Coordination Polymers from Calixarene-Like [Cu2(Dicarboxylate)2]4

Green crystals of 1a were obtained from slow diffusion of an ethanolic solution (4 mL) of 1,3-H2bdc (166 mg, 0.999 ..... Moulton, B.; Zaworotko, M. J...
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Coordination Polymers from Calixarene-Like [Cu2(Dicarboxylate)2]4 Building Blocks: Structural Diversity via Atropisomerism Heba Abourahma,† Graham J. Bodwell,‡ Jianjiang Lu,† Brian Moulton,† Ian R. Pottie,‡ Rosa Bailey Walsh,† and Michael J. Zaworotko*,†

CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 4 513-519

Department of Chemistry, University of South Florida, 4202 East Fowler Avenue (SCA 400), Tampa, Florida 33620, and Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X7 Received March 1, 2003;

Revised Manuscript Received April 24, 2003

ABSTRACT: The self-assembly of [Cu2(dicarboxylate)2]4 nanoscale secondary building units (nSBUs) into coordination polymers can occur in such a way that a range of calix-like atropisomers can exist. Eight compounds have been prepared and crystallographically characterized via reaction between Cu(NO3)2 and angular dicarboxylate ligands: {[Cu2(bdc)2(py)2]4•guest}n (1a), {[Cu2(bdc)2(py)2]4•4nitrobenzene•2EtOH}n (1b), {[Cu2(bdc)2(4-pic)2]4•4o-dichlorobenzene}n (2), {[Cu2(5-OEt-bdc)2(py)2]4•8H2O}n (3), {[Cu2(5-OPr-bdc)2(py)2]4•guest }n (4), {[Cu2(pdc)2(py)2]4•4MeOH}n (5), {[Cu2(pdc)2(4-pic)2]4•4H2O}n (6), and {[Cu2(tdc)2(MeOH)2]4•4naphthalene•8MeOH}n (7). The following atropisomers, all of which have also been observed in calixarenes, were observed: (a) cone (1a, 3, and 4); (b) partial cone (1b); (c) 1,2-alternate (2 and 5-7); and (d) 1,3-alternate (1a, 3, and 4). The similarities and differences between the crystal structures of 1-7 are detailed herein. Introduction Supramolecular approaches to synthesis1,2 provide an alternate paradigm to sequential multistep synthesis for the design and synthesis of artificial chemical structures. In particular, supramolecular synthesis can be highly efficient since it relies on the self-assembly of selected building blocks with complementary functionalities and geometries and is typically accomplished in a one-pot reaction. Coordination polymers, in the context of porous3-25 and magnetic26-32 materials, exemplify the power of modular self-assembly33 to generate infinite networks in which a metal moiety typically acts as a node, and multifunctional organic ligands act as spacers that propagate this node. The node and spacer8,10,16,34-36 strategy has been remarkably successful at producing a diverse range of predictable network architectures, some of which have no precedent in natural systems. The modularity of this approach is particularly attractive since, being inherently formed from more than one component, it facilitates systematic fine-tuning of structural and functional features. Another reason that there exists such a degree of structural diversity in coordination polymers is that it is possible for a given set of molecular building blocks to generate more than one possible superstructure, and supramolecular isomerism may therefore arise.3,37-39 The dimetal tetracarboxylate secondary building unit (SBU)40 illustrated in Figure 1 is a ubiquitous moiety as evidenced by the presence of over 1000 entries involving 21 transition metals in the Cambridge Structural Database (CSD).41 Its use as a building block for the generation of infinite and discrete superstructures is a relatively recent phenomenon. Nevertheless, it is already clear that its structural and functional versatil* Corresponding author. Fax: (813) 974-1733. E-mail: [email protected]. † University of South Florida. ‡ Memorial University of Newfoundland.

