculated solvent accessible volume in 1 is 4251.8 3, which corresponds to 20.0% of the structure. This (a) (b) Figure 2.2 Crystal structure and schematic representation of compound 1* Crystal structure presented in stick m ode. Coordinated apical ligands are presented as single oxygen atoms. Noncordinated guest molecule s are removed for clarity

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33 would be increased to 11791.9 3 (55.5%) upon removal of guest. Each sphere of 1 has 14 closest neighbors: eight of the closest neighbors are 23.98 away, as measured by centroid to centroid separation, whereas the ot her six are 27.69 away. This results in body-centered cubic (bcc) packing as shown in Figure 2.3. For the nanometric size of compound 1 and its spherical shape, it is termed nanoball .144 Microcrystals of 1 can also be obtained quantita tively by direct mixing of the reactants and allowing the solution to stand. Th e crystals are stable in solution, but lose crystallinity when removed from the mother liq uor. The nanoball is sparingly soluble in organic solvents such as THF, EtOAc, DMSO, DMF and MeOH, which facilitated its characterization by mass spectrometry. Elect rospray ionization F ourier transform ion cyclotron resonance mass spectrometry (ES FT-ICR) of 1 confirmed the persistence of the nanoball structure in solution as a span of m/z values corresponding to isotope clusters of the +3 cationic species, [Cu24(bdc)24(MeOH)m(H2O)n(Li)3]+3, were detected (M+= (a)(b) Figure 2.3 Crystal structure packing and schematic representation for compound 1* Non-coordinated guest molecule s have been removed for clarity. Coordina ted apical ligands are presented as single oxygen atoms; remaining atoms were removed for clarity

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34 2094.67 m/z (calc’d 2091.56) for m=24, n=0). The MS spectrum of nanoball 1 can be found in Appendix 1. Zaworotko et al reported two other phases of this nanoball, a triclinic and a monoclinic phase, in addition to another nanobal l structural isomer that differs in the connectivity of the SBUs.144 Yaghi et al also reported the same nanoball structure, which was obtained via hydrothermal synthesis, and described it as a cuboctahedron.193 A derivative of parent nanoball 1 was obtained from the reaction of a derivative of bdc, namely 5-OH-bdc with Cu(NO3)2.180 An alternative synthe tic approach to slow diffusion of methanolic solutions of the st arting materials was needed to produce X-ray quality single crystals of this nanoball. Since the targeted product is a spherical structure with 24 OH-groups decorating the surface, slow diffusion of methanolic solutions of the starting materials did not produce crystals or precipitate. The limited solubility of the starting materials to polar solven ts, however, restricted the choi ce of solvent. As a result, the reaction was performed in methanol and di ethyl ether was allowed to slowly diffuse into the reaction mix to ai d crystallization. Small cr ystals, unsuitable for X-ray crystallography formed via this method. Cons equently, a different synthetic approach was implemented that ultimately led to the fo rmation of single crystals suitable for X-ray crystallography, efficiently and reproduc ibly. Equimolar amounts of Cu(NO3)2 2.5H2O and 5-OH-H2bdc were mixed in methanol to yield a blue solution. Upon addition of 2 equivalents of 2,6-lutidine, an immediate color change (fro m blue to deep blue-green) was observed. Addition of diethyl ether resulted in a precipitate, which upon recrystallization from hot DMSO yielded cr ystals suitable for X-ray crystallography within hours. Contrary to the synthesis of nanoball 1 the use of template molecules did

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35 not appear to have an effect on the formation of nanoball 2 Performing the reaction in methanol in the presence or absence of nitrobenzene follo wing the procedure outlined above produced the same result, as confirme d by X-ray crystallography. Performing the reaction directly in DMSO also yields the de sired product; however, crystals form after 10-14 days in a small amount. Figure 2.4 depicts the crystal structure of nanoball 2 There are four crystallographically independent copper ions in the structure each set of two comprising an SBU (dCu1-Cu2= 2.654(5), dCu3-Cu4= 2.641(3) ). The average Cu-O distance for the four oxygen atoms of the four 2 5-OH-bdc ligands coordinated in the basal positions is 1.953(10) . The apical position is coordinated by DMSO (dCu3-O= 2.121(6) ), MeOH (dCu2-O= 2.09(3)) or H2O (dCu1-O= 2.06(3), dCu4-O= 2.138(16)). Of the 12 exterior copper ions four are coor dinated by MeOH molecules a nd eight are coordinated by DMSO molecules. All the interior copper ions are coordinated by water molecules. (a) (b) Figure 2.4 Crystal structure of compound 2* Crystal structure of 2 presented in stick mode (a) and space-filling mode (b). Coordinated apical ligands are presented as single oxygen atoms with the remaining atom s removed for clarity. Noncoordinated gue st molecules were removed for clarity

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36 A close look at the 5-OH-bdc ligands to determine whether they are distorted reveals that two types are present: ones that constitute the square windows and ones that constitute the triangular windows. The 5-OH-bdc ligands constituting the square windows have their carboxylate moieties twisted from planar ity with respect to one another by = 8.61, whereas the ones constituting the triangular windows have their carboxylate moieties twisted by 5.71. The bend angle was found to be 4.89 and 5.13 for the ligands in the square and triangular windows, respectively. The average twist and bend angles observed in this nanoball is smaller than that observed in compound 1 (9.65 and 5.33, respectively). This nanoball is not perfectly spherical as evident by two measured lengths for the diameter. The external and internal diameters measured from two opposite copper ions gave two values each: 21.229, 21.239 and 15.947, 15.931, respectively. The effective external diameter of nanoball 2 measured from opposite hydroxyl group oxygen atoms is 2.99 nm. A single molecule of 2 occupies a volume of 17,422 3 (including the solvent sphere) and has a mo lecular weight of 7.43 kDa. The average internal diameter of nanoball 2 (measured from opposite c opper ions) is 15.94 . However, taking into account the volume occupied by the coordi nated axial ligands results in an effective diameter of 9.0 2 measured between opposite coordinated oxygen. This affords a cavity with a volume of 384 3 (slightly bigger than that of nanoball 1 339 3). The solvent accessibl e volume per one nanoball of 2 is 8241 3, which corresponds to 47.3% of the structur e. This could be increased to 25,611 3 (73.5%) upon removal of guest molecules. Disordered solvent molecules occupy the cavity of the nanoball and the interstitial space between the nanoballs, which pack in a

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37 body-centered cubic fashion as illustrated in Figu re 2.5. It is important to note that not all solvent and/or guest molecules we re located in the crystal structure, which is reflected in the high value of the calculated solvent acc essible volume and the low density of the structure ( = 0.708 g/cm3). Crystals of 2 persist in solution indefin itely; however, they are highly deliquescent, turning into liquid, briefly upon e xposure to the atmosphere. It is possible that this is due to excess DMSO on the surface of the crystals, since rinsing with an abundance of ether or placing the crystals under vacuum overnight allows the isolation of microcrystals of nanoball 2 The stability of nanoball 2 to acidic and basic conditions was evaluated, qualitatively. The blue me thanolic solution of nanoball 2 was found to change color and produce a white precipitate in strongly acidic (pH 3) and strongly basic (pH 9) media, suggesting the deformation of the nanoball. Figure 2.5 Perspective view of the bcc pa cking in the crystal structure of 2* *Apical coordinating ligands are presented as single oxyge n atoms with the remaining atoms removed for clarity. Non-coordinated guest molecule s were removed for clarity

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38 Nanoball 2 is readily soluble in polar protic solvents (MeOH, EtOH, i -PrOH), and sparingly soluble in hot DMF a nd hot acetonitrile. The persis tence of the ball structure in methanol solution was confirmed by ES FT -ICR mass spectrometry, where a span of m/z values corresponding to isotope clusters of the +4 cationic species, [Cu24(5-OHbdc)24(MeOH)m(H2O)n(Li)4]+4, was detected in the spectrum (M+= 1670.35 m/z (calc’d 1661.20) for m=24, n=0). The mass spectrum of 2 can be found in Appendix 2. The enhanced solubility of compound 2 in MeOH facilitated microcrystals growth on the surface of mica and gla ss via evaporation from metha nol solution and study of its surface properties by atomic force microscopy (AFM). AFM has become one of the most widely used tools for studying crysta l growth and behavior on surfaces.194-199 For example, AFM has been used for the study of size control of na nocrystals on LangmuirBlodgett films,194 protein crystal growth,195 molecular and nanotribology,196 statistical analysis of 2-D crystal sizes,197 dopant effects on crystal growth,198 and annealing effect on crystallization.199 AFM studies revealed th at microcrystals of nanoball 2 have uniform dimensions and that they are stab le even after mild heating. The images obtained on a mica surface, without thermal treatment, show increasing density of microcrystals with increasing concentration (Figure 2.6). The microcrystals have an average size of 1.3 (0.4) nm, average height of 140 (30) nm a nd roughness (RMS) of 56 nm. In the case of films prepared on glass, it was found that thermal treatment (at 37 C and 75 C for 24hrs) was necessary to rem ove a contaminant film formed by residual solvent. Annealing at 37 C and 75 C re sulted in microcrystals having RMS of 236 and 261 nm, respectively. Image analysis shows th at the average crystal size of 1.4 (0.4) nm is the same for both temperatures; height of crystals was found randomly distributed

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39 around 500 nm for the sample annealed at 37 C whereas the sample annealed at 75 C, showed a clear statistical distribu tion of heights at 300, 600 and 900 nm. Synthesis of a second derivative of the parent nanoball was feasible from the reaction of another derivative of bdc, namely 5-NO2-bdc with Cu(NO3)2. The slow diffusion of a methanolic solution of pyrid ine into a methanolic solution containing Cu(NO3)2 2.5H2O, 5-NO2-H2bdc and nitrobenzene template molecules yielded bluegreen cubic crystals suitab le for X-ray crystallography. (a) Annealing at 37C (b) Annealing at 75C Figure 2.6 Microcrystals of 2 annealed for 24 hrs on mica (a)(b) Figure 2.7 Crystal structure of compound 3* Crystal structure of 3 presnted in stick mode (a) and space filling mode (b). Coordinated apical ligands are presented as single atoms in the space filling mode with the remaining of the atoms removed for clarity. Noncoordinated guest molecules were removed for clarity

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40 The crystal structure of compound 3 is depicted in Figure 2.7. There are 12 crystallographically independent copper ions in the crys tal structure of 3 each having a square pyramidal geometry. The basal plane is defined by four oxygen atoms of the four 2 5-NO2-bdc moieties (dCu-O(avg.)= 1.9428(124), 1.9549(124), 1.9632(90), 1.9713(87) ) and the apical position is occ upied by coordinating pyridine (dCu5-N= 2.097(7), dCu7-N= 2.102(6), dCu9-N= 2.089(8), dCu11-N= 2.103(8) ), water (dCu4-O= 2.150(8) ) or methanol (dCu1-O= 2.150(8), dCu2-O= 2.154(8), dCu4-O=2.150(8), dCu6-O=2.164(8), dCu8-O=2.168(8), dCu10-O= 2.159(9), dCu12-O= 2.165(8) ). The Cu-Cu distance is in the range of 2.6265(18) and 2.6583(18) with an average of 2.6480(120) . All bond distances are in agreement with the average bond distances calculated from the CSD search. Of the 12 exterior copper ions of nanoball 3 two are coordinated by MeOH molecules, two by water molecules and eight by pyridine molecules. All interior copper ions are coordinated by disordered methanol molecules. Nanoball 3 is not perfectly spherical as the diameter of the equators diffe red. The external diameter measured from opposite copper ions ranged between 21.180 and 21.385 with an average of 21.298(83) . The internal diameter measured from opposite copper ions ranged between 15.896 and 16.105 with an average of 16.003(89) . In order to determine the effective volume of the cavity available to guest molecu les, one must take into account the volume occupied by internal coordinating axial lig ands. The effective shortest diameter measured from opposite coordinated oxygen atom s is 8.80 , which affords a cavity with a volume of 357 3 (compared to 339 and 384 3 for nanoballs 1 and 2 respectively). The average effective external diameter meas ured from opposite nitro groups is 3.13 nm.

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41 A single molecule of 3 occupies a volume of 12,645 3 (including the solvent sphere) and has a molecular weight of 7.76 kDa (excluding guest molecules). Guest molecules are encapsulated in th e square and triangular windows of the ball, but not in the in ternal cavity. Figure 2.8 illustrates the host-guest inclusion complex that results from nanoball 3 Three types of square windows were identified. (a) One square window hosts pyridine and a nitrobenzen e molecule that interact with each other by face-to-face stacking interactions (dcentroid…centroid= 3.682 ). This is within the acceptable range of 3.4-3.8 for centroid-centroi d distances of aromatic rings involved in face-to-face stacking interactions.200,201 Additionally, the pyridine molecule interacts with one of the 5-NO2-bdc ligands constituting th e wall of the square window (dcentroid…centroid= 3.449 ). Figure 2.9(a) illustrates these interactions. (b) The second type of square window hosts two anti-para lleled nitrobenzene mo lecules that do not interact with each other, but each interacts with one of the 5-NO2-bdc ligands constituting Figure 2.8 Nanoball 3 encapsulates guest molecules in the triangular and square windows* The wire-frame of the nanoball is presented in stick mode and colored yellow for clarity. Gu est molecules are presented in space filling mode

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42 the walls of the window (dcentroid…centroid= 3.757, 3.904 ). Figure 2.9(b) illustrates these interactions. (c) The third type of square window hosts disordered water and methanol molecules (Figure 2.9(c)). The guest metha nol molecules appear to be involved in Hbonding interactions. Four types of triangular windows were id entified. (a) One ty pe of triangular window hosts two nitrobenzene molecules th at interact with each other by CHinteractions (dCH… = 2.568 ). Additionally, the nitroben zene molecule parallel to the wall interacts with the 5-NO2-bdc constituent of the wall by face-to-face stacking interactions (dcentroid…centroid= 3.568 ). These interactions are illustrated in Figure 2.10(a). (b) The second type of triangular window hosts a methanol and water molecule that sit inside the cav ity in addition to a nitrobenzene mo lecule that sits above the cavity with the nitro group pointing in so that it intera cts with the water molecule by weak hydrogen bonding (dO…O= 3.138 ). This is illustrated in Figure 2.10(b). (c) The third type of window hosts one water molecule, as illustrated in Figure 2.10(c). The shortest (a)(b) (c)Figure 2.9 Host-guest interactions in the square windows of compound 3* *Three types of square windows identified: (a) nitrobenzene and pyridine guest molecu les interact with each other by faceto-face stacking interactions. (b) Two nitrobenzene guest molecu les interact with the walls of the square window by face-to-face stacking interactions. (c) Guest water and methanol mo lecules in the third type of square window. Guest molecules are presented in space-filling mode whereas the windows are presented in s tick mode. Apical coordinated ligands are presented as single atoms with the remaining atoms removed for clarity

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43 O…O contact between the guest water molecule and the coordinated methanol ligands is 3.630 . (d) The fourth type of triangular window hosts disordered water molecules, as illustrated in Figure 2.10(d). Different values for the bending and twisting angles of the 5-NO2-bdc ligands in nanoball 3 were determined depending on whether they are in the square or triangular window, but in general ranged between 4.13 and 6.08 with an average of 5.31 for and 2.92 and 16.0 with an average of 11.2 for In general, these distortions appear to be influenced by interactions with guest molecu les present in the windows. In addition to guest molecules occupying the windows of the nanoball, guest nitrobenzene, pyridine and methanol were located in th e interstitial space between the nanoballs as illustrated in Figure 2.11. (a)(b)(c)(d) Figure 2.10 Host-guest interactions in the triangular windows of compound 3* Four types of triangular windows identified: (a) Two nitrobenzene guest molecules in teract with each other by edge-to-face CHinteractions. (b) Three guests are present: water, metha nol and nitrobenzene. The latter sits above the cavity and interacts with the water molecule via w eak hydrogen bonding. (c) Guest water molecule occupies the third type of triangular window. (d) Disordered water molecules are present in the four th type of triangular window. Guest molecules are presented in space-filling mode whereas the windows are presented in stick mode. Apical coor dinated ligands are presented as single atoms with the remaining atoms removed for clarity

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44 The solvent accessible volume per a single nanoball of 3 is 932.3 3, which corresponds to 7.4% of the structur e (compared to 20.0% and 47.3% for 1 and 2 respectively). This could be increased to 6586 A3 (52.1%) upon removal of guest molecules. Each nanoball is surrounded by tw elve (8+4) closest nei ghbors with distances in the range of 24.204-26.384 for the former and 28.017 for the latter. As a result, the geometry around each nanoball can be descri bed as a distorted cuboctahedron, which results in a packing arrangement simila r to cubic closestpacking (ccp). It is worth noting that our laboratory has generated eight other variants of nanoball 3 that differ in the identity of the coordi nating apical ligands, and exhibit similar inclusion properties.202 The three nanoball structures have a numb er salient features that are worth highlighting. (1) They are nanometric in size. The exterior diameters of nanoballs 1 2 and 3 are ca. 2.89, 2.99 and 3.13 nm, respectively. (2) The use of substituted bdc results Figure 2.11 Aromatic guest molecules occupy the interstitial space between nanoballs of compound 3* Guest molecules that occupy the inte rstitial space between the nanoballs is pr esented in space filling mode and color coded for clarity. The nanoballs are presented in stick mode. Guest molecules that sit in the winows have been removed for clarity. Methanol mol ecules have been removed

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45 in highly decorated nanoballs with 24 functi onal groups on the surface. This enhances the solubility of the nanoballs in addition to introducing a handle on the molecule that facilitates further derivatization. (3) They c ontain 24 copper ions that can be utilized, for instance, for catalysis, redox or magnetism. The modular nature of the structures makes it feasible to exchange the copper ions with different tr ansition metals for which the SBU molecular square is known.19 In turn, a number of different properties can be incorporated just by varying the metal ion. (4) They are porous containing a hollow core in addition to square and triangular windows. Compound 3 clearly demonstrates the ability of the nanoball to encapsulate guest molecules in its triangular and square windows via supramolecular interactions. The shape and size of windows results in different host/guest and guest/guest interacti ons. Similar properties are expected for the other two nanoballs; however, crystal structure data for 1 and 2 did not allow analysis of their inclusion properties. The effec tive internal cavity of the nanoball is ca. 0.37 nm3, which is large enough to encapsulate a C60 molecule.144 Characteristic features of nanoballs 1 3 are summarized in Table 2.1. Table 2.1 Summary of characterisitic features of of compounds 1 3 .* Compound 1 2 3 MW (Da) 6,494 7,430 7,760 Diameter (nm) 2.89 2.99 3.13 Volume (nm3) 10.1 17.4 12.7 Effective cavity volume (nm3) 0.367 0.384 0.357 % Porosity (maximum) 20.0 (55.5) 47.3 (73.5) 7.40 (52.1) The molecular weight is based on the formula [Cu2(5-R-bdc)2(L)2]12 and excludes guest and uncoordina ted solvent molecules. The % porosity is calculated per a single molecule. Maximum porosity represents the maximum porosity possible upon removal of guest a nd solvent molecules. The triangular and square windows of nanoballs 1 3 are curved due to the angularity of the bridging bdc ligands. The dimensions of the windows are nanometric.

