Coordination Polymers of Tetra (4-carboxyphenyl) porphyrins

Apr 10, 2004 - School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv ... network an open three-dimensional architecture, wherein each zinc ...
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CRYSTAL GROWTH & DESIGN

Coordination Polymers of Tetra(4-carboxyphenyl)porphyrins Sustained by Tetrahedral Zinc Ion Linkers

2004 VOL. 4, NO. 3 633-638

Michaela Shmilovits, Mikki Vinodu, and Israel Goldberg* School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv University, 69978 Ramat Aviv, Tel Aviv, Israel Received October 30, 2003

ABSTRACT: Reactions between the free base tetra(4-carboxyphenyl)porphyrin and the corresponding platinum or palladium metalloporphyrin derivatives with zinc acetate dihydrate under solvothermal conditions in a basic environment (in the presence of pyridine and ammonium hydroxide) yielded extended supramolecular networks. These polymeric arrays consist of fully deprotonated porphyrin carboxylate units interconnected to each other by Zn(H2O)22+ auxiliaries. The zinc ion linkers adapt a tetrahedral coordination environment, imparting to the polymeric network an open three-dimensional architecture, wherein each zinc binds to two adjacent porphyrin units and two water ligands, while every porphyrin entity is linked to four different metal centers. In a reaction involving the free base macrocycle, the zinc ions were inserted into the porphyrin core as well, forming a five-coordinate entity with pyridine as an axial ligand. The solid state syntheses yielded either one-dimensional ladder type coordination polymers that pair in an interlocking manner or three-dimensional diamondoid arrangements with interpenetrating polymeric networks. Both types of frameworks are further interlinked to each other by weak hydrogen bonds from the zincbound water ligands of one array to the carboxylate functions of another. The previously reported porphyrin-based polymer tessellated by Zn2+ linkers that coordinate at a given binding site to four (rather than two) porphyrin units is also discussed. Introduction Porphyrins and metalloporphyrins are among the most widely studied chemical systems in solution and in the solid state due to their remarkable structural robustness, high relevance to catalysis, and to electron and energy transfer processes. The coordination properties of the metal entity in the center of the porphyrin ring, along with tailored functionality of the molecular periphery by suitable substituents, provide diverse programming elements for the design of new network solids and porous materials, which may bear structural as well as functional resemblance to the inorganic zeolites. We have introduced recently the tetra(4-carboxyphenyl)porphyrin (TCPP or M-TCPP in its coremetalated form) moiety as a uniquely versatile building block to this end.1 The most attractive algorithms for the self-assembly of the porphyrin building blocks into extended two-dimensional (2D) and three-dimensional (3D) architectures involve their tessellation to each other by a concerted mechanism of hydrogen bonding and intercoordination through external metal ion templates.2 The latter can readily interact with, and bridge firmly between, the deprotonated carboxylic functions (deprotonation of the TCPP molecules occurs during the reaction to account for charge balance) of adjacent building blocks. This often yields extended coordination polymers of “porous” crystalline architecture. Different metal ion auxiliaries have been successfully applied to this end. In coordination polymers based on the carboxyphenyl porphyrins, these include the Na+,3-6 K+,4 Ca2+,7 Co2+,8 Cu2+,9 and Zn2+ 3 ions or ion clusters, as well as complexes such as [Cu(NH3)6]2+.9 The dimen* To whom correspondence should be addressed. Tel: +972-36409965. Fax: +972-3-6409293. E-mail: [email protected].

Scheme 1

sionality of the polymeric networks that form is affected to a large extent by the composition (whether single ions, metal ion clusters, or metal complexes) and preferred coordination geometries of the bridging auxiliaries. Insertion of additional metal ions into the porphyrin core may serve to enhance the structural rigidity of the TCPP framework but is not essential to the network formulation. Similar metal ion auxiliaries were also found useful in the formulation of coordination polymers with other porphyrin entities in solution as well as in the crystalline state.2,10-12 As part of our continuing effort to formulate new materials of this type,2 we focus in this paper on coordination polymers of the TCPP, applying divalent zinc ions as bridging sites for intercoordination between the porphyrin building blocks. A preliminary account of this type of network material (involving Zn-TCPP and Zn2+ bridges and referred to as compound 1) has already been published.3 Here, we expand on this subject by reporting new compounds 2, 3, and 4, which have been obtained by reacting, respectively, the free base H2TCPP, Pt-TCPP, and Pd-TCPP moieties with Zn2+ linkers (Scheme 1) and by presenting a comparative discussion.