Figure 1. Building blocks utilized in this study. (a) The ubiquitous dimetal tetracarboxylate secondary building unit (SBU) (top) can be viewed as a molecular square (bottom). (b) Angular benzene-1,3-dicarboxylate, bdc, (top) and 5-ethoxybenzene-1,3-dicarboxylate (bottom) have the carboxylate moieties predisposed at a 120° angle. (c) Angular thiophene-2,5dicarboxylate, tdc, (top) and 1-methylpyrrole-2,4-dicarboxylate, pdc, (bottom) have the carboxylate moieties predisposed at 157 and 145°, respectively.

ity makes it a particularly attractive molecular building block: the metal can be varied with resulting functional changes; the carboxylate moiety has many permutations; and the axial ligand can be varied almost at will. Two approaches to the use of this SBU have thus far been delineated: as a linear spacer when coordination occurs at the axial positions,42-46 or as a molecular square linked at the equatorial sites (as suggested when viewing the SBU down the 4-fold axis in Figure 1).47,48 To most effectively utilize the SBU as a square building block polycarboxylate ligands can be used, which link the SBUs at their vertexes. Benzene-1,4dicarboxylate49 (a linear dicarboxylate), benzene-1,3dicarboxylate47,48,50-52 (an angular dicarboxylate), and benzene-1,3,5-tricarboxylate53 (a three-directional car-

10.1021/cg0340345 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/14/2003

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Figure 2. Schematic illustrating how square SBUs can selfassemble into a triangular nSBU (a) or a square nSBU (b).

Abourahma et al.

possibility that square nSBUs might exhibit atropisomerism and demonstrate that they do indeed form atropisomers in the solid state and that there is a profound effect upon the resulting polymeric structures. The following dicarboxylates were exploited in the studies reported herein (Figure 1): bdc (benzene-1,3-dicarboxylate); derivatives of bdc in which the two carboxylate moieties remain disposed at 120° (5-OEt-bdc and 5-OPrbdc); and tdc (thiophene-2,5-dicarboxylate) and pdc (1methylpyrrole-2,4-dicarboxylate), both of which represent dicarboxylates in which the two carboxylate moieties are disposed at angles larger than 120° (157 and 145°, respectively). Experimental Procedures

Figure 3. Four atropisomers of a calix[4]arene: (a) cone, (b) partial cone, (c) 1,2-alternate, and (d) 1,3-alternate.

boxylate) have thereby all afforded self-assembled structures with rational, if not predictable, topologies. We have focused our efforts on angular ligands and have demonstrated that the use of benzene-1,3-dicarboxylate (bdc), in which the two carboxylate moieties are rigidly predisposed at 120°, facilitates the selfassembly of the square SBUs into square (cluster of four SBUs)47 or triangular (cluster of three SBUs)52 nanoscale SBUs, nSBUs (Figure 2). The square SBUs are linked in an angular fashion, and the resulting nSBUs therefore possess curvature, and in principle, torsional flexibility. These nSBUs can further self-assemble into discrete faceted polyhedra that contain both types of nSBUs (square and triangular)48,51 or infinite networks that contain only square nSBUs47 or only triangular nSBUs.52 Calixarenes represent a large and continually growing area of research in the context of supramolecular chemistry.54-62 It occurred to us that the shape and even the chemical nature of the square nSBU illustrated in Figure 2b resembles that of a calix[4]arene molecule and that square nSBUs therefore might be subject to atropisomerism.63 Atropisomerism, a long known phenomenon for calixarenes, arises when rotation around a covalent bond is impeded enough to allow for the isolation of different structures in the solid state. There are four possible atropisomers of calix[4]arenes64-68 that were designated by Gutsche54 as the cone, partial cone, 1,2-alternate, and 1,3-alternate, all illustrated in Figure 3. Similar behavior has been observed for larger calixarenes.69-73 The nomenclature refers to the orientation of the arene rings with respect to one another. In the cone conformation, all arenes point up and form a cone-like structure, whereas in the partial cone three arenes point up and one points down. The square nSBU has been previously observed in the cone and 1,3alternate conformations47 and can be regarded as being a metal-organic calix. In this paper, we address the