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46 The sides of the triangular and squa re windows are all equal in compound 1 measuring 10.613 and 8.613 at the wide and narrow ends respectively. The angles around the copper corners of the triangles and square s are perfect 60.00 and 90.00, respectively. The windows in compound 2 are slightly distorted and smaller compared to compound 1 The sides of the square windows are not a ll equal with length s of 10.636 and 10.606 at the wide end and 8.003 and 7.955 at the narrow end. The triangular window is an isosceles triangle with side lengths equal to those of the square window. The angles also deviate from those observed in compound 1 The angles of the squares are 89.56 and 90.143, whereas the angles of the triangle are 59.91 and 60.18. The distortion is further pronounced in compound 3 where all the sides of th e triangles and squares are variable in length (10.69910.588 for th e wide end and 8.0557.876 for the narrow end). The angles also vary ranging betw een 60.32359.825 for the triangles and 91.2588.88 for the squares. These nanometric tr iangular and square windows, which can be viewed as trigonal and tetragonal clusters of SBUs, could be considered the secondary building units comprising the nanoball struct ures (Figure 2.12) and hence have been termed nSBUs.166 In other words, we can consider the nanoballs the result of selfassembly of trigonal and tetragonal nSBUs at the vertices. This will become more relevant in the next two chapters.

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47 Characterization of the nanoball structures is complicated. X-ray crystallography is the best tool for definite structural determination, but even then due to the large number of atoms, high disorder of solvent, guest a nd coordinated ligands it is difficult to locate atoms of guest and solvent molecu les, as was the case in compounds 1 and 2 In addition, due to the high porosity of the structures, ther e is low electron density and in turn poor diffraction which complicates data collection. Consequently, definite determination of guest molecules and thorough structural analysis of host-guest interactions could not be performed. X-ray powder diffr action (XPD) typically repres ents a good tool to confirm that the bulk sample has the same crystal structure as the single crystal from which data was collected. However, the nanoballs we re found to produce poor XPD patterns, making it difficult to ascertain the identity of the bulk powder. A typical XPD spectrum obtained of a nanoball structure showed broa d, indefinite peaks suggesting no long-range order in the crystals. Furthermore, si nce these nanoballs were prepared from Cu(II), a d9 paramagnetic metal, NMR could not be utilized. Figure 2.12 Trigonal and tetragonal nSBU constituents of the nanoball structures

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48 2.3 Conclusions We have demonstrated herein a design stra tegy to generate a molecular version of the faceted polyhedron the small rhombihexahedron that involves linking molecular squares (SBU) at the vertexes at the require d angle of 120 using the angular ligand bdc. This design strategy is different from ones previously implemented to generate metalorganic Platonic and Archimedean polyhe dra, which involved edge-sharing of polygons98,131,143,191 that generates a closed-face pol yhedron, or the use of nodes and spacers with the appropriate geometries190 and resulted in the wire-frame of the polyhedron. Compounds 1 3 are synthesized from simple reac tions of bdc and its derivatives with Cu(II) under thermodynamic equilibrium conditions. These compounds are spherical and nanometric in size and hence are termed nanoballs. The average diameter of compounds 1 3 is 3.05 nm and the average mo lecular weight is > 6500 Da. The nanoballs are porous materials with an internal cavity and open square and triangular window. It was demonstrated with compound 3 that they are able to encapsulate guest molecules in their windows via supramolecular interactions and display rich host-guest chemistry. These triangular and square windows, which have sides measuring 1.06 and 0.80 nm at the wide and na rrow ends, respectively, could be viewed as the “SBUs” that constitute the nanoballs and hence are termed nSBUs. The nanoballs could be envisioned as building blocks of even larger superstructures. Zaworotko et al have already demonstrated192 that it is indeed possible to use the nanoballs as nodes for larger structures, suprasupermolecules.203,204 With their large windows and cavity and their exhibited inclusion properties, the na noballs could be envisi oned as carriers for

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49 other molecules such as pharmaceuticals, and hence can have potential application as drug delivery systems. 2.4 Experimental General methods: All materials were used as receive d; solvents were purified and dried according to standard methods. Mass spect roscopic data were obtained on a 9.4 Tesla FT-ICR mass spectrometer by the National Hi gh Magnetic Field Labor atory at Florida State University. TGA data were obtained on a TA instruments 2950 TGA at high resolution with N2 as purge gas. Synthesis of [Cu2(bdc)2(MeOH)2]12, (1): A methanolic solution (5mL) of H2bdc (0.066 g, 0.40 mmol) and 2,6-dimethylpyridine (0.12 mL, 1.0 mmol) was layered on top of a methanolic solution (5 mL) of Cu(NO3)2. 2.5H2O (0.093 g, 0.41 mmol) containing nitrobenzene (3mL). Upon slow diffusion of the two solutions, blue-green square crystals of 1 formed. Synthesis of [Cu12(5-OH-bdc)12(DMSO)4(MeOH)2(H2O)6]2, (2): To a methanolic solution (100 mL) of 5-OH-H2bdc (5.092 g, 28.0 mmol) and Cu(NO3)2. 2.5H2O (6.390 g, 27.5 mmol) was added 6.40 mL (54.9 mmol) of 2,6-dimethylpyridin e and the solution was stirred for 15 minutes. Upon addition of 150 mL of diethyl ether, a precipitate formed, which was filtered and air dried over night. Crystallization of a sample of the crude (60.0 mg) from DMSO (14 mL) afford ed greenish-blue plates, which were identified by X-ray crystallography as the t itle compound. The crystals were isolated by

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50 pipetting into a clean vial and placing unde r vacuum over night to yield 28.5 mg, (50 % yield by weight). Direct filtering of the crystals failed to isolate the product in a solid form, rather the product turned liquid. Rinsing with ether helped maintain the crystallinity of the product; however, a la rge portion of the sample was lost. Synthesis of [Cu12(5-NO2-bdc)12(MeOH)7(Py)4(H2O)]2, (3): Slow diffusion of a methanolic solution (10 mL) of Cu(NO3)2. 2.5H2O (124 mg, 0.533 mmol) into a methanolic solution (10 mL) of 5-NO2-H2bdc containing pyridine (0.080 mL, 0.989 mmol), naphthalene (25 mg, 0.195 mmol) and nitrobenzene (3 mL) yielded large square green blue crystals within months. Singl e crystal X-ray crysta llography confirmed the identity of the crystals as the title com pound. Yield could not be determined as the crystals dissolved and new crystals formed (see compound 14 Chapter 4). Crystal Structure determination: Single crystals suitable for X-ray crystallographic analysis were selected following examinati on under a microscope. Intensity data were collected on a Bruker-AXS SMART AP EX/CCD diffractometer using Moka radiation ( = 0.7107 ). The data were corrected for Lo rentz and polarization effects and for absorption using the SADABS program. The struct ures were solved using direct methods and refined by full-matrix least-squares on |F|2. All non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in geometrically calc ulated positions and refined with temperature factors 1.2 tim es those of their bonded atoms. All crystallographic calculations were conducted with the SHELXTL 5.1 program package. Crystallographic data for compounds 1 3 are presented in Table 2.2.

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51Table 2.2 Crystallographic data for compounds 1 3 1 2 3 Empirical formula C245.84 H96 Cu24 O120 C200 H72 Cu24 O206.40 S4 C398 H360 Cu24 O218 Formula weight 6494.21 7430.18 11072.36 Temperature (K) 200(2) 200(2) 100(2) Wavelength() 0.71073 0.71073 0.71073 Crystal system Cubic Tetragonal Monoclinic Space group Im-3m I4/mmm C2/c a () 27.6895(17) 31.111(4) 37.510(3) b () 27.6895(17) 31.111(4) 41.627(3) c () 27.6895(17) 35.999(6) 32.430(2) () 90 90 90 () 90 90 92.721(2) () 90 90 90 Volume (3) 21230(2) 34844(8) 50580(6) Z 2 2 4 (Mg/m3) 1.016 0.708 1.545 (mm-1) 1.235 0.778 1.088 F(000) 6454 7366 22584 Crystal size (mm3) 0.1 x 0.1 x 0.33 0.2 x 0.3 x 0.3 0.25 x 0.20 x 0.10 range for data collection () 2. 75 to 23.25 0.87 to 21.04 0.98 to 25.05 Limiting indices -30<=h<=27 -26<=k<=30 -29<=l<=30 -30<=h<=31 -31<=k<=31 -36<=l<=21 -44<=h<=34 -49<=k<=49 -38<=l<=38 Reflections collected 36316 49928 134287 Independent reflections 1501 5170 44734 R(int) 0.0865 0.0800 0.1682 Completeness to = 23.25 99.6 % 99.4 99.8 % Data/restraints/parameters 1501 / 0 / 100 5170/ 0 / 296 44734 / 114 / 2415 Goodness-of-fit on F2 1.138 1.679 0.918 Final R indices [I>2sigma(I)] R1, wR2 0.0784, 0.2725 0.1531, 0.4259 0.0992 0.2685 R indices (all data) R1, wR2 0.1069, 0.2953 0.2128, 0.4702 0.2243, 0.3019 Largest diff.peak and hole (e.-3) 0.831 and –0.438 1.051 and -0.592 0.960 and –0.625

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52 Chapter 3 TETRAGONAL 2D SHEETS 3.1 Preface The last chapter demonstrated that trigonal and tetragonal nSBUs self-assemble into discrete nanoscale spherica l structures (nanoballs) that conform to the shape of the faceted polyhedron, the small rhombihexahedron It is possible, however, to obtain other structures that are based on one type of nSBU exclusively. In this chapter we will present examples of 2D infinite structures that are based on tetragonal nSBUs only (Fig 3.1). Since the molecular square SBUs are linke d at the angle of 120 subtended by the carboxylate moieties of the bridging bdc liga nd, the resulting nSBUs possess curvature and torsional flexibility. It was demonstrated in the previ ous chapter that these nSBUs exhibit inclusion properties and can host mol ecules via supramolecula r interactions. It occurred to us that the shape and chemical na ture of the tetragonal nSBU resembles that of a calix[4]arene molecule and that squa re nSBUs therefore might be subject to atropisomerism.205

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53 3.1.1 Square Grids 44 square grids are the most co mmon topology of 2D structures.206 Square grids are clay mimics and hence are capable of inte rcalating guest molecules between the grids, and additionally they can inte rcalate guest molecules in the square cavities present in the plane of the grid.10,12 The first reported 2D grid s were based on cyano ligands;81-84 since then a wide range of N-bound spacer ligands such as pyrazines,85-87 bipyridine32,74,88-92 and extended bipyridines90,93 have been used to generate sq uare grids with different size cavities. Despite great success in enlarging the size of the cavity by using longer spacers, interpenetration remains a problem that prec ludes space and diminishes the potential of the square grids as porous host materials. In addition, since the framework of the grid is charged, counter ions typically occupy the cavities of the grid, preventing porosity. Attempts to remove or exchange ions from the cavities have almost consistently, with few exceptions,162,207,208 resulted in collapse of the framework. The use of the SBU as a node for construc ting square grids presents an excellent alternative.166,168,209,210 Instead of one metal ion serving as a node in the framework (as in (b) (a) Figure 3.1 Tetragonal 2D sheets sustained by tetragonal nSBU building blocks

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54 metal-bipyridine compounds) the SBU moiety serves as a rigid node. Linking the SBU nodes with rigid spacers (terephthalate) ha s been shown to afford robust, stable superstructures with necessarily larger pores than possible with traditional M-N bonds.181 Furthermore, since the SBU is neutral, count er ions that typically occupy space in the crystal structure are eliminated. Such stru ctures have been show n to be highly porous and capable of sorption of large volumes of gas. The use of flexible dicarboxylate has also been shown to generate extended porous st ructures with large degree of diversity due to the rotational flexibility about single C-C bonds.162 The angular bdc, being a ditopic ligand, is also able to link the SBUs in 2D infinite sheets and form 44 square grid-like structures. The angularity of the bdc ligand, however, is expected to affect the planarit y of the resulting 2D sheets and generate undulating ones in order to reliev e the strain w ithin the plane of the grid. In addition, the cavities of the grid, which are the cavities of the nSBUs, would be expected to be bowlshaped. 3.1.2 Calixarenes and Metallacalixarenes Calixarenes are macrocyclic compounds prepared by the condensation of n molecules of formaldehyde and n molecules of phenol, which results in n phenyl rings bridged by n methylene groups. The size of the resu lting macrocycle is specified in its name by inserting the number n in brackets between calix and arene Thus a cyclic tetramer is designated calix[4]arene. Ca lixarenes are known to complex small guest molecules in their bowl-shaped cavity and hence they represent a large and continually growing area of research in the context of supramolecular chemistry.211-219

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55 Atropisomerism220 is a phenomenon that arises wh en rotation around a covalent bond is impeded enough as to allow for the isolation of different isomers. This phenomenon has been observed in a variety of systems incl uding porphyrins, biaryls and calixarenes. There are four possible atr opisomers of calix[4]arenes221-225 that were designated by Gutsche and coworkers211 as the cone, partial cone, 1,2alternate and 1,3-alternate, all illustrated in Figure 3.2.226,227-229 The nomenclature refers to the orientation of the arene rings with respect to one anot her. In the cone conformation, all arenes point up and form a cone-like structure, whereas in the partia l cone three arenes point up and one points down. Similar behavior has been observed for larger calixarenes.230 Due to the curvature of the tetragonal nSBU imparted by the angular ity of the bdc ligands it can be regarded as being a metal-organic calix, or a “metalla[4]calix”, where the CH2 bridging groups have been replaced by the SBU. Metallcalixarenes231-237 are metal-organic analogous of the organic calixarenes that result from the a ssembly of metal nodes a nd organic spacers. Examples based on Pt(II) and Pd(II) complexes of 2-hydroxypyrimidine have been shown to resemble calix[4]arenes in their shape, conformation and chemical properties.233 This chapter will demonstrate that tetragonal nSBUs do indeed exhibit atropisomerism in the solid state. We w ill present seven structures with the general formula {[Cu2(aryldicarboxylate)2L2]4}n (aryldicarboxylate= bdc 5-OEt-bdc, 5-OPr-bdc, (a) Cone (d) 1,3-alternate (c) 1,2-alternate (b) Partial cone Figure 3.2 Atropisomers of a calix[4]arene

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56 pdc; L= pyridine, 4-picoline) that are based on the self-assembly of tetragonal nSBUs. We will demonstrate that atropisomerism has a profound effect on the polymeric structures in terms of crystal struct ure, porosity and inclusion properties. 3.2 Results and Discussion In this study, we employed bdc and two of its derivatives, 5-OEt-bdc and 5-OPrbdc, all of which have the carboxylate moieties predisposed at 120. In addition, we used the wider angle dicarboxylat e ligand pdc (1-methylpyrrole-2,4-dicarboxylate), which has the two carboxylate moieties predisposed at an angle of 157. The ligands are illustrated in Figure 3.3. The slow diffusion of an ethanolic solution of H2bdc and pyridine into an ethanolic solution of Cu(NO3)2 2.5H2O containing benzene template molecules afforded blue green crystals of 4a {[Cu2(bdc)2(py)2] 4}n. The crystal structure of 4a is depicted in Figure 3.4 and reveals that it is a 44 undulating grid that result s from the self-assembly of tetragonal nSBUs at the vertices. There is one crystallographically independent copper ion having a square pyramidal geometry (dCu1-Cu1= 2.6676(7) ). The basal plane is (a) bdc (b) 5-OEt-bdc (c) 5-OPr-bdc(d) pdc Figure 3.3 Angular aryldicarboxylates employed for making tetragonal 2D sheets

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57 defined by four oxygen atoms of the four bdc moieties (dCu-O= 1.9959(20), 1.9500(20), 1.9417(19), 2.0063(20) ) and the ap ical position is occupied by a pyridine coordinating ligand (dCu-N= 2.1582(23) ). All bond distances are within agreement with the average bond distances calculated from the CSD search (section 1.7.1). The carboxylate moieties of the bdc ligand are twisted out of plane with respect to each other ( = 12). This is larger than the twisting observed in the carboxylate moieties of bdc in the nanoball structure, compound 1 ( = 9.65). The carboxylate moietie s of the bdc ligand also bend towards each other from the plane of the benzene ring by an angle = 4.36 resulting in an undulated sheet. A closer l ook at the tetragonal nSBUs in 4a reveals that they adopt two conformations in an alternating fashion: the cone and 1,3-alternate conformations. Each cone has a diameter of 15.13 (measured from opposite copper ions) and a depth of 10.01 (measured from the center of the botto m of the bowl to the midpoint of a line joining the top hydrogen atoms on opposite bdc moieties). Each type of nSBU contains highly disordered guest benzene molecule s. The undulating sheets stack eclipsed (interlayer separation of 8.44 ) so that the cones stack inside one another and the 1,3alternate nSBUs define hourglass shaped channels that run para llel to [001]. The solvent accessible volume in the unit cell of 4a is only 0.5%, and would be increased to 27.5% upon removal of guest molecules.238