10.1021/cg0342009 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/10/2004

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Table 1. Crystal Data and Experimental Parameters of the Structural Analysis compounda

2

3

4

formula weight crystal system space group (no.) T (°C) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z µ(Mo KR) (mm-1) Fcalcd (g cm-3) 2θmax (deg) reflections collected unique reflections reflections with I > 2σ R (I > 2σ) R (all data) Rw (I > 2σ) Rw (all data)

1286.67 triclinic P1 h -163 13.2820(8) 14.0630(7) 16.9090(10) 75.991(4) 89.705(3) 81.997(3) 3033.2(3) 2 1.24 1.409 50.0 23 492 10 518 6591 0.075b 0.121b 0.183b 0.203b

1222.16 monoclinic P21/c -163 18.2660(3) 10.2780(3) 12.5540(6) 90.0 103.976(1) 90.0 2287.1(1) 2 4.16 1.775 55.7 20 993 5398 4071 0.053 0.082 0.097 0.107

1133.47 monoclinic P21/c -163 18.2790(4) 10.2850(4) 12.5310(8) 90.0 103.938(1) 90.0 2286.5(2) 2 1.50 1.646 55.7 17 881 5424 3298 0.064 0.128 0.128 0.149

a Chemical formulas: 2, C H N O Zn‚2Zn(H O) ‚1.5(C H N)‚ 53 29 5 8 2 2 5 5 2(H2O)2;3,C48H24N4O8Pt‚2Zn(H2O)2‚0.5(C5H5N);and4,C48H24N4O8Pd‚ b 2Zn(H2O)2‚0.5(C5H5N). On the basis of diffraction data after the squeeze/bypass procedure (see text).16

Experimental Section Synthesis. The free base, Pt-, and Pd-TCPPs were procured commercially from Porphyrin Systems GbR. They were used without further purification. The polymeric materials were synthesized under solvothermal conditions, placing a mixture of the reactants and solvents in a small sealed reactor, continuously heating the reaction mixture at about 125-150 °C for 3-6 days, and then gradually cooling the resulting products to ambient room temperature over a period of several hours. The reacting components included about 5 mg (0.0063/ 0.0056/0.0051 mmol) of the porphyrin (either H2-TCPP, PtTCPP, or Pd-TCPP), ∼6 mg (0.0027 mmol) of Zn(OAc)2‚2H2O (dissolved in methanol)sthe zinc ion source intended to coordinate to and between the carboxyphenyl groupss0.5 mL of NH4OH as the base reagent to assist in deprotonation of the carboxylic functions, and 0.8-1.0 mL of pyridine as porphyrin solubilizing agents. The yields of the crystalline products 2-4 (Table 1) were generally rather low, on a less than 1 mg scale in a single reaction run, which was inadequate for reliable elemental or thermogravimetric characterizations. Their composition, as {(M-TCPP)4-‚2[Zn(H2O)22+]}∞, was precisely determined by X-ray diffraction analysis. The identity of the formed crystal lattices in a given reaction was confirmed in each case by repeated measurements of the unit cell dimensions from different single crystallites. The above preparative conditions are different than those used to obtain compound 1. The latter involved dissolving Zn-TCPP in a hot mixture of methanol and ethylene glycol in the presence of zinc acetate dihydrate and yielded a differently structured coordination polymer formulated as {(Zn-TCPP)2-‚Zn2+}∞ (see below).3 X-ray Crystallography. The diffraction measurements were carried out on a Nonius KappaCCD diffractometer, using graphite monochromated Mo KR radiation (λ ) 0.7107 Å). The crystalline samples of the analyzed compounds were covered with a thin layer of light oil and freeze-cooled to ca. 110 K in order to minimize solvent escape, structural disorder, and thermal motion effects and increase the precision of the results. The crystal and experimental data for all of the compounds are summarized in Table 1. These structures were solved by direct and Patterson methods (SIR-97, DIRDIF-96)13,14 and refined by full-matrix least-squares on F2 (SHELXL-97).15 Intensity data of all compounds were routinely corrected for absorption effects. All nonhydrogen atoms were refined aniso-