General Methods. All materials were used as received; solvents were purified and dried according to standard methods. TLC plates were visualized using a short wave (254 nm) UV lamp. Melting point (mp) data were obtained on a FischerJohns apparatus and are uncorrected. Infrared (IR) spectra (cm-1) were recorded as Nujol mulls using a Mattson Polaris FT instrument. 1H NMR spectra were obtained from DMSOd6 solutions on a Bruker instrument operating at 250 MHz for 5-OR-H2bdc and on a Bruker Avance instrument operating at 500.1 MHz for N-methyl H2pdc. Chemical shifts are relative to internal TMS standard. Coupling constants are reported in Hz. Reported multiplicities are apparent. 13C NMR spectra are recorded at 125.8 MHz; chemical shifts are relative to DMSO (δ 39.5). Low- and high-resolution mass spectroscopic data were performed by the University of Ottawa Mass Spectrometry Centre. TGA data were obtained on a TA instruments 2950 TGA at high resolution with N2 as purge gas. Formulations of the coordination polymers are based upon nSBUs rather than empirical units. No attempts to optimize the yields were performed. Synthesis of Alkoxy Dimethyl Isophthalate Esters. Established literature procedures74 were followed except for using acetone as the solvent instead of DMF. Saponification of Alkoxy Dimethyl 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 was acidic (pH 3-4). Cooling the solution to 4 °C overnight yielded a white, crystalline product. Synthesis of 5-Ethoxybenzene-1,3-dicarboxylic Acid (5-OEt-1,3-H2bdc). Dimethyl 5-hydroxyisophthalate (5.01 g, 24.1 mmol) was dissolved in acetone (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 yield 5.48 g of the yellowish ester product. The ester was then saponified according to the general procedure to yield the title compound in quantitative yield (4.77 g). 1H NMR (250 MHz, DMSO): δ ) 7.95 (s, 1H, CH arom), 7.55 (s, 2H, CH arom), 4.09 (q, 2H, CH2), 1.29 (t, 3H, CH3); 13C NMR (250 MHz, DMSO): δ ) 166.84 (CO2H), 159.00 (C-O), 132.94 (C-CO2H), 122.48 (CH), 119.31 (CH), 64.12 (OCH2), 14.86 (CH3). Synthesis of 5-Propoxybenzene-1,3-dicarboxylic Acid (5-OPr-1,3-H2bdc). Dimethyl 5-hydroxyisophthalate (5.11 g, 24.1 mmol) was dissolved in acetone (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 yield 4.51 g of an oily yellow residue, the TLC of which (silica, EtOAc/hex 3:2) indicated the 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 (250 MHz, DMSO): δ ) 7.95 (s, 1H, CH arom), 7.55 (s, 2H, CH arom), 4.09 (q, 2H, CH2), 1.29 (t, 3H, CH3); 13C NMR (250 MHz, DMSO): δ ) 166.84 (CO2H), 159.00 (C-O), 132.94 (C-CO2H), 122.48 (CH), 119.31 (CH), 64.12 (OCH2), 14.86 (CH3).