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58 Compound 4b {[Cu2(bdc)2(py)2]4}n, was obtained from an ethanolic solution containing H2bdc, pyridine, Cu(NO3)2 2.5H2O and nitrobenzene. There are two crystallographically independent copper ions in the crys tal structure of 4b (dCu1-Cu1= 2.6373(15), dCu2-Cu2= 2.6351(17) ). The geometry around each copper is square pyramidal. The basal plane is defined by four oxygen atoms of the four bdc moieties (dCu1-O= 1.957(5), 1.961(5), 1.964(4), 1.988(5); dCu2-O= 1.954(4), 1.954(5), 1.976(5), 1.977(5) ) and the apical position is occ upied by a pyridine coordinating ligand (dCu1-N= 2.152(6); dCu2-N= 2.137(6) ). All bond distances ar e in agreement with the average bond distances calculated from the CS D search. Crystal structure of 4b (Fig 3.5) reveals that it is another undulating 2D sheet structure. A closer look at the nSBU constituents of 4b reveals that they adopt the partial cone c onformation. The partial cone is defined by one of the bdc ligands in the te tragonal nSBU lying in a plan e almost perpendicular to the plane defined by the other three bdc ligands (dihedral angle = 97.0). The partial cone has a diameter of 14.08 (measured from oppo site copper ions) and a depth of 10.53 (a)(b) Figure 3.4 Crystal structure of compound 4a* (a) top-view of the sheet illustrating the cone and patial cone nSBU constituents. (b) si de-view illusrtating the eclipsed stacking of the sheets. Coordinated pyridine ligands are pr esented as a single nitrogen atom with the remaining atoms deleted for clarity. Guest molecules were removed for clarity

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59 (measured from the center of bottom of the bowl to the line joining the top hydrogen atoms of opposite bdc moieties that are in th e same plane). In order for the SBUs to afford this partial cone conformation, the bdc ligands are more significantly distorted compared to the previous structures. The car boxylate moieties of th e bdc ligands that lie in plane in the partial cone are twisted out of plane w ith respect to one another by = 5 and 44, while the carboxylate moieties of the fourth bdc ligand pointing down are twisted by = 19. The bdc ligands bend by an angle that ranges between 2.61-4.48. Each partial cone in 4b contains a disordered nitr obenzene molecule (Fig 3.6). The nitrobenzene molecules appear to be involved in stacking interactions with the walls of the cavity, however, centroid-centroid distances could not be calculated due to the high disorder of the nitrobe nzene molecules. Within the sheet, the nitrobenzene guest molecules arrange in such a way that thei r dipole moments cancel. The sheets stack eclipsed so that the partial cones sit inside one another with an in terlayer separation of ca. 9.40 . Disordered ethanol molecules are inte rcalated between the layers. There is no (a)(b) Figure 3.5 Crystal structure of 4b* (a) Top-view illusrtating the partial c one nSBU hosting disordered nitrobenzen e guest molecules. (b) Side-view of 4b which packs in an AAA fashion

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60 solvent accessible area in the unit cell of 4b ; however, upon removal of guest the accessible area would be 12.0%.238 Since the framework in 4b has an identical chemical formula to 4a it can be considered a supramolecular isomer. Compound 5 {[Cu2(bdc)2(4-pic)2]4}n, was obtained by the slow diffusion of a methanolic solution of H2bdc and 4-picoline into a methanolic solution of Cu(NO3)2 2.5H2O containing o -dichlorobenzene. There are two crystallographically independent copper ions in the SBU. The basal plane is occupied by four oxygen atoms of the bdc moieties (dCu1-O= 1.958(3), 1.966(3), 1.972(3), 1.996(4); dCu2-O= 1.965(4), 1.966(4), 1.974(4), 2.005(4) ), whereas the api cal position is occupied by a coordinated picoline molecule (dCu1-N= 2.154(4), dCu2-N= 2.145(4) ). The copper ions in the SBU are separated by 2.6682(6) . The bond dist ances are comparable to those of 4a and 4b and in agreement of those calculated from th e CSD search. The crystal structure of compound 5 is shown in Figure 3.7 and rev eals that it is also a 2D 44 corrugated grid structure. Figure 3.6 Nitrobenzene guest molecules occupy the cavities in compound 4b

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61 A closer look at the crystal structure re veals that the tetragonal nSBUs adopt the 1,2-alternate conformation. In order for the SBUs to afford this conformation of the tetragonal nSBU, the bdc ligands are distorte d with the two carboxylate ligands of the bdc moieties twisting out of plane by = 47. This is the greatest distortion observed in the bdc ligands for all the structures discu ssed so far. The bend angle, however, is minimal ranging between 0.30 and 0.83 2D grids of 5 propagate in the bc-plane and stack along the a-axis in an ABAB fashion w ith an interlayer separation of 9.81 . The effective dimensions of th e grids measure 1.2 x 1.0 nm2 (distance from Cu-Cu mid point of opposite SBU units in the nSBU taking in to account the van der Waals radii of copper). Each nSBU contains a guest molecule of o -dichlorobenzene, which interacts with three of the bdc moieties that co nstitute the walls of the cavity via CH… interactions (dCH…centroid= 2.686, 2.921, 3.424 ), as can be seen in Figure 3.8. There is no solvent accessible area in the unit cell of 5 but upon removal of guest the potential solvent accessibl e area is 25.9%.238 (a)(b) Figure 3.7 Crystal structure of compound 5* (a) Top-view of the 44 grid in 5 The 1,2-alternate nSBU constituent on the 2D sheet is highlighted for clarity. Guest o diclorobenzene guest molecules occupy the nSBU. (b) Side-view illustrating the stack ing of the sheets in an ABAB fashion

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62 Structures 4a 4b and 5 collectively exhibit all four atropisomers of the nSBU likely to be generated by the angularity that results from bdc serving as a bridging ligand. Figure 3.9 illustrates the nSBU cons tituents of polymeric structures 4a 4b and 5 The cone isomer (a) has C4 v symmetry where all four di carboxylate ligands orient in the same direction (the 5-position pointi ng towards the viewer). The partial cone (b) has Cs symmetry with three dicarboxylate ligands oriented up and one down. The 1,2alternate isomer (c) also exhibits Cs symmetry with two adjace nt dicarboxylate ligands oriented up and two down. Finally, the 1,3-alternate isomer (d) exhibits C2 symmetry Figure 3.8 Guest o -dichlorobenzene interacts with the walls of the cavity via CHinteractions (c) 1,2-alternate (a) Cone(b) Partial cone(d) 1,3-alternate Figure 3.9 Atropismerism in nSBU constituents of tetragonal 2D sheets based on bdc

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63 with opposite dicarboxylate ligands oriented in the same direction. We have reported a set of isostructural analogues based on Zn(II)166 that exhibit the sa me atropisomerism phenomenon.239 When derivatives of bdc were investig ated, namely 5-OEt-bdc, and 5-OPr-bdc, two isostructural compounds were obtained:{[Cu2(5-OEt-bdc)2(py)2]4}n, 6 and {[Cu2(5OPr-bdc)2(py)2]4}n, 7 Crystal structure of 6 is depicted in Figure 3.10. There is one crystallographically inde pendent copper ion in the SBU having a square pyramidal geometry. The basal plane is defined by four oxygen atoms of the four bdc moieties (dCu1-O= 1.938(5), 1.939(5), 1.987(5), 2.016(5) ) and the apical position is occupied by a pyridine coordinating ligand (dCu1-N= 2.146(7) ). The two copper ions of the SBU are separated by 2.6662(19) . All bond distances are in agreement with the average bond distances calculated from the CS D search. As can be seen from the crystal structure of 6 (Fig. 3.10), the structure is related to 4a consisting of undulating sheets that result from the self-assembly of alternating cone and 1,3-alternate nSBUs. The structure (a) (b) Figure 3.10 Crystal structure of compound 6* *(a) Top view of the 2D corrugated sheet presented in space filli ng mode. The coordinated pyridi ne ligand is presented as a single nitrogen atom with the remaining atoms removed for cl arity. The cone and 1,3-alternate nSBUs are highlited. (b) Side view of 6 presented in stick mode illustrating the eclipsed stacking of the sheets

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64 is different, however, in the f act that only one type of cavity is present, which results from the cone nSBUs. Each cone hosts f our water molecules. The hour-glass shaped channels present in 4a which result from the 1,3-alternat e nSBUs, are occupied by the ethyl chain substituents. This is reflected in the solvent-accessible volume in the unit cell of 6 which was calculated to be 1%, but w ould be increased only to 9.7% upon removal of solvent molecules compared to an increase to 27.5 % for 4a .238 The cone nSBUs have the same outer diameter as those of 4a (0.94 nm), however, they are obviously deeper due to the alkyl chain substituents and measure 1.30 nm from the center of the base to the midpoint of a line joining oppos ite terminal hydrogen atoms of the ethyl substituents. Distortion in the bdc ligand is also observ ed in this structure where the carboxylate moieties twist out of plane with re spect to each other by an angle = 16 and 20, which is greater than that observed in 4a ( =12) and any of the nanoball structures, which also contain tetragonal nSBUs in the cone conf ormation. The carboxylate moieties of 5-OEtbdc are also bent by an angle = 4.37, which is comparable to that observed in 4a The undulating sheets in 6 stack eclipsed so that each cone contains disordered solvent molecules and the bottom of a cone from the ad jacent sheet. The interlayer separation is 8.68 . Compound 6 formed reproducibly from a variety of different solvents (MeOH, EtOH, DMSO), in the presence of different base s (pyridine, 2,6-lutidi ne), in the absence of template molecules and in the presence of a variety of them (nitrobenzene, benzene, o dichlorobenzene, naphthalene). It is theref ore not unreasonable to assume that the alkyl chain substituent plays a template role that directs the formation of this structure exclusively.

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65 The crystal structure of compound 7 reveals that it is al so a 2D corrugated sheet that results from the self-assembly cone and 1,3-alternate nSBUs. The sheets stack eclipsed so that the cones sit inside one a nother and the 1,3-alternate nSBUs define hourglass shaped channels. These channels are occupied by the alkyl chain substituents of the bdc ligands. Structure 7 could not be refined below Rf = 0.2925 due to the high thermal motion and/or disorder and consequently could not be thoroughly analyzed. 7 as in the case of 6 was isolated in the absence of template molecules as well as in the presence of various ones including nitrobenzene, benzene, o -dichlorobenzene and naphthalene. It was also isolated from a variety of solven ts including MeOH, EtOH or DMSO and in the presence of different bases, in cluding pyridine or 2,6-lutidine. When the wider angle ligand pdc was em ployed, two related structures were obtained under different reaction conditions. The reaction of H2pdc, pyridine and Cu(NO3)2 2.5H2O in methanol yielded bl ue green crystals of 8 {[Cu2(pdc)2(py)2]4}n, overnight whereas crystals of 9 {[Cu2(pdc)2(4-pic)2]4}n, were obtained by the slow diffusion of a methanolic solution of H2pdc and 4-picoline into a methanolic solution of Cu(NO3)2 2.5H2O containing nitrobenzene. Crystal structure of 8 which is illustrated in Figu re 3.11, reveals that it is a 44 square grid. There is one crystallographical ly independent copper ion in the SBU having a square pyramidal geometry.

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66 The basal plane is defined by four oxyge n atoms of the four pdc moieties (dCu-O= 1.953(4), 1.971(4), 1.971(4), 1.9781(4) ) and th e apical position is occupied by a pyridine coordinating ligand (dCu-N= 2.164(6) ). The two coppe r ions of the SBU are separated by 2.6688(16) . Ther e is no significant effect on bond distances with the use of the wider angle dicarboxylat e, pdc. All bond distances are in agreement with those of structures based on bdc and with the averag e bond distances calculated from the CSD search. The effective dimensions of the grid measure 10.98 x 12.17 (distance from CuCu mid point of opposite SBU units in the nS BU taking into account the van der Waals radii for copper). The grids stack slipped in an ABCABC fashion with an interlayer separation of 6.499 . This sta ggered stacking facil itates the inclusion of the coordinated pyridine molecules from one grid in the cav ities of the neighbor ing grids where they interact with the pdc ligands constituti ng the walls of the grid by face-to-face stacking interactions (dcentroid…centroid = 3.592 ). This is within the acceptable range of 3.4-3.8 for centroid-centroid distances of aromatic rings involved in face-to-face stacking interactions.200 Guest methanol molecules are in tercalated between the grids in (a)(b) Figure 3.11 Crystal structure of compound 8* (a) Top-view illustrating the square gr id. Coordinated pyridine molecules are presented as single nitrogen atoms with the remaining atoms deleted for clarity (b) Sheets of 8 stack in an ABC-fashion. The la yers have been color coded for clarity.

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67 8 There is no solvent accessible volume in the structure of 8 but upon removal of guest 7.1% volume is available for guest molecules.238 A closer look at th e tetragonal nSBU in the structure of 8 reveals that it is in the 1,2-alte rnate conformation. To adopt this conformation, the carboxylate moieties of the pdc ligand twist out of plane with respect to one another by an angle = 24, which is much smaller than that observed for the bdc ligand in the tetragonal nSBU adopting the 1,2-alternate ( = 47 ) This is not surprising, however, as this ligand has a wider angle and in turn is more flexible. The bend angle, on the other hand ( = 4.7), is much greater than that in 5 ( = 0.31-0.83 ) Compound 9 has the same crystal structure as that of 8 shown in Figure 3.11. There is one crystallographically independe nt copper ion having a square pyramidal geometry (dCu1-Cu1= 2.6598(15) , slightly shorter than that observed in 8 2.6688(16) ). The basal plane is defined by four oxyge n atoms of the four pdc moieties (dCu-O= 1.957(4), 1.962(4), 1.966(4), 1.9781(4) ) and th e apical position is occupied by a pyridine coordinating ligand (dCu-N= 2.153(5) ). All bond distances are in agreement with the average bond distances calculated from the CSD search. The effective dimensions of the grids measure 10.80 x 11.84 (distance from Cu-Cu mid point of opposite SBU units in the nSBU taking into account the van der Waals radii for copper). As in 8 the grids stack parallel and slipped in one direction so that every fourth layer repeats, i.e. ABCABC packing (Fig. 3.11), with an in terlayer separation of 6.940 . This is slightly longer than the interlayer separation in 8 (6.499 ), as 4-picoline is slightly longer than pyridine. This packing arra ngement facilitates the inclusion of the coordinated picoline liga nds into the cavities of neighbori ng grids (Fig 3.12) where they interact with the pdc ligands constituti ng the walls of the cavity by face-to-face

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68 stacking interactions (dcentroid…centroid = 3.632 ). This is within the acceptable range of 3.4-3.8 for centroid-centroid distances of aromatic rings involved in face-to-face stacking interactions. Disorder ed water molecules are inter calated between the grids. There is no solvent accessible volu me in the crystal structure of 9 but upon removal of guest there would be 5.2% solvent accessible volume.238 Nitrobenzene was not used in the synthesis of 8 but was used in the synthesis of 9 Since the same coordination polymer forms in both cases, and nitrobenzene is not incorporated in the structure of 9 it is reasonable to assume that additional templa te molecules are not necessary to direct the formation of this atropisomer of pdc and that coordinated pyridine ligands play a templating role. This distortion in the bridging pdc ligand in this structure is comparable to that in 8 The use of the wider angle dicarboxylate pdc results in the 1,2-alternate nSBU atropisomer only (Figure 3.13). Examination of the SBU moieties in the nSBUs of structures 8 and 9 revealed that the “ trans ” carboxylate moieties are planar ( = 0), which could account for the formation of this nSBU exclusively. The larger angle at (a) (c) (b) Figure 3.12 Packing of square grids in 9 facilitates stacking interactions* *Perspective view of two stacked grids in stick mode (a) and space-filling mode (b). Packing of the sheets facilitates stacking interactions between coordina ted pyridine molecules and pdc ligand constituting the wall of the cavity (c).