tropically. The hydrogens of the porphyrin were located in idealized positions and were refined using a riding model with fixed thermal parameters [Uij ) 1.2Uij (eq.) for the atom to which they are bonded]. Those bound to the water O-atoms were located either in difference Fourier maps or in calculated positions. Compounds 2-4 yielded tiny crystals in the form of very thin plates, requiring extended X-ray exposure times. Correspondingly, the fraction of weak reflections below the intensity threshold in the lighter atom compounds 2 and 4 is rather high. Compounds 3 and 4 reveal isomorphous structures with the porphyrin molecules located on centers of inversion. They contain pyridine solvent molecules (with partial occupancy) located on inversion and orientationally disordered about it. Compound 2 contains in the asymmetric unit 1.5 molecules of pyridine and two molecules of water as crystallization solvent, which are mostly disordered. The atomic positions of the disordered solvent could be approximately located in electron density maps but not refined with satisfactory precision and acceptable geometries. While conventional refinements of 3 and 4 converged reasonably well, that of 2 converged poorly at a relatively high R value of R1 ) 0.12. Correspondingly, the contribution of the disordered solvent in 2 (one molecule of water and 1.5 molecules of pyridine) was subtracted from the corresponding diffraction patterns by the squeeze/bypass procedure16 in order to improve the structural characterization of the ordered fragments. This method is widely used in crystallographic analyses of compounds containing substantial amounts of disordered solvent. The calculated solvent accessible volumes of about 837 Å3/unit cell along with the assessed residual electron densities in these voids16 match reasonably well the assumed solvent stoichiometry. Despite the solvent disorder, the refinement calculations based on the reduced data provided a reliable model of the molecular structure and the porphyrin supramolecular organization. The Ortep molecular plots of 2 and 3 are shown in Figure 1 (molecular structure of 4 is nearly identical to that of 3). Incidentally, similar synthetic experiments with Pt-TCPP and Pd-TCPP, in which the pyridine solvent was replaced by ethylene glycol, afforded crystal structures 5 and 6 that are, respectively, isomorphous (although of lower X-ray quality) to 3 and 4. Structural analyses confirmed that they contain identical polymeric architectures. In 5 and 6, however, molecules of the ethylene glycol are trapped in the lattice, being located on and severely disordered about, the centers of inversion. The corresponding crystal data at ca. 110 K are as follows: Compound 5. C48H24N4O8Pt‚2Zn(H2O)2‚(ethylene glycol)x, monoclinic, space group P21/c, a ) 12.3800(8), b ) 10.1620(5), c ) 18.3560(12) Å, β ) 103.269(3)°, V ) 2247.8(2) Å3. Compound 6. C48H24N4O8Pd‚2Zn(H2O)2‚(ethylene glycol)x, monoclinic, space group P21/c, a ) 12.3390(9), b ) 10.1570(10), c ) 18.3430(15) Å, β ) 103.162(5)°, V ) 2238.0(3) Å3.

Results and Discussion The assembly mechanism of supramolecular motifs of meso-carboxyphenyl porphyrins by metal ion templates was demonstrated by us and others to be a very effective method for the formulation of porphyrin-based coordination polymers.2-9 The stereochemistry of divalent closed shell d10 zinc in inorganic and organometallic compounds is characterized by high versatility, revealing coordination numbers from 2 to 8 and diverse geometries.17 When inserted into the porphyrin core, the zinc ions commonly have coordination numbers 4 (squareplanar), 5 (pyramidal), and 6 (octahedral), with 5 being especially common.18 In its function as an external bridging auxiliary between the large square-shaped TCPP building blocks, we find that the zinc stereochemistry is characterized by four coordination and tetrahedral geometry.

Coordination Polymers of Tetra(4-carboxyphenyl)porphyrins

Figure 1. Ortep representations of the molecular structures of (a) 2 and (b) 3. The atom ellipsoids represent thermal displacement parameters at the 50% probability level at ca. 110 K. Molecules of 3 are located on crystallographic inversion.