Coordination Polymers from [Cu2(Dicarboxylate)2]4 Synthesis of 1-Methylpyrrole-2,4-dicarboxylic Acid (1Me-2,4-H2pdc). The dimethyl ester was prepared according to the literature procedure.75 A solution of 1-methylpyrrole2,4-dicarboxylic acid dimethyl ester (1.23 g, 6.24 mmol) in 4:1 20% aqueous NaOH solution/methanol (25 mL) was heated at reflux for 1 h. The reaction was cooled to room temperature and made acidic using 1 M HCl, and a white precipitate formed. The solid was extracted using EtOAc (2 × 50 mL) and the aqueous layer was salted by the addition of solid NaCl and then extracted with another portion of EtOAc (50 mL). The combined organic layers were dried over MgSO4, filtered, and concentrated under reduced pressure to afford 1-methylpyrrole-2,4-dicarboxylic acid (1.01 g, 95%) as a white solid: mp 261-262 °C, IR (Nujol) 3230-2443 (s, br), 1669 (s), 1284 (s) cm-1; 1H NMR (DMSO-d6, 500.1 MHz) δ 12.39 (br s, 2H), 7.63 (d, J ) 1.5 Hz, 1H), 7.06 (d, J ) 2.1 Hz, 1H), 3.88 (s, 3H); 13C NMR (DMSO-d6, 125.1 MHz) δ 164.5, 161.6, 133.3, 123.8, 117.7, 114.9, 36.9; EI-MS m/z (%) 169 (M+, 84), 162 (14), 152 (100), 125 (14), 108 (21); HRMS (EI) calcd. for C7H7NO4: 169.0375, found: 169.0381. Synthesis of {[Cu2(bdc)2(py)2•Guest]4}n (1a). Green crystals of 1a were obtained from slow diffusion of an ethanolic 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 copper nitrate hemipentahydrate (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 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 those calculated from single-crystal diffraction data. Synthesis of {[Cu2(bdc)2(py)2]4•4Nitrobenzene•2EtOH}n (1b). Green crystals of 1b were obtained upon standing of an ethanolic solution of 1,3-H2bdc (254 mg, 1.53 mmol), pyridine (0.38 mL, 4.70 mmol), copper nitrate hemipentahydrate (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 a second weight loss of 37% between 280 and 400 °C. Further heating results in decomposition of sample. Synthesis of {[Cu2(bdc)2(4-pic)2]4•4o-Dichlorobenzene•8MeOH}n (2). Green crystals of 2 were obtained from the slow diffusion of a methanolic solution (7 mL) of 1,3-H2bdc (166 mg, 0.999 mmol) and 4-picoline (0.30 mL, 3.1 mmol) into a methanolic solution (7 mL) of copper nitrate hemipentahydrate (233 mg, 1.00 mmol) containing 5 mL of o-dichlorobenzene. Crystals formed after 3-5 months in 50% yield. The crystals are thermally stable to above 200 °C after which the TG curve shows a weight loss of 58% between 250 and 300 °C and a second weight loss of 16% between 350 and 400 °C. Further heating of the sample results in decomposition. Synthesis of {[Cu2(5-OEt-bdc)2(py)2]4•8H2O}n (3). Green crystals of 3 were obtained within days from the slow diffusion of a methanolic solution (5 mL) of 5-OEt-1,3-H2bdc (87.8 mg, 0.424 mmol) and 2,6-dimethyl pyridine (0.10 mL, 0.85 mmol) into a solution of copper nitrate hemipentahydrate (98.6 mg, 0.424 mmol) in 5 mL of methanol containing template molecules (nitrobenzene, benzene, naphthalene, o-dichlorobenzene, or none). TG analysis shows removal of guest molecules at 88 °C, followed by removal of coordinating pyridine molecules at 223 °C, and then decomposition above 305 °C. Synthesis of {[Cu2(5-OPr-bdc)2(py)2]4•Guest }n (4). Green crystals of 4 were obtained within days from the slow diffusion of a methanolic solution (5 mL) of 5-OPr-1,3-H2bdc (113 mg, 0.511 mmol) and pyridine (0.20 mL, 2.5 mmol) into a methanolic solution of copper nitrate hemipentahydrate (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 loss 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.