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69 which the two carboxylates are predisposed results in flexibility and in turn allows for planarity in the SBU moiety. 3.3 Conclusions We have demonstrated herein that tetragonal nSBUs can self-assemble exclusively into 2D structures. The torsional flexibility in the tetr agonal nSBU results in greater degree of structural dive rsity in the solid state, where extended structures with the same topology (44) differ due to atropisomrism in their nSBU constituents. The shape, chemical nature and conformational flexibil ity of the tetragonal nSBUs make them reasonable analogues of calix[4]arenes. They can be considered metallacalix[4]arenes where the bridging CH2 groups are replaced by the SBU moiety. Indeed, all four atropisomers that are known to exist for ca lixarenes were observed in the tetragonal nSBU components of the coordination polymers pr esented herein. In particular, the types of cavities present and the relative solvent or guest accessible volumes change quite dramatically depending upon which atropisomer is present in the structures. Template molecules appear to have an effect on which atropisomer is formed. In the absence of substituents on the bdc ligand, all atropiso mers are observed and their formation is dependent on what template molecules are us ed. The cone/1,3-alternate atropisomers 1,2-alternate Figure 3.13 Tetragonal nSBU constituent of compounds 8 and 9

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70 form in the presence of benzene, the partial cone forms in the pres ence of nitrobenzene, whereas the 1,2-alternate forms in the presence of o -dichlorobenzene. Guest molecules interact with the walls of the nSBUs via ar omatic stacking interactions. In order to accommodate the formation of the different at ropisomers, the bdc ligands are distorted by the carboxylate moieties twisting out of plane. The most tw isting occurs in the case of the 1,2-alternate nSBUs, in wh ich the carboxylates are out of the plane by an angle of 47. However, this nSBU exhibits the l east distortion in term s of bending. When substituents are present on the bdc ligand, they appear to play a templating effect as the presence of a variety of templates or their ab sence does not appear to have an effect on the resulting structure. The wider angle dicarboxylate pdc yielded one atropisomer, namely the 1,2-alternate. Some of these 2D structures, namely 8 and 9 are well suited for pillaring, a commonly used strategy for the generation of 3D porous materials.240-242 Future studies will be directed towards the preparation of discrete molecular versions of the nSBUs, which would se rve as even more direct analogues of calix[4]arenes. We shall also focus upon the inherent modularity of the nSBU and seek to customize it so that, depending on which va riable is changed, one might control size, shape and chemical nature of the nSBU a nd the cavities and channels thereby formed. 3.4 Experimental General methods: All materials were used as receive d; solvents were purified and dried according to standard methods. TLC plates we re visualized using a short wave (254 nm) UV lamp. 1H and 13C NMR spectra were obtained from DMSOd6 solutions on a Bruker instrument operating at 250 and 62.5 MHz, respectively. TGA data were obtained on a

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71 TA instruments 2950 TGA at high resolution with N2 as purge gas. Formulations of the coordination polymers are based upon nSBU s rather than empirical units. 1Methylpyrrole-2,4-dicarboxylic acid was provided by G. J. Bodwell at Memorial University of Newfoundland. It was synthesi zed according to the literature procedure243 and published elsewhere.164 Synthesis of alkoxy dimethyl isophthalate esters: Established literature procedures244 were followed except for using acetone as the solvent instead of DMF. Saponification of alkoxy di methyl isophthalate esters: A sample of the ester was heated at 50 C in methanol/ 20%NaOH (aq.) until TLC indicated the completion of the reaction (ca. 45 min). The reaction mixture was then cooled in an ice bath and concentrated HCl was added dropwise until the solution is acidic (pH 3-4). Cooling the solution to 4 C over night yielded a white, crystalline product. Synthesis of 5-ethoxybenzene-1,3dicarboxylic acid (5-OEt-1,3-H2bdc): Dimethyl 5hydroxyisophthalate (5.01 g, 24.1 mmol) was dissolved in ace tone (100 mL) and treated with potassium carbonate (10.9 g, 10 equiv) and ethyl iodide (2.10 mL, 1.1 equiv) according to the literature procedure to yiel d 5.48 g of the yellowish ester product. The ester was then saponified according to the ge neral procedure to yield the title compound in quantitative yield (4.77g). 1H NMR: = 7.95 (s, 1H, C H arom), 7.55 (s, 2H, C H arom.), 4.09 (q, 2H, C H2), 1.29 (t, 3H, C H3); 13C NMR: = 166.84 ( C O2H), 159.00 ( C O), 132.94 ( C -CO2H), 122.48 ( C H), 119.31 ( C H), 64.12 (O C H2), 14.86 ( C H3).

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72 Synthesis of 5-propoxybenzene-1,3dicarboxylic acid (5-OPr-1,3-H2bdc): Dimethyl 5hydroxyisophthalate (5.11 g, 24.1 mmol) was dissolved in ace tone (100 mL) and treated with potassium carbonate (11.4 g, 3.5 equiv) and propyl iodide (2.60 mL, 1.1 equiv) according to the literature procedure to yiel d 4.51 g of an oily yellow residue, TLC of which (Silica, EtOAc/ Hex 3:2) indicated th e presence of starting material. The crude product was chromatographed (Silica, gradient elution EtOAc/ Hex) to yield an off-white solid (1.29 g), which upon saponification according to the general procedure yielded 0.512 g (44 %) of the title compound. 1H NMR: = 7.95 (s, 1H, C H arom), 7.55 (s, 2H, C H arom.), 4.09 (q, 2H, C H2), 1.29 (t, 3H, C H3); 13C NMR: = 166.84 ( C O2H), 159.00 ( C -O), 132.94 ( C -CO2H), 122.48 ( C H), 119.31 ( C H), 64.12 (O C H2), 14.86 ( C H3). Synthesis of {[Cu2(bdc)2(py)2 Guest]4}n (4a): Blue-green crystals of 4a were obtained from slow diffusion of an etha nolic solution (4 mL) of 1,3-H2bdc (166 mg, 0.999 mmol) and pyridine (0.240 mL, 2.97 mmol) into an aqueous solution (4 mL) of Cu(NO3)22.5H2O (233 mg, 1.00 mmol). Crystals form within days in 40% yield. The crystals are thermally stable to above 150 C after which the TG curve shows a mass loss of about 33% between 180 and 300 C, which is consistent with and corresponds to the loss of benzene and pyridine (determined by TG-MS). Further heating leads to decomposition above 400 C. The most intense peaks observed in the X-ray powder diffraction (XPD) patterns from the bulk sample are consistent with t hose calculated from single crystal diffraction data.

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73 Synthesis of {[Cu2(bdc)2(py)2]4 4Nitrobenzene 2EtOH}n (4b): Blue-green crystals of 4b were obtained upon standing of an ethanolic solution of 1,3-H2bdc (254 mg, 1.53 mmol), pyridine (0.38 mL, 4.70 mmol), Cu(NO3)22.5H2O (346 mg, 1.49 mmol) and nitrobenzene (3 mL) at room temperature for 3-6 months in 7.5% yield. The crystals are thermally stable to ca 100 C after which the TG curve shows a weight loss of 40% between 110 and 200 C and second wei ght loss of 37% between 280 and 400 C. Further heating results in d ecomposition of the sample. Synthesis of {[Cu2(bdc)2(4-pic)2]4 4 oDichlorobenzene 8MeOH}n (5): Blue-green crystals of 5 were obtained from the slow diffusion of a methanolic so lution (7 mL) of 1,3-H2bdc (166 mg, 0.999 mmol) and 4-picoline (0.30 mL, 3.1 mm ol) into a methanolic solution (7 mL) of Cu(NO3)22.5H2O (233 mg, 1.00 mmol) c ontaining 5 mL of o dichlorobenzene. Crystals formed after 35 months in 50% yield. The crystals are thermally stable to above 200 C after whic h the TG curve shows a weight loss of 58% between 250 and 300 C and second wei ght loss of 16% between 350 and 400 C. Further heating of the samp le results in decomposition. Synthesis of {[Cu2(5-OEt-bdc)2(py)2]4 8H2O}n (6): Blue-green crystals of 6 were obtained within days from the slow diffusion of a methanolic solu tion (5 mL) of 5-OEt1,3-H2bdc (87.8 mg, 0.424 mmol) a nd 2,6-dimethylpyridine (0.10 mL, 0.85 mmol) into a solution of Cu(NO3)22.5H2O (98.6 mg, 0.424 mmol) in 5 mL of methanol containing template molecules (nitrobenzene, benzene, naphthalene, o -dichlorobenzene or none). TG

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74 analysis shows removal of guest molecules at 88 C followed by removal of coordinating pyridine molecules at 223 C then decomposition above 305 C. Synthesis of {[Cu2(5-OPr-bdc)2(py)2]4 Guest }n (7) : Blue-green crystals of 7 were obtained within days from the slow diffusion of a methanolic solu tion (5 mL) of 5-OPr1,3-H2bdc (113 mg, 0.511 mmol ) and pyridine ( 0.20 mL, 2.5 mmol) into a methanolic solution of Cu(NO3)22.5H2O (117 mg, 0.503 mmol ) in 5 mL of methanol that contains template molecules (3 mL) (nitrobenzene, benzene, o -dichlorobenzene, naphthalene or none) in 18.0% yield. TG analysis reveals lo ss of guest molecules below 200 C followed by a weight loss of 80.1% between 280 and 350 C. Further heating of the sample resulted in decomposition. Synthesis of {[Cu2(pdc)2(py)2]4 4MeOH}n (8): To a methanolic solution (5 mL) containing N -Me 2,4-H2pdc (111 mg, 0.656 mmol) and pyridine ( ca. 0.30 mL, 2.6 mmol) was added Cu(NO3)22.5H2O (145 mg, 0.623 mmol) in methanol (5 mL). An immediate color change occurred and blue-green crys tals formed overnight in 19.2% yield. TG analysis reveals loss of guest/solvent belo w 200 C after followed by a mass loss of about 65% between 235 and 300 C. Further heating resulted in decomposition of the sample. The most intense peaks observed in the X -ray powder diffraction (XPD) patterns from the bulk sample are consistent with those cal culated from single crystal diffraction data.

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75 Synthesis of {[Cu2(pdc)2(4-pic)2]4 4H2O}n (9): Blue-green crystals of 9 were obtained by slow diffusion of a methanol ic solution (10 mL) containing N -Me 2,4-H2pdc (96 mg, 0.57 mmol) and 4-picoline (0.17 mL, 1.7 mmol) in to a methanolic solution (10 mL) of Cu(NO3)22.5H2O (131 mg, 0.563 mmol) containing ni trobenzene (3 mL). Crystals formed within days in 36.4 % yield. TG an alysis shows loss of guest between 100 and 190 C followed by a mass loss of 64% between 235 and 300 C. Further heating resulted in decomposition of the sample. The most intense peaks observed in the X-ray powder diffraction (XPD) patterns from the bulk sample are consistent with those calculated from single crystal diffraction data. Crystal structure determination: Single crystals suitable for X-ray crystallographic analysis were selected following examinati on under a microscope. Intensity data were collected on a Bruker-AXS SMART AP EX/CCD diffractometer using Moka radiation ( = 0.7107 ). The data were corrected for Lo rentz and polarization effects and for absorption using the SADABS program. The struct ures were solved using direct methods and refined by full-matrix least-squares on |F|2. All non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in geometrically calculated positions and refined with temperature factors 1.2 tim es those of their bonded atoms. All crystallographic calculations were conducted with the SHELXTL 5.1 program package. Crystallographic data for compounds 4a 9 are presented in Table 3.1.

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76Table 3.1 Crystallographic data for compounds 4a 9 4a 4b 5 6 Empirical formula C36H20Cu2N2O10 C43H29Cu2N3O10.5 C34H26Cl2Cu2N2O8 C30H30Cu2N2O12 Formula weight 767.65 882.80 768.58 737.66 Temperature (K) 173(2) 100 (2) 100 (2) 200(2) Wavelength() 0.71073 0.71073 0.71073 0.71073 Crystal system Tetragonal M onoclinic Monoclinic Tetragonal Space group P4/ncc Cc P2(1)/m P4/ncc a () 18.7912 (8) 10.2527 (12) 19.5148 (14) 18.8743(16) b () 18.7912 (8) 18.973 (2) 12.7678 (9) 18.8743(16) c () 16.8886 (10) 16.977 (2) 14.3466 (10) 17.356(3) () 90 90 90 90 () 90 100.460(2) 114.1330 (10) 90 () 90 90 90 90 Volume (3) 5963.5(5) 3247.5 (7) 3262.2 (4) 6182.7 (13) Z 8 4 4 8 (Mg/m3) 1.924 1.554 1.606 1.581 (mm-1) 1.683 1.374 1.523 1.443 F(000) 3492 1548 1600 3008 Crystal size (mm3) 0.10 x 0.10 x 0.20 0.10 x 0.05 x 0.05 0.10 x 0.10 x 0.05 0.05 x 0.05 x 0.05 range for data collection () 1.5328 .27 1.2228.30 1.9628.29 1.5324.74 Limiting indices -23<=h<=24 -20<=k<=25 -21<=l<=22 -13<=h<=10 -25<=k<=20 -18<=l<=21 -25<=h<=25 -14<=k<=17 -15<=l<=18 -22<=h<=22 -16<=k<=22 -20<=l<=20 Reflections collected 33929 20125 10412 33929 Independent reflections 3632 7879 6730 2657 R(int) 0.0560 0.0993 0.0332 0.1942 Completeness to (%) 97.9 94.9 95.2 100.0 Data/restraints/parameters 3632/ 0/ 231 7879/ 0/ 526 6730/ 2/435 2657/ 0/ 205 Goodness-of-fit on F2 0.866 1.031 0.904 1.068 Final indices [I>2sigma(I)] R1, wR2 0.0407, 0.1063 0.0841, 0.1967 0.0545, 0.1103 0.0525, 0.1150 R indices (all data) R1, wR2 0.0680, 0.1139 0.1611, 0.2386 0.0530, 0.1173 0.1562, 0.1650 Largest diff. peak and hole (e.-3) 0.926 and -0.523 1.258 and -0.611 1.258 and -0.611 0.985 and -0.603

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77Table 3.1 Continued 7 8 9 Empirical formula C64 H60 Cu4 N4 O24 C25H24Cu2N4O9 C26H26Cu2N4O9 Formula weight 1523.32 651.56 665.59 Temperature (K) 100(2) 200 (2) 100 (2) Wavelength() 0.71073 0.71073 0.71073 Crystal system Monoclinic Monoclinic Monoclinic Space group P2(1)/c P2(1)/n P2(1)/n a () 18.964 (7) 8.2043 (13) 8.4851 (15) b () 19.029 (7) 14.970 (2) 14.6395 (2) c () 18.645 (6) 10.6012 (16) 11.1455 (19) () 90 90 90 () 91.605 (7) 90.191 (3) 93.392 (3) () 90 90 90 Volume (3) 6726 (4) 1302.0 (4) 1382.0(4) Z 4 2 2 (Mg/m3) 1.504 1.662 1.599 (mm-1) 1.329 1.695 1.599 F(000) 3120 664 680 Crystal size (mm3) 0.10 x 0.10 x 0.05 0.02 x 0.02 x 0.01 0.05 x 0.05 x 0.05 range for data collection () 1. 0723.49 2.3526.42 2.3528.26 Limiting indices -21<=h<=18 -21<=k<=16 -20<=l<=18 -10<=h<=10 -18<=k<=17 -13<=l<=7 -3<=h<=11 -12<=k<=17 -14<=l<=14 Reflections collected 27660 7450 2463 Independent reflections 9726 2656 2708 R(int) 0.1610 0.0832 0.0513 Completeness to (%) 97.8 99.0 97.0 Data/restraints/parameters 9726/0/869 2656/ 0/ 199 2708/0/209 Goodness-of-fit on F2 1.267 0.976 0.944 Final indices [I>2sigma(I)] R1, wR2 0.2522, 0.4630 0.0599, 0.1116 0.0643, 0.1120 R indices (all data) R1, wR2 0.2925, 0.4831 0.1331, 0.1358 0.1175, 0.1293 Largest diff. peak and hole (e.-3) 2.061 and -1.611 0.486 and -0.354 0.594 and -0.395

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78 Chapter 4 2D KAGOM LATTICES: NANOSCALE MOLECULAR MAGNETS 4.1 Preface The last decade has witnessed tremendous advances in our u nderstanding of and ability to manipulate molecula r and supramolecular assemblies.10 New paradigms for the design and synthesis of a new generation of porous and magnetic materials now exist. Such advances are a consequence of the fundamental understanding of intermolecular interactions and of structure and cooperativity in many aspects of molecular science. Thus, the prospects for design, control a nd manipulation of molecular materials, particularly in areas related to non-co valent bonding, self-assembly and nanoscale structures are now truly exceptional. Crystal engineering concepts can be a pplied to the design of novel framework topologies and nanostructures th at are based upon simple geom etric principles. The last two chapters discussed examples of discrete nanoballs that result from the self-assembly of trigonal and tetragonal nSBUs (Chapter 2), and of infinite 2D nanostructures that result from the self assembly of tetragonal nSBUs only (Chapter 3). This chapter will present examples of infinite 2D networks th at result from the self -assembly of trigonal nSBUs only (Figure 4.1).

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79 4.1.1 Molecular Magnetism Geometric frustration of magnetic materials has been theoretically predicted to lead to a variety of novel magnetic ground states.245 A triangular lattice represents an example of geometrically frustrated topology. Many such lattices are possible, but most attention has been directed at the 2D tr iangle, 2D Kagom, 3D Face centered cubic (FCC), and 3D pyrochlore lattices (Figure 4.2). In addition to the clear division of the f our lattices based on dimensionality, they can be further subdivided based on the conn ectivity of their cons tituent triangles: the FCC and triangle lattices result from edge-s haring of triangles, whereas the Kagom and Kagome triangular FCC pyrochlore Figure 4.2 Four lattices that exhibit spin frustration (a) (b) Figure 4.1 Trigonal nSBUs self-assemble into trigonal 2D sheet

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80 the pyrochlore lattices result from vertex-sha ring of triangles. Th e Kagom lattice has gained most attention out of the four lattices because of its inherent manifestation of spin active sites into a high degree of spin frustration. Molecular magnetism is a field that ha s been developing independently of metallic and oxide magnets, and has the potentia l to make significant contributions in the area of material science. Molecular magnets246-250 are materials in which molecules are the spin carriers or facilitate communicati on between spin carriers. One advantage of molecular magnets is that the spatial arrangeme nt of the spin carriers is not limited to the traditional metallic lattices, and that two-dime nsional networks are easily accessible. It therefore seems appropriate to target frustrated lattices by synthesizing molecular materials from magnetic components. 4.1.2 Examples of Molecular Kagom Lattices Few examples of molecular Kagom latt ices have been reported to date.165,209,251254 For the most part, experi mental studies have focused on inorganic materials. An example of such material is SrCr9xGa3xO19, which has been shown to have spin-liquid type behavior by neutron scattering.255 In this compound the Cr ions occupy the lattice points. One disadvantage of metallic Kago m compounds, however, is that ordering of the spins can be accommodated by interconnectiv ity of the layers thereby diminishing spin frustration within the layers.256 An example an organic Kagom lattice is a compound based on the radical cation m N -methylpyridinium -nitronyl nitroxide that ha s been shown to exhibit antiferromagnetic behavior at very low temperatures.251,257

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81 In the context of metal-organic structur es, three examples of a Kagom lattice have been reported to date. Zaworotko et al165,258 reported the synthesis and characterization of a Kagom lattice that resu lts from self-assembly of trigonal nSBUs at the vertexes into 2D sheets with trigonal a nd hexagonal cavities. The crystal structure of this compound is shown in Figure 4.3. The lattice is sustained by Cu2 dimers that occupy the lattice points and are bridged by bdc ligands. The copper ions ar e coordinated by pyridine in the apical positions. The trigonal nSBU constituents of the lattice have dimensions of 8.2 thereby generating hexagonal cavities with an effec tive diameter of 9.1 . The sheets stack eclipsed in an AAA fashion generating hexagona l channels with the same dimensions as the hexagons. Highly disordered solvent molecules occupy the trigonal and hexagonal cavities. Magnetic studies of this compound revealed that it does exhibit ferromagnetic behavior even at 300 K.165,259 (a) (b) Figure 4.3 Crystal structure of a 2D structure that exhibits the Kagom lattice topology* *(a) crystal structure of a compound reported by Zaworotko et al165 having that Kagom topology, (b) Schematic reprentation of a Kagom lattice de monstrating the sp in frustration

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82 Yaghi et al209 reported a relevant Kagom lattice that results from the selfassembly of trigonal nSBUs based on the wider-angle ligand 2,5thiophenedicarboxylate. This compound has the same 2D sheet structure as that described for the structure above. The resulting hexagonal cavitie s, however, are obviously larger having an effective diameter of 12.3 . The sheets stack stagge red in an ABCABC fashion decreasing the size of the channels that would result from the hexagonal cavities. Domasevitch et al254 recently reported a 3D structure that results from the pillaring of 2D Kagom lattices. Three examples of this structure based on Co, Zn and Cd were reported. The lattice is sustained by the metal ion sitting at the lattice points and bridged by the flexible organi c ligand 3,3’,5,5’-tetramethyl-4,4 ’-bipyrazolyl to yield a 2D sheet with trigonal and hexagona l cavities in the plane of the 2D sheet. The hexagonal cavity has an effective diameter of ca. 8 . The 2D sheets are pillared into the third dimension by dithionate ligands. These com pounds were prepared for the objective of using them as zeolite-like, porous materials. 4.1.3 Modification of Molecular Kagom Lattice Figure 4.4 depicts a schematic presentati on of the Kagom lattice reported by Zaworotko et al .165 The SBUs are represented by green squares the center of each is occupied by a Cu2 dimer, which is coordinated by axia l ligands (red and blue balls). The SBUs are bridged by bdc ligands (yellow balls) It is clear from the schematic that the structure is modular; hence, it should be feasible to modi fy it by varying its molecular components.