Some of these features are nicely demonstrated in 1 (CSD refcode WAWGOQ),18 a preliminary account of which has introduced the carboxylate-metal ion coordination synthon into the solid state synthesis of supramolecular porphyrin assemblies.3 It shows that the zinc ion residing within the porphyrin core is fivecoordinate, deviating outward toward a molecule of ethylene glycol as an axial ligand. On the other hand, the exocyclic zinc cation maintains an approximate tetrahedral geometry of coordination around it to four different TCPP units (Figure 2a). Details of the molecular structure are included in Table 2. Compound 1 was prepared in an essentially neutral environment. In these conditions, the porphyrin molecule was only doubly deprotonated affording in the crystal structure a continuous diamondoid array, characterized by a 1:1 porphyrin-zinc linker stoichiometry (where every porphyrin is connected to four zinc linkers and every linker to four porphyrin units). The basic motif in this array can be best described as a flat linear chain of the porphyrin macrocycles, which are connected to each other through the zinc ion auxiliaries by the cis-related carboxylic/ carboxylate arms of neighboring species along the chains. The interporphyrin voids are characterized by a van der Waals width of about 0.5 nm. Each such chain

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Figure 2. (a) Interporphyrin coordination scheme in compound 1, which yields tetrahedrally interconnected porphyrin networks. At each connecting node, the zinc ion linker associates tetrahedrally to four neighboring porphyrin units.3 Color code in the first network: Zn, pink; O, red; N, blue; and C, green. Concatenation of the network arrays is also shown, by depicting molecules of a second interpenetrating array in black/gray wire frame. The ethylene glycole axial ligands to the porphyrin cores and the H-atoms are not shown. (b) Spacefilling illustration of the porous crystalline architecture of 1, viewed down the hexagonal channel voids. In panel b, C is gray and H is white. Table 2. Coordination Bonding Distance Ranges (Å) in Solids 1-4 compounds

1a

2

Zn(TCPP)‚‚‚N(pyrrole) Zn(TCPP)‚‚‚O/N (axial ligand) Zn(bridge)‚‚‚O(porphyrin) Zn(bridge)‚‚‚O(water)

2.044-2.069(4) 2.108(4) 1.948-1.967(4) b

2.054-2.075(6) 2.151(5) 1.952-1.972(5) 2.003-2.042(6)

compounds

3

4

Pt/Pd(TCPP)‚‚‚N(pyrrole) Zn(bridge)‚‚‚O(porphyrin) Zn(bridge)‚‚‚O(water)

2.013-2.020(5) 1.945-1.975(4) 2.020-2.040(5)

2.010-2.022(5) 1.940-1.967(4) 2.030-2.037(4)

a See ref 3. b The zinc ion bridge connects tetrahedrally to four porphyrin units.

connects on both sides at regular intervals to adjacent arrays oriented in a roughly perpendicular manner to the original one. This creates a 3D grid. The polymeric

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Figure 3. Interporphyrin chain coordination scheme in compound 2. Note that two zinc ion linkers coordinate to the cis-related arms of two neighboring porphyrins along the chain. Color code: Zn, pink; O, red; N, blue; and C, green.

diamondoid frameworks interweave into one another during the crystallization process, forming a robust architecture (Figure 2a). It resembles a “molecular sieve” system, as it is perforated by wide parallel channel voids that propagate along the c-axis through the crystal. These channels have a hexagonal crosssection and an average van der Waals diameter of 0.75 nm (Figure 2b). In 1, these channels are filled with a large number of ethylene glycol solvent molecules, which are not coordinated to the polymeric lattice and can be readily exchanged by other guest components. The solvothermal reaction of the free base porphyrin with zinc acetate was associated with insertion of the zinc ions into the porphyrin core and also as exocyclic bridging auxiliaries between the porphyrin units. In the basic environment, a complete deprotonation of the tetra-acid occurred, yielding a 1:2 ratio between the porphyrin units and the zinc ion linkers. As in the previous case, the inner zinc is five-coordinate with a square-pyramidal geometry, linking to a pyridine molecule as an axial ligand (Table 2). The external zinc ions are four-coordinate with a tetrahedral geometry, each binding to carboxylate groups of two porphyrin units and two molecules of water. This leads to the formation of chain coordination polymers as depicted in Figure 3. In the crystals, adjacent chain arrays related by inversion lock into one another by the axial ligands of one chain inserting into the interporphyrin cavities of another chain in a zipper type manner (Figure 4a). The association of the bridging zinc ions with only two (rather than four in the previous example) porphyrin units allows for some stretching of the intermolecular space to a width of about 0.6 nm and a convenient accommodation of the penetrating pyridyl rings. Thus paired polymeric chains link to adjacent arrays only through weak hydrogen bonds, which involve the zincbound water molecules and the exposed O-atoms of the carboxylate functions (Table 3). As opposed to zinc, the platinum and palladium ions within the porphyrin core reveal high propensity for a square-planar coordination environment and have little affinity for axial ligands. When Pt-TCPP and Pd-TCPP are reacted with zinc acetate dihydrate under similar conditions, they form isomorphous structures 3 and 4 in which the metalloporphyrin macrocycles lie on centers of crystallographic inversion and are almost perfectly planar. Compounds 3 and 4 thus exhibit nearly identical molecular features and intermolecular interaction schemes. All four carboxylic functions are deprotonated as in 2. However, in this case, each carboxylate group of a given porphyrin is coordinated to a different zinc ion, while every metal ion is linked through CO2-‚‚‚Zn coordination to only two different TCPP molecules. Molecules of water again occupy the other two coordination sites of the tetrahedral zinc auxiliary.