Crystal Growth & Design, Vol. 3, No. 4, 2003 515 Synthesis of {[Cu2(pdc)2(py)2]4•4MeOH}n (5). 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 copper nitrate hemipentahydrate (145 mg, 0.623 mmol) in methanol (5 mL). An immediate color change occurred, and greenish-blue crystals formed overnight in 19.2% yield. TG analysis reveals loss of guest/solvent below 200 °C 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 calculated from single-crystal diffraction data. Synthesis of {[Cu2(pdc)2(4-pic)2]4•4H2O}n (6). Green crystals of 6 were obtained by slow diffusion of a methanolic solution (10 mL) containing N-Me 2,4-H2pdc (96 mg, 0.57 mmol) and 4-picoline (0.17 mL, 1.7 mmol) into a methanolic solution (10 mL) of copper nitrate hemipentahydrate (131 mg, 0.563 mmol) containing nitrobenzene (3 mL). Crystals formed within days in 36.4% yield. TG analysis 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. Synthesis of {[Cu2(tdc)2(MeOH)2]4•4Naphthalene•8MeOH}n (7). Green crystals of 7 were obtained from the slow diffusion of a methanolic solution (10 mL) of H2tdc (172 mg, 0.999 mmol) and naphthalene (256 mg, 2.03 mmol) into a methanolic solution containing pyridine (0.20 mL, 2.5 mmol), naphthalene (256 mg, 2.03 mmol), and nitrobenzene (3 mL). Crystals form within days in 12% yield. TG analysis shows a weight loss of 12% between 95 and 105 °C followed by another weight loss of 10% between 120 and 170 °C. The framework appears to be stable to above 250 °C, above which the TG curve shows a weight loss of 50% between 290 and 310 °C. Further heating of the sample results in its decomposition. 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 examination under a microscope. Intensity data were collected on a Bruker-AXS SMART APEX/CCD diffractometer using MoKR radiation (λ ) 0.7107 Å). The data were corrected for Lorentz and polarization effects and for absorption using the SADABS program. The structures 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 times those of their bonded atoms. All crystallographic calculations were conducted with the SHELXTL 5.1 program package. Table 1 provides crystallographic data for compounds 1-7. Atomic coordinates and thermal parameters have been deposited, and the CCDC numbers are given in Table 1. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Rd., Cambridge CB2 1EZ, UK; fax: (+44)1223-3363-033; or [email protected]).

Results and Discussion Figure 4 depicts the nSBU constituents of coordination polymers 1-7. The square nSBU consists of four dicarboxylate ligands and four SBUs. The cone isomer (Figure 4a) has C4v symmetry, and all four dicarboxylate ligands orient in the same direction (the 5-position pointing toward the viewer). The partial cone (Figure 4b) has Cs symmetry with three dicarboxylate ligands oriented up and one down. The 1,2-alternate isomer

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Table 1. Data Collection, Structure Solution, and Refinement Parameters for 1-7a compound

1a

1b

2

3

5

6

7

formula CCDC deposit no. MW color, habit

C36H20Cu2N2O10 162 957

C43H29Cu2N3O10.5 205 360

C34H26Cl2Cu2N2O8 205 361

C30H30Cu2N2O12 205 359

C25H24Cu2N4O9 205 357

C26H26Cu2N4O9 205 358

C26H24Cu2O12S2 205 362

767.65 blue-green plates tetragonal P4/ncc, 8 18.7912 (8) 18.7912 (8) 16.8886 (10) 90 90 90 5963.5(5) 173(2) 1.683 1.53-28.27 33 929

882.80 blue-green plates monoclinic Cc, 4 10.2527 (12) 18.973 (2) 16.977 (2) 90 100.460(2) 90 3247.5 (7) 100 (2) 1.374 1.22-28.30 20 125

768.58 blue-green plates monoclinic P2(1)/m, 4 19.5148 (14) 12.7678 (9) 14.3466 (10) 90 114.1330 (10) 90 3262.2 (4) 100 (2) 1.523 1.96-28.29 10 312

737.66 blue-green plates tetragonal P4/ncc, 8 18.8743(16) 18.8743(16) 17.356(3) 90 90 90 6182.7 (13) 200(2) 1.443 1.53-24.74 28 119

651.56 blue-green plates monoclinic P2(1)/n, 2 8.2043 (13) 14.970 (2) 10.6012 (16) 90 90.191 (3) 90 1302.0 (4) 200 (2) 1.695 2.35-26.42 7450

665.59 blue-green plates monoclinic P2(1)/n, 2 8.4851 (15) 14.6395 (2) 11.1455 (19) 90 93.392 (3) 90 1382.0(4) 100 (2) 1.599 2.30-28.26 2463