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83 This could be accomplished in one of four ways. (1) The metal component could be exchanged, especially since the SBU is known for a large number of transition metals.19 This would be expected to not onl y alter the magnetic properties in the structure, but also incorporate new prope rties such as luminescence and catalysis depending on the metal used. (2) The brid ging ligand could be either modified or replaced. Modifying the ligand by introducing substituents with different electronic and steric properties is expected to alter the ma gnetic properties and inte rlayer separation in the structure. Electron donating or withdraw ing substituents introduced onto the bdc ring are expected to enhance or reduce the magnetic properties, respectively, by altering the intradimer coupling. The use of a wider angl e bridging ligand has already been shown to alter the packing in the lattice.209 (3) The apical coordinatin g ligands could be varied, which is expected to alter the interlayer separation de pending on the bulkiness of the ligand, in addition to affecting the magnetic proper ties of the lattice. It has already been demonstrated that varying the apical liga nd has a measurable effect on the magnetism on Figure 4.4 Schematic representation of the molecular Kagom lattice reported by Zaworotko et al165

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84 the dicopper tetracarboxylate SBU.250 (4) Guest molecules with different properties (chirality, luminescence, magnetism, etc) could be incorporated in the cavities. This is expected to have a significant e ffect on the bulk physical properties. This chapter will demonstrate that it is indeed feasible to modify the Kagom lattice. We will present five new st ructures with the general formula {[Cu2(5-Rbdc)2L2]3}n (R= H, Ph, NO2; L = MeOH, H2O, pyridine, 4-picoline) that sustain the Kagom topology and examine the effect of modifying the axial ligand and the bdc ligand on the packing, interlayer separa tion and porosity of the structures. 4.2 Results and Discussion For this study, we employed bdc and two of its derivatives: 5-ph-bdc and 5-NO2bdc (Fig 4.5). Compound 10 {[Cu2(bdc)2L2]3}n, (L= MeOH and/or 4-picoline) was obtained by the slow diffusion of a methanolic solution of Cu(NO3)2 2.5H2O into a methanolic solution of H2bdc and 4-picoline in the presence of nitrobenzene template molecules. The crystal structure of 10 which is shown in Figure 4.6, can be described as bowl(a) bdc(b) 5-Ph-bdc(c) 5-NO2-bdc Figure 4.5 bdc ligands employed for making Kagom lattices

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85 shaped trigonal nSBUs self-assembled at the vertexes into a 2D Kagom lattice. The Cu2 dimers of the SBU sit on the lattice points and are bridged by the bdc ligands, which results in trigonal and he xagonal cavities within the layer. There is one crystallographically independent copper ion having a square pyramidal geometry. The basal plane is defined by four oxygen atoms of the four 2 bdc moieties (dCu-O= 1.953(8), 1.954(8), 1.961(8), 1.962(8) ) a nd the apical position is occupied by disordered MeOH a nd/or 4-picoline ligands (sta tistical probability of MeOH to 4-picoline in the crystal structure is 8:2; dCu-O= 2.166(12), dCu-N= 2.16(3) ). The two copper ions of the SBU are separated by 2.637(3) . All bond distances are in agreement with the average bond distances calculated from the CSD search (section 1.7.1). The apical coordinating ligands point towards the trigonal cavities, wher eas the 5-position of the bdc ligands points, in an alternati ng up and down fashion, towards the hexagonal cavity. This structure could be considered a supramolecular isomer of that published by Zaworotko et al as it has the same chemical formula {[Cu2(bdc)2(L2)]3}n, (L= pyridine for published structure, 4-picoline/methanol for 10 ). (a) (b)Figure 4.6 Crystal structure of compound 10* *(a) Top view of the stacked sheets of compound 10 Three layers have been color-code d to highlight the ABC stacking. (b) Side-view illustrating the ABC stacking. Apical coodinate d ligands are presented as single nitrogen atoms with the remaining atoms removed for clarity. Gues t molecules have been removed for clarity

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86 As the paddlewheels are confin ed in a trigonal cluster the bdc ligand distorts to accommodate this arrangement. The carboxylat e moieties of the bdc are twisted out of plane with respect to each other by an angle = 14.0, compared to the twisting of the carboxylates in the tetragonal nSBu of 4a of 12.0 and the nanoball in compound 1 ( = 9.65 ) The carboxylate functionalities are also bent towards each other from the plane of the benzene ring by an angle = 5.38, which is slightly great er than that seen in the tetragonal nSBU of 4a ( = 4.33) and more comparable to the nanoball structures ( avg.= 5.22). The trigonal nSBU could be described as a curved equilatera l triangle with sides measuring 10.623 and 7.977 , at the wide a nd narrow ends, respectively (measured from neighboring copper ions). The angles around the copper corner s are perfect 60.00. The dimensions of the nSBU are comparab le to the trigonal nS BU constituents of compound 1 Figure 4.7 depicts the trigonal nSBU in 10 The layers are undulating due to the cu rvature of the trigonal nSBU bowls imparted by the angularity of the bdc liga nd and stack in an ABCABC fashion along the Figure 4.7 Trigonal nSBU constituent of compound 10* Coordinated apical ligands are presented as a single nitr ogen atom with the remaining atoms removed for clarity. Disordered guest molecule in the cavity has been removed for calrity

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87 c-axis with an interlayer separation of 8.739 , contrary to the re ported structure which stacks in an AAA eclipsed fashion with an inte rlayer separation of 9.916 . It is likely that the slipped stacking of the sheets reduces steric interactions be tween the coordinated 4-picoline ligands and results in shorter in terlayer separation. De spite this staggered stacking, the structure is porous The calculated solvent acces sible volume in the unit cell of 10 was found to be 28.5%, and would be incr eased to 41.0%, upon removal of guest. Highly disordered guest molecules occ upy the trigonal and hexagonal cavities. Kagom lattice 11 {[Cu2(5-ph-bdc)2(py)2]3}n, was obtained by the slow diffusion of an ethanolic solution of Cu(NO3)2 2.5H2O into an ethanolic solution of 5-ph-H2bdc and pyridine in the absence of template molecules. Blue-green crystals of 11 form exclusively within hours. Figure 4.8 illustrates the crystal structure of 11 The sheet structure in 11 is consistent with th at described above for 10 and results from of the assembly of tri gonal nSBUs at the vertexes. Th ere is one crystallographically independent copper ion having a square pyramidal geometry (dCu1-Cu1= 2.656(2) ). The (a) (b) Figure 4.8 Crystal structure of compound 11* (a) top view of 11 in space filling mode. (b) side view of 11 illustrating the AAA stacking. Coordinated pyridine molecules in (b) are shown as a single nitrogen atom with the remaining atoms deleted for clarity. Guest molecules were removed for clarity.

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88 basal plane is defined by four oxygen atoms of the four 2 bdc moieties (dCu-O= 1.965(7), 1.963(7), 1.976(7), 1.977(8) ) and the apical position is occupied by a disordered coordinating pyridine ligand (dCu-N= 2.151(3) ). All bond dist ances are in agreement with the average bond distances calculated fr om the CSD search. The trigonal nSBU (Fig 4.9) can be described as a curved e quilateral triangle with the sides measuring 10.677 and 8.100 at the wide and narrow ends, respectively (measured from neighboring copper ions). The angles of the triangle are perfect 60.00. In order for the SBUs to accommodate this trigonal arrangement the 5-ph-bdc moieties are distorted with the two carboxylate functionalities twisting out of plane with resp ect to each other ( = 19.0) and bending towards each other from the plane of the benzene ring by = 6.18. This distortion is much more pronounced here than in 10 ( =14.0 and = 5.38). (a) (b) Figure 4.9 Trigonal nSBU constituent of compound 11* *(a) Trigonal nSBU constituent of 11 is an equilateral triangle with sides me asuring 10.677 and 8.100 , at the wide and narrow ends, respectively. (b) Stacki ng trigonal nSBUs are involved in CH... interactions (2.949 ) between the coordinated pyridine and th e Ph-substituents of the 5Ph-bdc. Guest molecules were removed for clarity

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89 A disordered ethanol molecule occupies the cavity of the trigonal nSBU. The phenyl substituents on the bdc ligands are ort hogonal to the plane of the benzene ring and point away from the trigonal cavity and toward s the hexagonal cavity that is generated as the nSBUs assemble into a 2D sheet, occupyi ng part of it. The layers stack eclipsed (AAA packing) with the bottom of the bowls in one sheet sitting inside the bowls of adjacent sheets. This facilitates CHinteractions between the coordinated pyridine ligands (bottom of one bowl) with the ph-s ubstituents on the bdc rings of another bowl (dCH…centroid 2.949 ), as seen in Figure 4.9. The interlayer separation in 11 is 9.968 . This is consistent with the interlayer separation of the pr ototype Kagom (9.916 ) which also has pyridine as the apical coordinati ng ligand. The eclipsed stacking results in hexagonal channels that run along the c-axis. Since the ph-substituents occupy part of the hexagonal cavity, an hour-glass shaped cavit y results with an effective diameter of 10.738 calculated based on the shortest contact from the center of the hexagon. This corresponds to a spherical ar ea with a volume of 648 3. This is substantially larger than the spherical cavity of th e nanoball structures (avera ge cavity volume = 369 3). The calculated solvent accessible volume in the unit cell of 11 was found to be 20.1%, and would be increased only to 27.4% upon removal of guest (compared to 41% for 10 ). This is not surprising since the bul ky phenyl substituents occupy part of the space in the channels. Microcrystals of 11 can also be obtained by direct mixing of the reactants and allowing the solution to stand. The powder diffraction pattern obtained from the powder sample is consistent with the one calculated from single crystal data. When crystals of 11 are removed from the mother liquor and exposed to the atmosphere, they are found to undergo a transformation into a new compound, 12 while

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90 maintaining crystallinity, as confirmed by single crystal X -ray. Crystal structure of compound 12 {[Cu2(5-Ph-bdc)2(py)(H2O)]3}n, is illustrated in Figure 4.10 and reveals that one of the apical coordinating pyridine ligands is displaced by a coordinating water molecule. This is a single-crystal-tosingle-crystal transformation, a phenomenon few examples of which have been reported to date.162,207,260-262 There are two crystallographi cally independent copper ions in the crystal structure of 12 resulting in an unsymmetrical SBU. Bo th copper ions have a square pyramidal geometry with the basal plane define d by four oxygen atoms of the four 2 5-Ph-bdc moieties (dCu1-O= 1.954(6), 1.956(6), 1.966(6), 1.971(6); dCu2-O= 1.960(6), 1.963(6), 1.970(6), 1.970(6) ). The apical position is occupied by a disordered coordinating pyridine ligand for one of the copper ions (dCu1-N= 2.151(3) ), whereas the other copper ion is coordinated by a water molecule (dCu2-O= 2.172(6) ). The two copper ions of the SBU are separated by 2.6353(12) . The bond distances are in agreement with the (a) (b) Figure 4.10 Crystal structure of compound 12* (a) top view of 12 in space filling mode. (b) side view of 12 in stick mode illustrating th e AA’AA’ stacking. Coordinated pyridine and water ligands in (b) are s hown as single nitrogen and oxygen atoms, respectively, with the remaining atoms deleted for clarity. Guest molecules were removed for clarity.

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91 average bond distances calculated from the CSD search. The trigonal nSBU constituent of 12 (Fig 4.11) is also a curved equilateral triangle with the sides measuring 10.615 and 7.935 at the wide and narrow ends, respec tively (measured from neighboring copper ions), which is comparable to the dimensions of the nSBU in 11 (10.677 and 8.100 ). The angles of the triangle ar e perfect 60.00. The 5-ph-bdc mo ieties are distorted, as in 11, to accommodate this trigonal arrangeme nt. The two carboxylate functionalities are twisted out of plane with respect to each other ( = 20.6) and bent towards each other from the plane of the benzene ring by = 6.00. The nSBUs assemble at the vertexes in to a 2D undulating sheet generating hexagonal cavities. The sheets stack eclipsed. A closer look at the stacking reveals that 12 packs in such a way that generates a bila yer structure: the coor dinated pyridines in neighboring layers point towards each othe r and the coordinated water molecules in neighboring layers point towards each other. This creates two dis tinct regions between Figure 4.11 Trigonal nSBU moiety in compound 12* *(a) Trigonal nSBU constituent of 12 is an equilateral triangle with sides me asuring 10.615 and 7.935 , at the wide and narrow ends, respectively. Coordinated water molecules are presented as single oxygen atoms. Guest molecules were removed for clarity

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92 the layers: a hydrophobic region containing co ordinated pyridines and a hydrophilic region containing coordinated water mol ecules (Figure 4.12). Consequently, two interlayer separations exist: 9.406 for the hydrophobic region and 6.905, for the hydrophilic region and the packing c ould be described as AA’AA’. Such packing results in distinct inte ractions within each region. In the hydrophilic region, guest water molecules are pr esent. For each nSBU, there are three water molecules each of which hydrogen bonds to two of the coordinated water molecules in a bifurcated fashion (dO…O = 2.908, 2.913 ). These nSBU bowls sit inside neighboring bowls in such a way that they are in close proximity to the coordinated water molecules of the neighboring bowl. No Hbonding interactions were found, however. In the hydrophobic region, the coordina ted pyridine ligands of each nSBU interact with one another via CHinteractions so that each pyridi ne acts as a donor once and as an 9.406 6.905 Figure 4.12 Bilayer packing of compound 12* Two distinct layers with different interlayer separations are observed in 12 Coordinated pyridine and water ligands are presented in space filling mode.

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93 acceptor once (dCH… = 2.548 ). These nSBU bowls s it inside neighboring bowls so that the coordinated pyridine s are in close proximity to the Ph-substituents. van der Waals interactions stabilize this packi ng arrangement. Figure 4.13 illustrates the interactions for the nSBUs presen t in each of the two regions. The eclipsed stacking generates hour-glass channels from the hexagonal cavities that run along the c-axis. The effectiv e diameter of the channel ca lculated from the shortest contact to the center of the hexagon is 8.136 , significantly shorter than that of 11 (10.738 ). This corresponds to a smalle r spherical area with a volume of 282 3, which is reflected in the calculated solvent a ccessible volume in the unit cell of 9.27% (compared to a volume of 648 3 and calculated solvent a ccessible area of 20.1% in 11) The accessible volume would be increased to 12.6% upon removal of guest molecules (compared to 27% in 11 ). (a) (b) Figure 4.13 Supramolecular interactions in trigonal nSBU of 12* *(a) the coordinated water molecules hydrogen bond to guest water molecules in the hydrophilic region (dO...O= 2.908, 2.913 ). (b) Coordinated pyridine ligands interact via CHinteractions (dCH... = 2.548 ). The bowl in (b) sits inside (a) so that the pyridine ligands are within close proximity to the Ph-substituents of the bdc moiety maximizing the hydrophobic interactions

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94 The transformation of 11 into 12 happens for a crystalline sample, as well as a powder sample. The powder diffraction patt ern of a freshly-collected sample of 11 was consistent with that calculated from single crystal data of 11 however, upon leaving the sample to stand in a closed vial and taking an XPD of the same powder sample after 35 days produces a pattern that is consistent with that calculated from single crystal data of 12 Compound 12 is thermally stable to temper atures in excess of 300 C and decomposes after losing guest molecules at 174 C and at 286 C. The other derivative of bdc, namely 5-NO2-bdc produced two Kagom lattices under different reaction conditions. The slow diffusion of a methanolic solution of pyridine into a methanolic solution of Cu(NO3)2 2.5H2O and 5-NO2-H2bdc containing hexamethylbenzene (HMB) and nitrobenzene afforded compound 13 {[Cu2(5-NO2bdc)2(MeOH)2]3}n. The crystal structure of 13 is illustrated in Figure 4.14. There are two crystallographi cally independent copper ions in the crystal structure of 13 each comprising an SBU. The geom etry around each copper ion is square (a)(b) Figure 4.14 Crystal structure of compound 13* (a) Top view illustrating the Kagome lattice. (b) Sideview illustrating the ABAB packing. Coordinated methanol molecules are presented as single atom with the remaining atoms removed for clarity.