Figure 4. Crystal structure of 2. (a) View of the inversionrelated chains locking to one another through the pyridyl axial ligands. Note the polar surface on both sides of the double chains. (b) Space-filling illustration of the structure, showing an intercalate type arrangement. The pyridine molecules, which accommodate in a disordered manner the space between the polymeric arrays, are not shown. In panel b, C is gray and H is white.

Direct coordination of the carboxylate groups to four different zinc centers results in a formation of an open 3D polymeric array spanning through the crystal structure (Figure 5a). The cyclic environment that surrounds a given interporphyrin void space now consists of four metalloporphyrin units and four zinc ion linkers, as opposed to two porphyrins and two linkers in 2. Within this array, adjacent zones of the porphyrin building blocks are related by glide plane/screw axis symmetry. Optimization of the coordination scheme dictates wide spacing between neighboring porphyrins (characterized by perpendicular distance between them of about 8.2 Å) in a given zone. Concatenation of two such networks into one another occurs in the crystals of 3 and 4, to minimize void space and preserve crystallinity. The interpenetrating networks are further stabilized by

Coordination Polymers of Tetra(4-carboxyphenyl)porphyrins Table 3. Hydrogen Bonding Parametersa D-Hwater

Acarboxylate/water

D-H H-A (Å) (Å)

D‚‚‚A (Å)

D-H‚‚‚A (°)

OH(53a) OH(53b) OH(54a) OH(54b) OH(55b) OH(57a) OH(57b)

compound 2 O8 (-x, 3 - y, -z) 1.00 O3 (1 - x, 2 - y, 1 - z) 1.02 O2 (x, y, z) 1.07 O3 (1 - x, 2 - y, 1 - z) 0.97 b O57 (x - 1, 1 + y, z - 1) 0.96 O5 (1 - x, 1 - y, -z) 0.95 O2 (x, y - 1, z) 1.00

2.870(8) 2.921(7) 3.088(8) 3.054(9) 2.931(8) 2.786(7) 2.859(7)

180 180 128 150 170 163 179

OH(5a) OH(5b) OH(6a) OH(6b)

compound 3 O4 (x, y - 1, z - 1) 1.05 1.97 2.953(7) O4 (1 - x, 1 - y, 1 - z) 0.97 2.19 3.113(6) O2 (x, 0.5 - y, z - 0.5) 1.03 2.03 3.032(6) O2 (1 - x, y + 0.5, 0.5 - z) 0.98 1.98 2.922(7)

154 159 163 160

OH(5a) OH(5b) OH(6a) OH(6b)

compound 4 O4 (x, y - 1, z - 1) 1.05 1.98 2.963(6) O4 (1 - x, 1 - y, 1 - z) 0.97 2.19 3.117(6) O2 (x, 0.5 - y, z - 0.5) 1.03 2.03 3.027(6) O2 (1 - x, y + 0.5, 0.5 - z) 0.98 1.99 2.930(6)

154 160 163 159

1.87 1.90 2.31 2.18 1.98 1.87 1.85

a For atomic labels of the carboxylate groups and of the coordinated water species, see Figure 1. b O57 represents water molecules included in the lattice of 2 but not coordinated directly to the zinc ion bridges.