719.65 blue-green plates monoclinic C2/c, 4 15.637 (4) 15.430 (4) 13.332 (3) 90 116.692 (4) 90 2874.1 (13) 200 (2) 1.668 1.97-28.30 8435

3632 (0.0560)

7879 (0.0993)

6730 (0.0332)

2657 (0.1942)

2656 (0.0832)

2708 (0.0513)

3360 (0.0848)

0.0407, 0.1063

0.0841, 0.1967

0.0530, 0.1173

0.0525, 0.1150

0.0599, 0.1116

0.0643, 0.1120

0.0767, 0.1539

crystal system space group, Z a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) T (K) M (mm-1) θ range reflns collected ind. reflns (Rint) R (F), Rw (F)

a 4: P2(1)/c, a ) 18.964(7), b ) 19.029(7), c ) 18.645(6) Å; R ) γ ) 90, β ) 91.605(7)°; V ) 6726(4) Å3, and Z )4. Because of high thermal motion and/or disorder, the structure could not be refined below Rf ) 0.2925.

Figure 4. nSBU constituents of coordination polymers presented herein are illustrated in space-filling mode. (a) Cone present in 1a, (b) partial cone present in 1b, (c) 1,2-alternate present in 2, (d) 1,3-alternate present in 1a, (e) 1,2-alternate present in 5 and 6, and (f) 1,2-alternate present in 7. 3 and 4 contain cone and 1,3-alternate nSBUs similar to panels a and d with the addition of OR groups on the 5-position. nSBUs (a-d) as compared to calixarenes (a-d), respectively, in Figure 3.

(Figure 4c, e, and f) also exhibits Cs symmetry with two adjacent dicarboxylate ligands oriented up and two down. Finally, the 1,3-alternate isomer (Figure 4d) exhibits C2 symmetry with opposite dicarboxylate ligands oriented in the same direction. The geometry around the copper ions is that of a distorted square pyramid. The basal positions are occupied by carboxylate moieties, and the apical positions are occupied by pyridine in 1a, 1b, 3, 4, and 5, by 4-picoline in 2 and 6, and by MeOH in 7. The observed structures of compounds 1-7 are discussed individually below. Compound 1a, {[Cu2(bdc)2(py)2] 4}n, contains two atropisomeric nSBUs, namely, the cone and the 1,3alternate (Figure 5). The nSBUs self-assemble, alternating between the cone and the 1,3-alternate conformations and thereby yield an undulating sheet structure. Each cone has an outer diameter of 0.94 nm and a depth of 0.84 nm (measured from the center of the base to the midpoint of a line joining the top hydrogen atoms on

Figure 5. Space-filling and schematic representation of the undulating sheet in 1a. Guest molecules and coordinating pyridine ring (except for the nitrogen atom) have been removed for clarity. The cone outer diameter is 0.94 nm, and its depth is 0.84 nm.

opposite bdc moieties). When the sheets stack, they eclipse one another in such a way that the cones stack inside one another and generate close cavities whereas the 1,3-alternate nSBUs define hourglass-shaped channels. Disordered guest molecules occupy both types of void space. The solvent accessible volume in one unit cell is 0.5% but would be increased to 27.5% upon removal of guest molecules.76 We have previously reported an isostructural Zn analogue47 in which Zn nSBUs self-assemble into undulating sheets containing both conformers of the nSBU. Compound 1b, {[Cu2(bdc)2(py)2]4}n, results from the self-assembly of partial cone nSBUs into an undulating sheet structure (Figure 6). The partial cone is the result of a bdc ligand lying in a plane almost perpendicular to the plane defined by the other three bdc moieties (θ ) 97.0°). The outer diameter of the partial cone (measured from opposite bdc ligands that lie in one plane) is 0.98 nm, and the depth is 0.42 nm (measured from the centers of the lines joining the top and bottom hydrogen atoms of opposite bdc moieties that are in the same plane). Each partial cone in 1b contains a disordered nitrobenzene molecule. The sheets stack staggered in an ABCABC fashion with an interlayer separation of ca. 0.94 nm. Disordered ethanol molecules are present

Coordination Polymers from [Cu2(Dicarboxylate)2]4

Figure 6. 2-D undulating sheet in 1b. (a) Top view showing the 2-D framework in space-filling mode and disordered guest nitrobenzene in line mode. (b) Side view showing the undulation of the sheets. Interlayer separation is 0.94 nm. Note that the coordinating pyridine rings (except for the nitrogen atom) as well as disordered methanol molecules have been removed for clarity.