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95 pyramidal (dCu1-Cu1= 2.6312(18), dCu2-Cu2= 2.6231(13) ). The basal plane is defined by four oxygen atoms of the four 5-NO2-bdc moieties (dCu1-O= 1.954(7), 1.956(6), 1.964(5), 1.979(6); dCu2-O= 1.943(7), 1.943(7), 1.966(7), 1.966(7) ) and the apical position is occupied by a coordinati ng methanol molecule (dCu1-O= 2.145(7); dCu2-O= 2.147(1) ). The bond distances are in agreement with th e other Kagom structures and the average bond distances calculated from the CSD search. The trigonal nSBU constituent of 13 consists of three centrosymmetryic SBUs two of which are equivalent, as il lustrated in Figure 4.15. The 5-NO2-bdc ligand bridging the two equivalent SBU moieties is different from the other two 5-NO2-bdc ligands. The nSBU could be described as a curved isos celes triangle with sides measuring 10.526 and 10.468 at the wide ends and 7.699 and 7.772 at the narrow ends. The angles of the triangle are 59.64 and 60.18. These dimensions are consistent with those in compound 3 NO2-nanoball. Figure 4.15 Trigonal nSBU constituent of compound 13* The nSBu resembles an isosceles triangle with sides e qual to 10.526 and 10.468 at the wide end and 7.699 and 7.772 at the narrow end. The angles of the triangle measure 59.64 a nd 60.18. Hexamethlybenzene and methanol are present in the cavity. Coordinated methanol is represented as a sing le atom with the remaining atoms removed for clarity.

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96 The 5-NO2-bdc ligands are distorted to accomm odate this trigonal arrangement. The twist angle in the two equivalent 5-NO2-bdc ligands is comparable to that in compounds 11 and 12 measuring 21.9, but much smaller in the third 5-NO2-bdc bridging ligand measuring 5.5, only. The bending angle in the two equivalent bridging ligands is also comparable to 11 and 12 measuring 5.65, whereas in the third ligand the bending is more pronounced with a bend angle of 7.47. A closer look at the crystal structure reveals that this bending is due to a strong stacking interaction (dcentroid…centroid 3.356 ) between the 5-NO2-bdc ligand and a HMB guest molecule that is present in the cavity (Fig. 4.15). Each trigonal nSBU also hosts a disordered methanol molecule. The other two 5-NO2-bdc ligands are involved in stacking interactions with guest nitrobenzene molecules. The trigonal nSBU bowls assemble at the vertexes into an undulated 2D sheet generating hexagonal cavities. The nitro su bstituents of the bdc point towards the hexagonal cavities, whereas the coordinated methanol molecules point towards the trigonal cavities. The hexa gonal cavities are occupied by two disordered nitrobenzene molecules, which are involved in stacking interaction with the two equivalent 5-NO2bdc ligands (dcentroid…centroid=3.870 ). Figure 4.16 illustrates one hexagon in the 2D sheet of 13

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97 The effective diameter of the hexagon base d on the closest contac t to the center of the hexagon is 8.450 , comparab le to that in structure 12 (8.136 ) and NO2-nanoball compound 3 (8.80 ). The undulating sheets stack st aggered in an ABAB fashion with an interlayer separation of 10.328 . Figure 4.17 demonstrates that viewing the stacked sheets at a slight incline down the a-axis reveals the presence of large channels. Figure 4.17 Large channels in crystal structure of compound 13* View down the a-axis at an incline rev eals large channels in the st ructure. Coordinated methanol is presented as single oxygen atom with the remaining atoms removed for cl arity. Guest molecules were removed for clarity. Figure 4.16 Perspective view of a hexagon extracted from the crystal structure of 13* HMB and nitrobenzene are presented in space filling mode. Coordinated methanol molecu les are presented as single oxygen atom with remaining atoms removed for clarity

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98 This is reflected in the calculated accessible volume which was found to be 17.1%, and would be increased to 52.0% upon removal of guest molecules. It is worth noting that the reaction that yields compound 13 yields another product (blue crystals), which were found to change quantitatively while in solution within a period of ca. 10 days into crystals of 13 This other product (compound 15 ) will be discussed in detail in Chapter 5. Another Kagom structure based on 5-NO2-bdc was obtained, interestingly from the same reaction vial as that affording nanoball 3. Compound 14 {[Cu2(5-NO2bdc)2(H2O)2]3}n, forms upon leaving the reaction vial over a period of two weeks where the nanoball crystals dissolved and new hexagona l blue-green crystals form. The identity of the crystals was confirmed as Kagom la ttice by single crystal Xray crystallography. The crystal structure of 14 is depicted in Figure 4.18. There is one crystallographically independe nt copper ion in the crystal structure of 14 having a square pyramidal geometry. The basal plane is defined by four oxygen (a)(b) Figure 4.18 Crystal structure of compound 14* (a) Tope view in space filling mode illustrating the Kagom la ttice. (b) Side view illustring the ABC packing. The layers have been colored for clarity. Coordinated water molecules are presented as single oxygen atom with the hydrogen atoms removed for clarity. Guest molecules have been removed for clarity.

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99 atoms of the four 2 5-NO2-bdc moieties (dCu-O= 1.947(5), 1.948(5), 1.962(5), 1.964(5) ) and the apical position is occupied by a water molecule (dCu-O= 2.153(5) ). The two copper ions of the SBU are separated by 2.6490( 17) . Bond distances are comparable to those of the other four structures reporte d herein and in agreem ent with the average bond distances calculated from the CSD searc h. The apical coordi nating ligands point towards the trigonal cavities, wh ereas the nitro substituents on the bdc ligands point, in an alternating up and down fashion, towards the hexagonal cavity. The 5-NO2-bdc ligands are distorted to accommodate this trigonal arrangement of the paddlewheels. The carboxylate moieties of the 5-NO2-bdc are twisted out of plan e with respects to each other by an angle = 10.2. The carboxylate functionalit ies are also bent towards each other from the plane of the benzene ring by an angle = 5.18, which is comparable to the bend angle in the nanoball structures ( avg.= 5.22) and compound 10 ( = 5.38). The trigonal nSBU could be described as a curved equilateral triangle with sides measuring 10.554 and 7.773 , at the wide and narrow ends, respectively (measured from neighboring copper ions). The angles around the copper ion corners are perfect 60.00. Figure 4.19 depicts the trigonal nSBU in 14 Disordered methanol molecules occupy the trigonal nSBU.

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100 The trigonal nSBUs assemble at the ve rtexes into an undulating 2D sheet generating hexagonal cavities. The hexagonal cavities are occupied by two disordered nitrobenzene molecules in addition to a methanol molecule. The diameter of the hexagonal cavity measured based on the clos est contact to the cen ter of the hexagon is 10.492 , which is larger than that of compound 13 based on NO2-bdc (8.450 ) and comparable to that of 11 based on 5-Ph-bdc (10.738 ). The sheets in 14 stack staggered in an ABCABC fa shion along the c-axis with an interlayer separation of 10.437 (corresponds to 1/3 translation along the c-axis). This packing style eliminates hexagonal channels in the structure that would arise from the hexagonal cavities; however, channels are pres ent in the structure as can be seen in Figure 4.20, when the structure is viewed at an incline down the c-axis. This is reflected in the calculated solv ent accessible volume of 14 which was found to be 9.06%, and Figure 4.19 Trigonal nSBU constituent of compound 14

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101 would be increased to 60.2% upon removal of guest molecules (highest of all the structures described herein). Even though there is no structural diversit y within the sheet structure of the 5 Kagom compounds presented herein, they ar e significantly different in their packing style, interlayer separations, and porosity. From the results presented herein, it appears that the electronic properties of the substituent of the bdc ligand influence the packing style: electron donating groups ( e.g. Ph in 11 and 12 ) result in eclipsed AAA-type packing whereas electron withdrawing groups ( e.g. NO2 in 13 and 14 ) result in staggered ABAB or ABCABC-type packing. This is further supported by eight other Kagom structures generated by our group that will be the subject of a future publication. The three packing styles are i llustrated in scheme 4.1. Figure 4.20 Crystal structure of compound 14 reveals large channels* Crystal structure of 14 is presented in stick mode with coordinated wa ter molecules presented as single oxygen atoms. Guest molecules have been removed for clarity

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102 The eclipsed stacking results in hexagonal ch annels that run through the structure. If bulky substituents are present on the bdc moiety, they occupy the hexagonal channels minimizing their accessible volume. This is re flected in the calculated solvent accessible volumes of compounds 11 and 12 (Ph substituent), which have significantly lower volumes than the other structures. Staggered stacking precludes the presence of channels that would result from the hexagonal cavities; however, another type of channels is present and attributes to the large calculated solven t accessible volume in compounds 10 13 and 14 The size of the substituent on the bdc ligand does not affect the interlayer separation. This is not surp rising since the substituents point towards the hexagonal channels and occupy the space therein. Guest/solvent molecules present in the structure interact with the framework resulting in greater dist ortion in the bdc moieties, as seen in compound 13 The strong interactions between the HMB and 5-NO2-bdc increase the bend angle and thus distort the resulting hexagonal cavity. (a) AAA packing(c) ABCABC packing (b) ABAB packing (a) AAA packing(c) ABCABC packing (b) ABAB packingScheme 4.1 Schematic representation of the three packing styles observed in the Kagom structures

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103 4.3 Conclusions We presented herein 5 new compounds that have a topology that has been much sought after by material scient ists and physicists, namely the Kagom lattice. These structures result from the self-assembly of trigonal nSBU bowls into sheet structures. The sheet in each compound is sustained by Cu(II) dimers that are linked by 5-R-bdc ligands (R= H, Ph, NO2) thereby generating trigonal a nd hexagonal cavities within the layer. Although there is no di versity in the sheet of these structures, there are dramatic differences in terms of the interlayer separati on, the packing style and porosity. It can be concluded from the structures pr esented herein that the electr onic and steric properties of the bdc moiety affect the packing style a nd porosity, respectively. All five Kagom lattices presented have significant volumes accessible to guest molecules. The bend angles in the bdc moieties appear to be st rongly influenced by guest molecules, which interact with the walls of the trigona l nSBU by supramolecular interactions. With the potential magnetic properties of these compounds and their demonstrated porosity, they represent good ca ndidates for multifunctional metamaterials. It is worth emphasizing that such dramatic changes were effected by simple modification of the reactants. A study of the effect of these modifications on the magnetic properties is currently underway.

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104 4.4 Experimental General methods: All materials were used as receive d; solvents were purified and dried according to standard methods. TGA data were obtained on a TA instruments 2950 TGA at high resolution with N2 as purge gas. Synthesis of {[Cu2(bdc)2(4-picoline)2]3}n (10): The slow diffusion of a methanolic solution (7 mL) of H2bdc (166 mg, 1.0 mmol) and 4-pi coline (0.3 mL, mmol) into a methanolic solution of Cu(NO3)2. 2.5H2O (232 mg, 1.0 mmol) c ontaining naphthalene guest molecules (256 mg, 2.0 mmol) yielded hexagonal blue green crystals. The most intense peaks observed in the XPD pattern from the bulk sample are consistent with those calculated from single crystal diffraction data. Synthesis of {[Cu2(5-Ph-bdc)2(pyridine)2]3}n (11) : To a sample of 5-ph-H2bdc (59 mg, 0.24 mmol) in ethanol (10 mL) was added pyr idine (60 L, 0.74 mmol). The solution was heated and stirred to maximize dissoluti on then filtered and layered on top of an ethanolic solution (5mL) of Cu(NO3)2. 2.5H2O (57 mg, 0.24 mmol). Blue-green crystals of the title compound form within hours at the interface (45.4% yield). The title compound can also be obtained in 43.0 % yiel d by direct mixing of the ingredients in ethanol, as confirmed by XPD. TG analysis reveals that 11 is thermally stable to ca 315C and loses guest and solvent molecu les at 174.0C in 6.78% and at 286.2C in 8.9%. The most intense peaks observed in the XPD pattern from the bulk sample are consistent with those calculated fr om single crystal diffraction data.

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105 Synthesis of {[Cu2(5-Ph-bdc)2(H2O)(pyridine)]3}n (12) : Crystals of 12 were obtained from 11 by removing from the mother liquor a nd allowing the sample to stand under ambient conditions. 12 can be obtained quantitatively from a sample of 11 upon standing, as confirmed by XPD which shows a pattern consistent wi th that calculated from single crystal X-ray data. Synthesis of {[Cu2(5-NO2-bdc)2(MeOH)2]3}n (13) : Slow diffusion of a methanolic solution (5 mL) of pyridine into a me thanolic solution (10 mL) of Cu(NO3)2. 2.5H2O (239 mg, 1.03 mmol) and 5-NO2-H2bdc (200 mg, 0.947 mmol) cont aining hexamethylbenzene (33 mg, 0.20 mmol) and nitrobenzene (1 mL ) yielded, within 24 hrs, two types of crystals: blue ( 15 ) and blue-green ( 13 ) hexagons. Upon allowing the solution to stand for a period of ca. 10 days, 15 disappeared and only 13 prevailed. Single crystal X-ray crystallography of blue green crystals confir med their identity as the title compound. The most intense peaks observed in the XPD pattern from the bulk sample are consistent with those calculated from single crystal diffr action data. %yield = 63.0% based on Cu(II). Synthesis of {[Cu2(5-NO2-bdc)2(H2O)2]3}n (14) : Slow diffusion of a methanolic solution (10 mL) of Cu(NO3)2. 2.5H2O (124 mg, 0.533 mmol) into a me thanolic solution (10 mL) of 5-NO2-H2bdc containing pyridine (0.080 mL, 0.989 mmol), naphthalene (25 mg, 0.195 mmol) and nitrobenzene (3 mL) yielded large sq uare green blue crystals within months. Single crystal X-ray crystallography determin ed the identity of the crystals as compound 3 (see chapter 2). Upon reexamining the reactio n vial after a period of three weeks, the

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106 large square crystals disappeared and small hexagonal blue green crystals appeared. Xray crystallography determined the identity of the crystals as the title compound. Crystal structure determination: Single crystals suitable for X-ray crystallographic analysis were selected following examinati on under a microscope. Intensity data were collected on a Bruker-AXS SMART AP EX/CCD diffractometer using Moka radiation ( = 0.7107 ). The data were corrected for Lo rentz and polarization effects and for absorption using the SADABS program. The struct ures were solved using direct methods and refined by full-matrix least-squares on |F|2. All non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in geometrically calc ulated positions and refined with temperature factors 1.2 tim es those of their bonded atoms. All crystallographic calculations were conducted with the SHELXTL 5.1 program package. Crystallographic data for compounds 10 14 is presented in Table 4.1.

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107Table 4.1 Crystallographic data for compounds 10 14 10 11 12 13 14 Empirical formula C22.22 H14.44 Cu2 N0.44 O9.78 C40 H38 Cu2 N2 O8 C33 H25 Cu2 N O10 C43 H43 Cu3 N3.50 O23 C12.44 H14.67 Cu N1.33 O7.89 Formula weight 571.20 801.80 722.62 1167.43 372.68 Temperature (K) 200(2) 100(2) 100(2) 100(2) 100(2) Wavelength() 0.71073 0.71073 0.71073 0.71073 0.71073 Crystal system Trigonal Trigonal Trigonal Monoclinic Trigonal Space group R-3 P-3 P-3 C2/m R-3 a () 18.600(4) 18.7764(16) 18.5186(10) 20.661(2) 18.3270(16) b () 18.600(4) 18.7764(16) 18.5186(10) 18.240(2) 18.3270(16) c () 26.218(7) 9.9682(17) 16.3112(18) 18.668(2) 31.312(6) () 90 90 90 90 90 () 90 90 90 122.342(2) 90 () 120 120 120 90 120 Volume (3) 7855(3) 3043.5(6) 4844.3(7) 5943.8(12) 9108(2) Z 9 3 6 4 18 (Mg/m3) 1.087 1.312 1.486 1.305 1.223 (mm-1) 1.255 1.098 1.375 1.136 1.110 F(000) 2584 1242 2208 2386 3434 Crystal size 0.10 x 0.10 x 0.05 0.10 x 0.10 x 0.05 0.10 x 0.10 x 0.05 0.05 x 0.05 x 0.02 0.08 x 0.04 x 0.03 range for data collection () 1.48 to 23.25 2.04 to 23.30 1.78 to 26.59 1.61 to 28.35 1.83 to 25.75 Limiting indices -20<=h<=20 -19<=k<=18 -28<=l<=23 -15<=h<=20 -20<=k<=20 -10<=l<=11 -19<=h<=23 -23<=k<=16 -20<=l<=18 -26<=h<=25 -24<=k<=23 -14<=l<=24 -20<=h<=22 -22<=k<=20 -37<=l<=30 Reflections collected 7941 13227 28680 17469 16455 Independent reflec tions 2490 2937 6774 7040 3883 R(int) 0.1222 0.1568 0.0881 0.0522 0.737 Completeness to 99.3 99.6 100.0 91.8 99.8 Data/restraints/parameters 2490 / 1 / 161 2937 / 18 / 278 6774 / 48 / 463 7040 / 0 / 329 3883 / 67 / 242 Goodness-of-fit on F2 1.052 1.028 1.131 1.107 1.034 Final R indices [I>2sigma(I)], R1, wR2 0.0936, 0.2742 0.0976, 0.2810 0.0784, 0.2301 0.1202, 0.3318 0.0894, 0.2365 R indices (all data), R1,wR2 0.1664, 0.2981 0.1555, 0.3101 0.1277, 0.2457 0.1641, 0.3648 0.1407, 0.2576 Largest diff. peak and hole (e.-3) 1.104 and –0.681 0.675 and – 0.630 1.450 and 1.730 1.664 and – 0.708 1.001 and –0.846

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108 Chapter 5 ADDITIONAL RELATED CHROMOPHORES 5.1 Preface 5.1.1 Self-assembly Under Thermodynamic Equilibrium Conditions A characteristic feature of a thermodyna mic self-assembly process is that all reaction steps are under equilibrium.7 This means that all reaction steps are reversible and the proportion of each product obtained is dependent on its relative thermodynamic stability. This reversibility is due to the lability of the coordination bond, which can dissociate and re-associate unt il the most stable thermodyna mic product is obtained. The slow diffusion technique typically employed in the synthesis of the metal-organic structures reported in the pr evious chapters is a ther modynamic equilibrium process where the reactants interact at a gradient of different concentrations as they slowly diffuse into one another. As the conditions vary, a number of different chromophores are possible. In addition to the binucle ar SBU chromophore forming, mononuclear chromophores are also possible.