Figure 7. (a) Top view of 2 showing the 2-D framework in space-filling mode and guest dichlorobenzene in line mode. Effective dimensions of the grids measure 1.2 × 1.0 nm2. (b) Side view of 2 showing the undulating sheets. The interlayer separation is 0.98 nm. Note that coordinating pyridine rings (except for the nitrogen atom) have been removed for clarity.

between the layers. The unit cell contains no residual solvent accessible area; however, the potential solvent area, upon removal of guest, is 12%.76 1b is a supramolecular isomer of 1a since the chemical formula of the coordination polymer network in the two is identical. In compound 2, {[Cu2(bdc)2(4-pic)2]4}n, the nSBU adopts the 1,2-alternate conformation and contains an o-dichlorobenzene molecule in the resulting cavity (Figure 7). The nSBUs self-assemble into 2-D corrugated square grids that propagate in the YZ-plane and stack along the X axis in an ABCDABCD fashion with an interlayer separation of 0.98 nm. The effective dimensions of the grids measure 1.2 × 1.0 nm2 (distance from Cu-Cu midpoint of opposite SBU units in the nSBU taking into account the van der Waals radii of copper). The unit cell contains no residual solvent accessible area; however, the potential solvent accessible area upon removal of guest is 25.9%.76 1a, 1b, and 2 therefore collectively exhibit all four atropisomers of the nSBU likely to be generated by the angularity that results from bdc serving as a bridging ligand. The curvature of the cone nSBU affects the trans carboxylate moieties in the basal position of the SBUs, which twist slightly from planarity (dihedral angle between planes of trans carboxylates θ ) -8.39°). Conversely, in the partial cone all trans carboxylates are planar (θ ) 0°), and in the 1,2-alternate form the SBU moieties undergo pronounced twisting (θ ) -17.7°) allowing neighboring benzene rings to be trans to one another.