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109 5.1.2 CSD Survey of Cu(II) Copper(II) has been extensively studied and is known to exist in a number of different geometries. It can be tetrahedral, square pyram idal, trigonal bipyramidal or octahedral. A CSD19 search of copper compounds c ontaining two carboxylate ligands returned 978 hits. Only 21% of the struct ures (201 hits) have the carboxylate moieties bound in a monodentate fashion, while the rema ining structures have the carboxylates chelating to the metal center. Of those 201 structures, 38 (19%) have the carboxylates syn around the metal center. The coordination number around the Cu(II) ion varies between four (93 structures), five (53 stru ctures), and six (55 structures) with one example of a 2-coordinated Cu(II) ion (Ref code PANFOZ). The Cu-O bond distance ranges between 1.821 and 2.451 with an averag e of 1.966(70) . Figure 5.1 illustrates some of the simple geometries possible for Cu(II). (a) linear (d) Square pyramidal; syn (c) octahedral; anti (b) Square planar; anti Figure 5.1 Some of the possible chromophores of Cu(II)

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110 5.1.3 Supramolecular Isomers Isophthalic acid is an angular ligand with the two functiona lities predisposed at an angle of 120. As it is possible for the buildi ng blocks to arrange into more than possible way in the solid state, a number of supramolecular isomers are possible.23 Figure 5.2 illustrates three possible supramolecular isomers that can be generated from the same set of 120 angular node and linear spacer: (a) hexagon, (b) helix and (c) zigzag chain. The hexagon represents a discrete planar species few examples of which have been reported to date. A number of hexagons based on isopht halic acid and its derivatives have been reported to date. Hamilton et al129 utilized the supramolecular hydrogen bonding interactions of the carboxylic acid functiona lity to assemble a supramolecular hexagon, whereas Moore et al263 assembled a molecular hexagon from functionalized isophthalic acids through acetylene bonds making polypheny leneacetylenes. Other examples of metal-organic supramolecular hexa gons have been reported by Lehn,124 Stang,264 Newkome,128 and Lin.126 Lehn used a flexible polyde ntate ligand, but directed the formation of the hexagon by using a template an ion that fits inside the cavity. All the others used an angular bifunc tional ligand that has the func tionalities predisposed at an angle of 120. In all cases, the reported hexagons are charged species. The zigzag polymer has been widely encountered, whereas the helix remains quite rare in the context of coordination polymers.265 Additional supramolecular is omers from the angular ligand and linear spacer are also possible and include larger ring systems (non-planar), catenane supramolecular isomers,266 and other discrete polyhedra. 267

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111 5.1.4 Isophthalic Acids Isophthalic acid268 is known to exist in the solid state as a zigzag chain sustained by the centrosymmetric carboxylic acid dimer. As isophthalic acid represents an angular ligand with the two carboxylic acid moieties predisposed at an angle of 120, other possible supramolecular isomers include a helix and a discrete hexagon. Strategies of directing the formation of the discrete self-a ssembled hexagon over the zigzag chain have been investigated for isophthalic acid. Hamilton et al129 demonstrated that placing a bulky substituent on the 5-position of the isopht halic moiety disrupts the packing of the chains and results in the formation of a self-assembled hexagon. The group reported the synthesis and crystal structure of a discre te hexagon based on the self-assembly of 5OC10H21-H2bdc. The planar hexagons are sustained by the 12 hydrogen bonding interactions (six centr osymmetric carboxylic acid dimers), and have an effective diameter of 1.2 nm (measured from opposite phthaloyl hydrogen atoms). The hexagons stack on top of one another creating channels. The long alkyl chains exte nd above and below the hexagon Zigzag chain helix Figure 5.2 Three supramolecular isomers possible fr om an angular node and a linear spacer

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112 plane of the hexagon in an alternating fashi on. This elegant strategy of directing the formation of the discrete macrocycle over th e infinite chain by adding bulky substituents was first employed by Whitesides et al ,269 who controlled the formation of a rosette structure over a crinkled tape in the cy anuric/melamine acid systems by adding bulky substituents that sterically hinder the formati on of the tape and favor the formation of the discrete structure. Vittal el al244,270 has also done some extensive studi es on the effect of alkyl chain substituents on the aggregation of isophthali c acids in the solid state. The studies revealed that the length of the alkyl chain ha s a significant effect on the resulting crystal structure. 5-OR-H2bdc were found to self-assemble via hydrogen bonding into zigzag chains that pack into lamella-type structur es when the alkyl chain, R, consists of 12 carbons or more. When the alkyl chain cons ists of 6-10 carbons, the isophthalic acids were found to self-assemble into discrete he xagons, where the alkyl groups extend above and below the plane of the rings. As in the structure reported by Hamilton et al ,129 the hexagons stack on top of one another generati ng channels. The cavities of the hexagons are filled by part of the alkyl chains (3-4 carbon atoms) of neighboring hexagons. Crystal structures of other simple iso phathlic acid derivatives that have been reported in the CSD include 5-nitro-, 5sulfoand 5-hydroxyisophathlic acids. 5nitroisophthalic acid (5-NO2-H2bdc) is known to self-assemble via hydrogen bonding into 1D chain structures. Two phases of 5-NO2-H2bdc have been reported to date, a hydrated and a non-hydrated phase. In the former, 1D linear chains result from the catemar synthon in which a water molecule bridges two carboxylic acid moieties. Thus the chains arrange in pairs that are sustained by bridgi ng water molecules with the nitro substituents

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113 in one chain aligned on the same side, but opposit e to that of the other chain of the pair. This could be described as a ta pe structure. The tapes clos e-pack into corrugated sheets that stack on top of one anothe r. In the other phase of 5-NO2-H2bdc, zigzag chains sustained by the centrosymmetr ic carboxylic acid dimer form. A CSD search of isophthalates coordinate d to transition metals returned 17 hits (search excluded structures that included the SBU chromophore). 10 of the structures are based on bdc, 6 on 5-OH-bdc and 1 on 5-SO3-bdc. All structures, with the exception of one, are polymeric. 5.2 Results and Discussion 5.2.1 Compound 15 The slow diffusion of a methanolic solution of pyridine into a methanolic solution of Cu(NO3)2 2.5H2O and 5-NO2-H2bdc containing HMB and nitrobenzene resulted in two types of crystals: blue and blue green hexagonal plates. The latter was identified as compound 13 (discussed in Chapter 4), which is based on the dimetal tetracarboxylate SBU. The crystal structure of the blue hexagonal plates, compound 15 is shown in Figure 5.3. The structure is a 1D tape that re sults from two 1D linear chains sustained by 5-NO2-bdc ligands coordinated to copper ions in a monodentate fashion. The chains arrange in such a way so that the nitro subst ituents of one chain are aligned in the same direction and opposite to that of the other chain in the tape.

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114 The Cu(II) ions have a square pyramidal geometry where the basal plane is defined by two anti monodentate carboxylate ligands (dCu-O1 1.9736(13) , dCu-O2 2.0264(17) ) and two orthogona l pyridine ligands (dCu-N1 1.9969(17) , dCu-N2 2.0204(17) ) and the apical position is occ upied by a carboxylate oxygen atom from the 5-NO2-bdc moiety in the opposite chain of the tape (dCu-O3 2.2550(14) ). This tape structures is similar to that of the hydrated form of the parent 5-NO2-H2bdc. The tape structure is reinforced by CHinteractions (dCH…centroid 2.905 ) between the coordinated pyridine ligands (Fig 5.4a). The tapes stack in su ch a way as to facilitate stacking interactions between the 5-NO2-bdc moieties of the stacked tapes (dcentroid…centroid 3.766 ) (Fig 5.4b). It is worth noting that we have previously reported an analogous structure based on bdc that has the same chromophore. The bond distances and interactions in that stru ctures are consistent with the one reported herein.271 Figure 5.3 crystal structure of compound 15

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115 Interestingly, when crystals of 15 are left in contact with the mother liquor, they turn into blue green crystals of 13 the Kagom structure, as confirmed by unit cell determination and XPD of the bulk sample. Closer examination of the chromophore in 15 reveals that this is not so surprising. The chromophore, illustrated in Fig 5.5, is bis(2-carboxylato)copper(II) in which two Cu ions are bridged by two carboxylate moieties and each is coordinated by a monode ntate carboxylate with the carbonyl oxygen of each available for further binding to the me tal centers. Figure 5.5 illustrates how the tetrakis(2-carboxylato)dicopper SBU can form when the two carbonyl oxygen atoms rotate and displace two of the coordinated pyridine ligands. In other words, bis(2carboxylato)copper(II) could be considered a pr ecursor to the tetrakis(2carboxylato)dicopper(II), SBU. A CSD search to de termine the abundance of this chromophore revealed that it is known for a wide number of transitions metals (209 hits), but in general, it appears to form when the dicarboxylate used is bulky ( e.g. 2,2dimethylpropanoate), or a ch elating ligand is employed ( e.g. ethylenediamine, N N N’ N’ (a) (b)Figure 5.4 stacking interactions in the crystal structure of compound 15* *(a) CHinteractions between the coordinate d pyridine ligands in the tape. (b) stacking interactions between 5-NO2-bdc moieties in the tape

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116 tetraethylethylenediamine). Only f our structures were reported for Cu(II) (JOZMAM, JEKLUG, PRTDCU, QEDQAR). The transformation of 15 into the 2D Kagom structure suggests that the paddlewheel chromophore is the more stable thermodynamic one. 5.2.2 Compounds 16a and 16b When 5-NO2-H2bdc is reacted with Cu(NO3)2 2.5H2O in the presence of a noncoordinating amine, 2,6-lutidine, a diffe rent chromophore is obtained. Figure 5.6 illustrates the square pyramidal Cu(II) chromophore observed in compounds 16a and 16b In order to determine how well-known this chromophore is, a CSD search was performed. A search for a five-coordinate copper with two monodentate carboxylates returned 53 hits. 40 of those have the carboxylate moieties planar with syn and anti conformation around the metal center. Only 6 of the hits had the carboxylates in an anti conformation, with the rest adopting a syn conformation. Surveying the bond distances in these structures revealed th at the four bonds in the basal plane are shorter than the one Figure 5.5 Possible rearrangement in chromophore of compound 15* Bis(2-carboxylato)copper(II) chromophore can rearrange to yield the tetrakis(2-carboxylato)dicopper(II) chromophore, SBU

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117 in the apical plane. This is in accordance with the Jahn-Teller di stortion exhibited by the d9 electron configuration of Cu(II) ion. The average bond distances in the basal plane were found to be dCu-O1 1.952(21) , dCu-O2 1.957(17) , dCu-L1 2.039(78) , dCu-L2 2.014(35) , dCu-L3 2.275(119) for the anti structures, and dCu-O1 1.952(22) , dCu-O2 1.955(27) , dCu-L1 2.029(75) , dCu-L2 2.058(106) , dCu-L3 2.201(112) for the syn structures. No significant difference in bond distances to copper of the basal ligands exists between the syn and anti conformations, but the apical ligand distance is significantly longer in the anti structures (2.275 compared to 2.201 ). The reaction of 5-NO2-H2bdc with Cu(NO3)2 2.5H2O and 2,6-lutidine in methanol yields a precipitate, crystallization of which fr om DMSO yields pale blue square plates, 16a X-ray crystal data of 16a revealed that it is an infinite 1D zigzag chain, crystal structure of which is illust rated in Figure 5.7a. There are two crystallographically independent copper ions in the structure. The basal plane of each is occupied by two syn monodentate 5-NO2-bdc ligands (dCu1-O1 1.962(3), dCu1-O2 1.959(3) ; dCu2-O1 1.954(3), dCu2-O2 1.954(3) ), one DMSO molecule (dCu1-O 1.966(3); dCu2-O 1.993(3) ) and one water molecule (dCu1-O 1.962(3); dCu2-O 1.977(3) ), but the apical position is occupied by Figure 5.6 Chromophore of compounds 16a and 16b *The chromophore consists of two syn carboxylate functiolities, in addition to MeOH, H2O and/or DMSO. The coordinated apical ligands are presented as single oxygen atom s with the remaining atoms removed for clarity

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118 water on one copper center (dCu1-O 2.227(3) ) and DMSO on the other (dCu2-O 2.216(3) ). The bond distances are all in agreement with the bond distances determined from the CSD survey. The chains extend along the a-ax is and associate in pa irs that stack along the b-axis. The stacked chains are sustained by stacking interactions between the aromatic 5-NO2-bdc moieties (dcentroid…centroid 3.700) and hydrogen bonding interactions between the coordinated ligands. Figure 5.7b demonstrates the arom atic stacking and Hbonding interactions. Each chromophore is involved in four hydrogen bonding interactions between the equatorially coordinated water molecule in one chain and the carbonyl oxygen atoms of the 5-NO2-bdc moieties of the neighboring chain (dO…O 2.666 and 2.679 ). This gives a total of four hydrogen bondi ng interactions per chro mophore (Figure 5.8), which enhances the stability of the chromophore. (a) (b) Figure 5.7 Crystal structure of zigzag chain 16a* (a) zigzag chain of compound 16a (b) Chains of 16a associate in pairs via hydrogen bonding interactions between the coordinated ligands and stacking interactions between the 5-NO2-bdc moieties

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119 Upon leaving crystals of 16a in the mother liquor for a period of few weeks, they disappear and new darker blue hexagonal crystals appear, 16b X-ray crystal data of 16b revealed that the new compound is a discrete hexagonal structure based on the same chromophore as in 16a There are two crystallographical ly independent copper ions in the structure of the hexagon. The basal plane of each is occupied by two syn monodentate 5-NO2-bdc ligands (dCu1-O1 1.965, dCu1-O2 1.909 ; dCu2-O1 1.936, dCu2-O2 1.936 ), one DMSO molecule (dCu1-O 1.955; dCu2-O 1.783 ) and one water molecule (dCu1-O 1.964; dCu2-O 1.966 ), but the apical position is occupied by methanol on one copper center (dCu1-O 2.239 ) and water on the other (dCu2-O 2.298 ). All bond distances in 16b are in agreement with those of the zigzag chain and the CSD survey, except for the distance of the coordinated DMSO on Cu2 (dCu2-O 1.783 ), which is significantly shorter than exp ected (shortest bond distance from CSD search is 1.952 ). Figure5.9 illustrates the crystal structure of 16b which results from the selfassembly of thirty molecular components: six 5-NO2-bdc moieties, six Cu(II) ions and eighteen coordinated solvent molecules. The hexagons are neutral and planar. The effective outer diameter of the hexagon meas ured from opposite nitro groups is 3.14 nm. The distance from the center of the hexagon to the closest contact, coordinated solvent Figure 5.8 Hydrogen bonding interactions between chains of 16a

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120 molecule, is 0.58 nm, which affords an internal cavity with an effective diameter of ca. 0.8 nm. This is 0.3 nm longer than the hexa gon resulting from the self-assembly of the alkyloxyisophthalic acid deri vative reported by Hamilton et al129 (effective inner diameter of hexagon measured from opposite ph thaloyl 2-H sites is 1.5 nm, compared to 1.2 nm for the hexagons formed by H2bdc-5-OC10H21. Molecules of 16b pack into sheets with unusua lly large void s between the hexagons (Figure 5.9a). The separation be tween two neighboring hexagons within a sheet is 3.5 nm (measured from centroid to cent roid). As the sheets pack, they stagger so that every fourth layer repeats, i.e. ABCABC packing (Figure 5.10b,c), creating hourglass shaped channels along [001] with an effective diameter equal to that of the hexagon (0.8 nm). This packing arrangement is contrary to eclipsed stacking of the other (a) (b) Figure 5.9 Crystal structure of compound 16b* *(a) Top view of hexagon 16b in space-filling mode. (b) Side view illustrating the planarity of 1 6b Coordinated ligands are presented as single oxygen atoms with the re maining atoms removed for calrity. Guest MeOH molecules have bee removed for clarity

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121 self-assembled hexagons based on derivatives of H2bdc, but can be explained upon examining the stacked hexagons more closely. A closer look at the packing intera ctions in the crystal structure of 16b reveals that each apically coordinated water molecule is involved in hydrogen bonding interactions (dO…O 2.637 ) with two carbonyl oxygen atoms of 5-NO2-bdc moieties in the adjacent sheet (Figure 5.11). Figure 5.11 Hydrogen bonding interactions between stacking hexagons of 16b (a) (b) (c) (a) (a) (b) (c) Figure 5.10 Crystal structure packing in 16b

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122 This results in 4 hydrogen bonding interactions per chro mophore and a total of 24 hydrogen bonding interactions for each hexagon, all of which contribute significantly to the stability of this packi ng arrangement. The hydrogen bonding pattern observed in the hexagons, 16b is the same as that observed in the supramolecular isomeric chain 16a (Fig 5.8), but the interacti ons are slightly stronger (dO…O 2.637 vs. 2.673 for the hexagon and chain, respectively). The stacked sheets of hexagons are also involved in stacking interactions between the aromatic rings of 5-NO2-bdc resulting in an interlayer separation is 3.34 .201 It would be expected from the crystal structure that 16b would be porous. The calculated solvent accessible void is ca. 10% of the total volume; however, this would be increased to 57% upon remova l of guest and replacing the coordinated DMSO and MeOH molecules with water molecules. 16b is readily soluble in MeOH and spari ngly soluble in hot DMSO and DMF. The enhanced solubility in methanol facil itated its characterizati on by electrospray mass spectroscopy, which indicated that 16b exists in solution with 4 coordinated DMSO molecules and 14 coordinated MeOH molecules (M+ = 2396 m/z ). The mass spectrum of 16b can be found in Appendix 3. The conversion of 16a into 16b upon leaving is solution fo r an extended period of time suggested that 16a is the kinetic product, while 16b is the thermodynamic product. To confirm this, the crude precipitate obtaine d from the initial reaction was heated in DMSO ( ca. 110 C) for 24 hours and the solution was allowed to stand. The most intense peaks observed in the X-ray powder diffrac tion (XPD) patterns from the bulk sample were consistent with those calculated from single crystal diffraction data confirming that

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123 the hexagon 16b forms exclusively under thermodynamic conditions. Thermal stability data shows that 16b is stable up to 220 C and lose s guest and coordinated solvent in 9.1% at 48.2 C (calc. 7.5%), and 31% at 91.0 C (calc. 28.5%). 5.3 Conclusions The results presented herein demonstrate that under thermodynamic equilibrium a number of chromophores are possible leading to a number of different products. The chromophore based on bis(2-carboxylato)copper(II) can be considered a precursor to the SBU, tetrakis(2-carboxylato)dicopper(II). The lability of the ligands facilitates the rearrangement of the chromophore into the desire d, thermodynamically stable SBU. When a non-coordinated amine is used (2,6-lu tidine), a square pyr amidal chromophore sustained by two syn monodentate carboxylate moieties and three coordinated solvent molecules results. The ability of the coor dinated solvent ligands to participate in hydrogen bonding, more specifically 4 hydroge n bonding interactions per chromophore, tremendously enhances its stability. Conse quently, the chromophore remains intact even when the kinetic 1D zigzag chain re-arranges into its su pramolecular isomeric hexagon. We have further demonstrated supr amolecular isomerism by presenting two structures that are based on th e same building units, yet they are dramatically different from one another. The 1D zigzag chain, 16a is the kinetic product forming first and then converting into the hexagon supramolecular isomer 16b which is the thermodynamic product. Hexagon 16b exhibits an unusual packing a rrangement where large voids are present within the sheets of hexagons (s eparation of 3.5 nm between centers of neighboring hexagons). Strong interlayer interactions betwee n the sheets, hydrogen

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124 bonding interactions (24 per hexagon) and stacking interactions, tremendously stabilize this packing arrangement. Future work with the hexagon could involve using as it a building block for even larger structures. For example, one could use linear ditopic ligands such as pyrazine, or bipyridine to stack the hexagons into nanotubes. 5.4 Experimental General methods: All materials were used as receive d; solvents were purified and dried according to standard methods. TGA data were obtained on a TA instruments 2950 TGA at high resolution with N2 as purge gas. Synthesis of {Cu(5-NO2-bdc)(py)2}n (15) : Slow diffusion of a methanolic solution (5 mL) of pyridine into a methano lic solution (10 mL) of Cu(NO3)2. 2.5H2O ( 239 mg, 1.03 mmol) and 5-NO2-H2bdc (200 mg, 0.947 mmol) containi ng hexamethylbenzene (33 mg, 0.20 mmol) and nitrobenzene (1 mL) yielded, with in 24 hrs, two types of crystals: blue ( 15 ) and blue-green ( 13 ) hexagons. No yield was determined as crystals of 15 turned into 13 upon standing in the mother liquor. Synthesis of {Cu(5-NO2-bdc)(DMSO)1.5(H2O)2(MeOH)0.5}n (16) : To a methanolic solution of 5-nitroisophtha lic acid (250 mg, 1.184 mmol) and copper nitrate (276 mg, 1.184 mmol) was added 2,6-luti dine (254 mg, 2.368 mmol). The resulting blue precipitate (371 mg) was recrysta llized from DMSO to afford 16a and 16b Alternatively, 16b was exclusively obtained by heating a solu tion of the crude precipitate (197 mg) in

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125 DMSO (15 ml) for 24 hours and allowing the solution to sit for 3 weeks to yield 128.3 mg (ca. 27%) of deep blue precipitate. Most intense peaks in the XPD are consistent with those calculated from the singl e crystal data. TG analysis showed that crystals of 16b are thermally stable to 220C and lose guest a nd solvent molecules at 48.2C in 9.1% (calc. 7.5%), and at 91.0C in 31% (calc. 28.5%). Crystal structure determination: Single crystals suitable for X-ray crystallographic analysis were selected following examinati on under a microscope. Intensity data were collected on a Bruker-AXS SMART AP EX/CCD diffractometer using Moka radiation ( = 0.7107 ). The data were corrected for Lo rentz and polarization effects and for absorption using the SADABS program. The struct ures were solved using direct methods and refined by full-matrix least-squares on |F|2. All non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in geometrically calculated positions and refined with temperature factors 1.2 tim es those of their bonded atoms. All crystallographic calculations were conducted with the SHELXTL 5.1 program package. Crystal structure data of compounds 15 16a and 16b are presented in Table 5.1.

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126Table 5.1 Crystallographic data for compounds 15 16a 16b 15 16a 16b Empirical formula C36 H26 Cu2 O12 C24 H38 Cu2 N2 O20 S4 C41 H40 Cu N6 O9 S0.08 Formula weight 861.71 929.88 827.00 Temperature (K) 100(2) 200(2) 200(2) Wavelength() 0.71073 0.71073 0.71073 Crystal system Monoclinic Monoclinic Hexagonal Space group I2/a P2(1)/n R3 a () 16.8839(14) 14.0608(12) 35.023(4) b () 10.2558(7) 13.8902(12) 35.023(4) c () 19.8799(16) 19.0465(16) 9.9711(13) () 90 90 90 () 92.245(2) 98.487(2) 90 () 90 90 120 Volume (3) 3439.7(5) 3679.2(5) 10592(2) Z 4 4 12 (Mg/m3) 1.664 1.679 1.556 (mm-1) 1.314 1.465 0.695 F(000) 1752 1912 5164 Crystal size (mm3) 0.10 x 0.10 x 0.05 0.10 x 0.10 x 0.15 0.15 x 0.05 x 0.01 range for data collection () 2.05 to 28.28 1.69 to 28.26 2.01 to 24.00 Limiting indices -20<=h<=21 -13<=k<=13 -17<=l<=26 -18<=h<=11 -17<=k<=18 -23<=l<=25 -40<=h<=37 -26<=k<=40 -11<=1<=11 Reflections collected 10280 22351 14506 Independent reflections 4012 8628 7267 R(int) 0.0372 0.0409 0.1249 Completeness to theta 93.8 94.6 99.9 Data/restraints/parameters 4012 / 0 / 263 8628 / 0 /477 7267 / 47 / 439 Goodness-of-fit on F2 1.052 0.981 0.992 Final R indices [I>2sigma(I)] R1, wR2 0.0345, 0.0874 0.0534, 0.1163 0.0950, 0.2176 R indices (all data) R1, wR2 0.0378, 0.0897 0.0534, 0.1163 0.1210, 0.2357 Largest diff. peak and hole (e.-3) 0.550 and –0.418 1.057 and –0.646 0.995 and –0.498

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127 Chapter 6 GENERAL CONCLUSIONS AND FUTURE DIRECTIONS 6.1 Summary This dissertation presented a series of related, yet structurally diverse supramolecular architectures that are obtaine d from simple starting materials in single step reactions under thermodynamic equilibrium conditions. The modular nature of the systems presented herein contributes greatly to the structural diversity observed. The lability of the metal-ligand c oordinate covalent bond that di rects the formation of these structures facilitated the formation and isol ation of a number of possible products. In more than one case we presented examples where the kinetic product converts into the thermodynamic product under thermodynamic equilibrium. We presented a new design strategy by which molecular squares, SBUs, are linked at the vertices to generate two nanos cale SBUs: a tetragonal (collection of four squares) and trigonal (collecti on of three squares) nSBUs. We demonstrated that these nSBUs self-assemble to generate architectures with different dimensionality, complexity and topology. Self-assembly of both types of nSBUs results in discrete spherical structures that conform to the shape of a faceted polyhedron, the small rhombihexahedron which we termed nanoballs. The nanoballs have the general formula

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128 [Cu2(5-R-bdc)2(L)2]12 and are distinguished by a number of features including their high molecular weight, large molecular volume and diameter and high porosity. The nanoballs exhibit inclusion pr operties in their windows, which are reminiscent of hostguest interactions in calixarenes. Self-assembly of the tetragonal nSBUs only results in tetragonal 2D sheets, 44 grids, which have the general formula {[Cu2(5-R-bdc)2(L)2]4}n, when bdc and its derivatives are employed and {[Cu2(pdc)2(L)2]4}n, when the wider angle pdc is employed. Further structural diversity in th e solid state in this series of structures is observed due to atropisomerism of the nSBU that arises from its conformational flexibility. The diversity is underlined by the presence of different sh ape and size cavities, different inclusion properties and by different solvent accessible volumes in the structures. This further strengthens the similarity of the nSBU to a calixarene. Self-assembly of the trigonal nSBUs only results in trigonal 2D sheets with the general formula {[Cu2(5-R-bdc)2(L)2]3}n. The topology of this seri es of structures is of one that has been much sought after by mate rial scientists and physicists, the Kagom lattice. Although there is no st ructural diversity within the sheet of the structures that belong to this series, there is diversity in the packing of the sheet s, in the interlayer separation and in the porosity. The differen ces between the Kagom structures were effected by varying the molecu lar components of the system. Molecular formulas for compounds belongi ng to the three structure topologies summarized above have been presented thr oughout this work in such a way as to

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129 emphasize their close relationship. All three to pologies have the same basic formula and hence could be viewed as structural supramolecular isomers of one another. Performing the synthesis under thermodynami c equilibrium conditions facilitated the generation and isolation of other chromophor es that ultimately resulted in related nanostructures. Two such chromophores were presented, one is bis(2carboxylato)copper(II), which is a kinetic product th at arranges in solution into tetrakis(2-carboxylato)dicopper(II), the SBU, to ultimately generate an SBU-based Kagom product. The other chromophore is [Cu(5-NO2bdc)(DMSO)1.5(MeOH)0.5(H2O)], which forms a polymeric chain in which it is highly stabilized by strong H-bonding in teractions. As a result, the chromophore stays intact as the polymer rearranges into a more ther modynamically stable product, a discrete nanoscale hexagon, which is one of the largest metal-organic molecular hexagons isolated and characterized by si ngle crystal X-ray crystallography. 6.2 Conclusions Angular dicarboxylate ligands can bridge the SBU, which represents a molecular square, at the vertexes into related discrete and infinite structures that are porous. The distortion in the bridging ligand as it confines the SBUs into these structures is influenced by guest molecules. Strong intramolecular inter actions result in grea ter distortion in the aryl ligand. The size of the building bloc ks facilitates the generation of nanoscale materials in single step reactions. Perf orming the reactions under thermodynamic equilibrium conditions is advantageous as it allows for the isolation of related products that otherwise would not be possible. Th e self-assembly process is influenced by a

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130 number of factors including template molecu les, solvent and concentration. Varying the reaction conditions can result in a number of related, yet structurally diverse supramolecular architectures. This diversity can arise from conformational flexibility, guest/host interactions and packing of molecules in the cr ystal lattice. The diversity is exemplified by different topologies which e ffect significantly different properties. Careful analysis of supramolecular interactions in the crystal structures and understanding the subtle molecu lar interactions can help be tter design experiments that would generate desire d products reproducibly. 6.3 Future Directions A thorough and systematic evaluation of the effect of reaction conditions on the resulting supramolecular struct ure could be conducted. The effect of the bdc ligand, coordinating base and solvent could be evaluate d. The steric and el ectronic properties of the bdc ligand are expected to affect the result ing supramolecular isomer. It is likely that a trend as that seen for isophthalic acid, in terms of the effect of bulky substituents on the resulting supramolecular structur e, would be seen in these systems. Bulky substituents are expected to preclude the formation of the 44 tetragonal sheets as they are sterically demanding and the crystal structure cannot acco mmodate them. They would not hinder the formation of the nanoball, however, as th ey would be decorating the surface and not interacting with each other, and they woul d not hinder the formation of the Kagom lattice as they would extend into the large, av ailable hexagonal channe ls, as seen from the results presented herein.

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131 As for the effect of the base, whether the base can coordinate or not can affect the resulting chromophore. Other bases with el ectron donating or withdrawing substituents or branched or long alky chains could be evaluated as they may lead to the formation of other interesting chromophores. Different solvents also need to be evaluate d as in conjugation with the employed base they can lead to different products, as illustrated in the mononuclear chromophore presented in chapter 5, if they able to coor dinate to the metal site and/or have hydrogen bonding capabilities. Synthesis of other derivatives of the sm all rhombihexahedron could be targeted by using other derivatives of bdc. Bdc ligands w ith different steric a nd electronic properties are expected to have direct effect on the bul k physical properties of the nanoballs in terms of solubility, stability, etc. Furthermore, thes e substituents are expected to have an effect on the trigonal and tetragonal windows and in turn on their inclusion properties. For example, the use of a bdc substituted with bulky aryl groups would alter the shape and electronic nature of the windows. The dept h of the window would be greater and its hydrophobicity would be enhanced. This coul d alter the selectivit y of the windows to guest molecules. A systematic evaluation of the selectivity of the windows to different guest molecules could be conducted to determin e how the shape, size or chemical nature affects the inclusion properties. The synthesis of other faceted polyhedra could also be targeted, specifically ones based on molecular squares only: the cubohemioctahedron and the small rhombidodecahedron Judicial control over the angle at which the squares are connected would control the formation of these polyhe dra. To generate the cubohemioctahedron,

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132 one needs to link the molecular squares at 90 angle. Obviously, su ch an angle is not readily obtained for organic molecules; however it is reasonable to assume that the use of ligands with slightly bigge r or smaller angles could yiel d the desired product as these ligands could sustain some distortion from their ideal angle. A possible dicarboxylate ligand that subtends such angles is N -methylpyrrole-3,4-dicarboxylate (72). To generate the small rhombidodecahedron, one needs to link the molecular squares at a wider angle (144). This could be effected by using the di carboxylate ligand N -methylpyrrole-2,4dicarboxylate, which subtends an angle of ca. 157, yet it is flexible to accommodate the formation of the desired product. Another li gand that could be used is the commercially available thiophene-2,5-dicarboxylate. This lig and subtends an angle of 145 and would also be suitable for the generation of the small rhombidodecahedron. The nanoballs could also be used as building blocks for even larger structures. The nanoballs could serves as nodes for extende d networks. This has already been demonstrated with a derivative of bdc.192 The nanoballs have two possible branching points: the 5-position of the bdc ligand, or the coordinated ap ical ligand. Compound 2 for example, is decorated with 24 OH-functiona lities that are suitable for derivatizing the nanoball further either by forming covalent bonds or supramolecula r interactions with hydrogen-bond acceptors. The use of labile apical ligands could facilitate their displacement with other polyt opic coordinating ligands to ge nerate extended structures. The nanoballs could also serve as a core for dendritic structures. The high multiplicity on the surface of the balls makes them prime candidates for such purpose. Such structures could be pr epared in one of two ways: using a pre-made nanoball and reacting it further with dendritic wedges (via covalent or supramolecular interactions) or

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133 using branched bdc ligands to self-assemble in situ into the dendritic product. The use of very highly branched dendrons, however, coul d sterically hinder the self-assembly process. The former strategy requires evaluating the stability of the nanoballs to various reaction conditions such as solvent effect, pH, temperature, etc. The preliminary results presented herein suggest that it would feasible to subj ect the nanoballs to further reactions. The Kagom structures are suitable fo r generating multifunctional metamaterials since they are lamellar and contain large he xagonal cavities. Guest molecules with different properties could be incorporated in the channels or between the layers and this would be expected to have a significant e ffect on the bulk physical properties of these materials. Magnetic studies need to be conducted to evaluate the effect of the substituents of the bdc on the magnetism.

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149 Appendixes

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150 Appendix 1. Mass spectrum for compound 1

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151 Appendix 2. Mass spectrum for compound 2

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152 Appendix 3. Mass spectrum for compound 16b

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ABOUT THE AUTHOR Heba Abourahma received her Bachelor’s degree with honours in chemistry from Saint Mary’s University in Halifax, Nova Sc otia in 1997. It was there where she had her first research experience and first publica tion. She earned her Master’s degree from the University of Ottawa in 1999 after working in the area of carbohydrate synthesis. While pursuing her Ph.D. at the University of South Florida, the author held a visiting assistant professor position at Eckerd College where she taught general, or ganic and inorganic chemistry, in addition to holding a senior inst ructor position at the University of South Florida. Heba has received honors and awards throughout her academic training including three Natural Sciences and E ngineering Research Counsel of Canada (NSERC) awards, academic and industrial, teaching awards a nd outstanding graduate student conference presentation awards. She has published a number of publications and presented contributed talks at re gional, national and international conferences.


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Structural diversity in metal-organic nanoscale supramolecular architectures
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ABSTRACT: Supramolecular synthesis has gained much attention in recent years. Such an approach to synthesis represents an attractive alternative to traditional, multi-step synthesis, especially for making complex,nanoscopic structures. Of particular interest, in the context of this work, is the use of metal-organic interactions to direct the self-assembly of nanoscopic architectures. These interactions are highly directional, relatively "strong" (compared to other supramolecular interactions) and kinetically labile, which allows for "self-correction" and in turn the production, often in high yield, of defect-free products. This also means that a number of related, yet structurally diverse products (supramolecular isomers) could be isolated. The work presented herein demonstrates the supramolecular synthesis of related, yet structurally diverse family of metal-organic nanoscale supramolecular architectures that are based on the ubiquitous paddle-wheel dimetal tetracarboxylate secondary building unit (SBU) and angular dicarboxylate ligands. It also demonstrates that the SBU self-assembles into clusters of four (tetragonal) and three (trigonal) nanoscale secondary building units (nSBU), which further self-assemble into nanoscale structures that include discrete (0D) faceted polyhedra, tetragonal 2D sheets and another 2D sheet that conforms to the so-called Kagom lattice. In addition, the work herein demonstrates that synthesis under thermodynamic equilibrium conditions facilitates "self-correction" so that the most stable thermodynamic product is obtained. Synthesis, characterization and crystal structure analysis of these structures is presented herein.
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coordination polymers.
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