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When derivatives of bdc were investigated, namely, 5-OEt-bdc and 5-OPr-bdc, two isostructural compounds were obtained: {[Cu2(5-OEt-bdc)2(py)2]4}n, 3, and {[Cu2(5-OPr-bdc)2(py)2]4}n, 4. Both structures contain cone and 1,3-alternate nSBUs that self-assemble into twodimensional undulating sheet structures related to 1a. As for 1a, the trans carboxylate moieties of the SBUs undergo twisting from planarity; however, it is more pronounced (θ ) 17.7 and 18.7° (av) for 3 and 4, respectively). 3 and 4 were isolated in the absence of template molecules and in the presence of various ones including nitrobenzene, benzene, o-dichlorobenzene, and naphthalene. 3 was also isolated from a variety of solvents including MeOH, EtOH, or DMSO and in the presence of different bases, including pyridine or 2,6lutidine. In 3 and 4, each cone contains disordered solvent molecules and the bottom of a cone from the adjacent sheet. The outer diameter of each cone is 0.94 nm. The 1,3-alternate nSBUs define an hourglassshaped cavity, but contrary to the 2-D network in 1a, no channels are present in 3 and 4. Rather, the alkyl chain substituents on the bdc moieties occupy the space that was present in 1a and preclude the existence channels. It should therefore be unsurprising that the solvent accessible volume is substantially lowered as compared to that of 1a. The solvent accessible area in 3 is 1% but would be increased only to 9.7% (as compared to 27.5% for 1a) upon removal of solvent molecules.76 For the dicarboxylates with wider angles between the carboxylate moieties, pdc and tdc, only the 1,2-alternate nSBU atropisomer was observed (Figure 4). Examination of the SBU moieties in the nSBUs of structures 5-7 reveals that the trans carboxylate moieties in the SBUs are planar (θ ) 0°). This could account for the formation of this nSBU exclusively. The larger angle at which the two carboxylates are predisposed allows neighboring dicarboxylate ligands to be trans to one another, while maintaining the planarity in the SBU moiety. Compounds 5, {[Cu2(pdc)2(py)2]4}n, and 6, {[Cu2(pdc)2(4-pic)2]4}n, result from the self-assembly of 1,2-alternate nSBUs into a corrugated 2-D sheet that is better described as an open framework square grid. The effective dimensions of the grids measure 1.3 × 1.1 nm2 (distance from Cu-Cu midpoint of opposite SBU units in the nSBU taking into account the van der Waals radii for copper). The grids stack parallel and slipped in one direction so that every fourth layer repeats (i.e., ABCABC packing) (Figure 8). Disordered guest molecules (methanol in 5 and H2O in 6) occupy the space between the layers. The interlayer separation between two adjacent grids is 0.65 nm. There are no solvent accessible areas in the unit cells of 5 and 6; however, the potential solvent accessible area, upon removal of guest, is 7.1 and 5.2%, respectively.76 Structure 7, {[Cu2(tdc)2(MeOH)2]4}n, can also be described as a corrugated grid. The grids propagate in the YZ plane and stack along the X axis in ABAB fashion so that every third layer repeats. The interlayer separation between two adjacent sheets is 0.78 nm. Each nSBU contains naphthalene molecule and has two methanol molecules that H-bond to the coordinated methanol molecules in the apical positions (O‚‚‚O 2.705(7) Å). The effective dimensions of the grids are the same

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Figure 8. 1,2-Alternate nSBUs from pdc and tdc selfassemble into 2-D sheets. (a) Side view of structure 5 illustrating the ABCABC packing with disordered MeOH molecules (in space-filling mode) present between the layers. The interlayer separation is 0.65 nm. (b) A 2-D sheet of 7 contains a naphthalene molecule and two independent methanol guest molecules in each cavity. Effective dimensions of the grid measure 1.3 × 1.1 nm2.

as for 5 and 6, 1.3 × 1.1 nm2. There is no residual solvent accessible area in the unit cell; however, the potential solvent area, upon removal of guests, is 45.8%.76 Conclusion We have demonstrated herein that square nSBUs that consist of dimetal tetracarboxylate moieties bridged by bdc can resemble a calix[4]arene in their shape, chemical nature, and conformational flexibility. Indeed, all four atropisomers that are known to exist for calixarenes were observed in the square nSBU components of the coordination polymers reported herein. The result of the atropisomerism in compounds 1-7 is the existence of a greater degree of structural diversity than would otherwise have occurred. In particular, the types of cavities present and the relative solvent or guest accessible volume change quite dramatically depending upon which atropisomer occurs. In addition, some of these 2-D structures, namely, ones based on atropisomers e and f (Figure 4) are better suited for the possibility of pillaring the 2-D structures described herein, a commonly used strategy for generation of 3-D porous materials.77-79 Future studies will be directed toward the preparation of discrete molecular versions of the nSBUs, which would serve as even more direct analogues of calix[4]arenas. We shall also focus upon the inherent modularity of the nSBU and seek to customize it so that, depending on which variable is changed, one might control size, shape, and chemical nature of the nSBU and the cavities and channels thereby formed. Acknowledgment. We gratefully acknowledge the financial support of the National Science Foundation (DMR-0101641). References (1) Lehn, J. M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, 1995. (2) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311-2327. (3) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 16291658. (4) Zaworotko, M. J. Chem. Commun. 2001, 1-9. (5) Evans, O. R.; Lin, W. B. Inorg. Chem. 2000, 39, 2189-2198.

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