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observations in closely related compounds.4,9,18 The porphyrin macrocycle is slightly saddled in 2 but essentially planar in 1 (excluding the zinc center, which deviates toward the axial ligand), 3, and 4. Bond lengths from the zinc ion in the porphyrin core to the pyrrole nitrogens are within 2.044(4)-2.069(4) Å in 1 and 2.054(6)-2.075(6) Å in 2. Those to the axial ligands are 2.108(4) and 2.151(5) Å, correspondingly. The respective bond lengths within the porphyrin cores of 3 and 4 are 2.013(5)-2.020(5) Å for Pt-N and 2.010(5)-2.022(4) Å for Pd-N. The corresponding CO-‚‚‚Zn coordination distances in 1-4 are all within 1.940(4)-1.975(5) Å, while the Zn‚‚‚OH2 distances in 2-4 are consistently slightly longer, within 2.003(6)-2.042(6) Å. In all of the structures, the exocyclic zinc ion auxiliaries exhibit preferred linkage to only one of the carboxylic/carboxylate Oatoms at each site, which allows for only partial delocalization within these groups.9 In the fully deprotonated porphyrins in 2-4, the C-O distances within the “hydroxylic” bonds linking to the zinc bridges are within 1.272-1.297(8) Å; those within the “carbonylic” bonds range from 1.237(8) to 1.254(8) Å. The hydrogen bond cross-links between the polymeric arrays, from the zincbound water ligands to the carboxylate groups, are rather weak; the OH(water)‚‚‚O(carboxylate) distances range from 2.8 to 3.1 Å (Table 3). Conclusion

Figure 5. (a) View of the open polymeric networks sustained by Zn(H2O)22+ linkers in 3 and 4. Adjacent vertical rows shown in this figure are related to each other by the glide/screw symmetry, being oriented in roughly perpendicular directions. The pyridine solvent trapped in the crystal lattice is not shown. Color code: Pt/Pd, pink; O, red; N, blue; and C, green. (b) Selfinterpenetration of inversion-related networks; molecules of different net zones are depicted by green and gray carbon wire frames, respectively.

roughly linear hydrogen bonds from the water ligands bound to zinc of one array to the porphyrin “carbonylic”O sites of the second array (Table 3). The remaining small localized voids are partly accommodated in the crystal by molecules of the pyridine solvent. Features of the molecular structure of compounds 1-4 are summarized in Table 2 and conform to earlier

Our efforts to crystal engineer porphyrin-based network materials and porous solids by utilizing diverse molecular recognition algorithms (interaction synthons) have led us to investigate typical aggregation modes the M-TCPP building blocks organized into supramolecular arrays by zinc ion auxiliaries. The assembly of the square-shaped porphyrin tetra-acid (that deprotonates readily) into extended frameworks through exocyclic metal ions can be accomplished without the need to incorporate additional counterions in the lattice. The uniqueness of the outer divalent zinc bridges (among other possible metal ion auxiliaries)4-9 lies in their propensity to form coordination patterns of tetrahedral symmetry.3 Exploiting these properties of the component species, the present work revealed three different types of polymeric multiporphyrin assemblies, effected by coordination of the four carboxylic/carboxylate functions to the exocyclic zinc linkers. The constructed ordered supramolecular patterns include one-dimensional double chain arrays and entangled diamondoid networks.19,20 The corresponding crystal structures of these polymers represent molecular sieve (1), intercalate (2), and clathrate (3, 4) organizations, which add to the fast-expanding database of supramolecular porphyrinbased materials of interesting properties, formulated with the aid of thermodynamically labile noncovalent interactions. Acknowledgment. This research was supported in part by The Israel Science Foundation (Grant No. 68/ 01) and by the U.S.-Israel Binational Science Foundation (BSF), Jerusalem, Israel (Grant No. 1999082). Supporting Information Available: Crystallographic data for compounds 2-4, in the crystallographic information file (CIF) format. This information is available free of charge via the Internet at http://pubs.acs.org.

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References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

Dastidar, P.; Stein, Z.; Goldberg, I.; Strouse, C. E. Supramol. Chem. 1996, 7, 257-270. (a) Goldberg, I. Chem. Eur. J. 2000, 6, 3863-3870. (b) Goldberg, I. Cryst. Eng. Comm. 2002, 4, 109-116. Diskin-Posner, Y.; Dahal, S.; Goldberg, I. Chem. Commun. 2000, 585-586. Diskin-Posner, Y.; Dahal, S.; Goldberg, I. Angew. Chem. Int. Ed. 2000, 39, 1288-1292. Diskin-Posner, Y.; Goldberg, I. New J. Chem. 2001, 25, 899-904. Diskin-Posner, Y.; Patra, G. K.; Goldberg, I. Eur. J. Inorg. Chem. 2001, 2515-2523. Kosal, M. E.; Chou, J.-H.; Suslick, K. S. J. Porphyrins Phthalocyanines 2002, 6, 377-381. Kosal, M. E.; Chou, J.-H.; Wilson, S. R.; Suslick, K. S. Nat. Mater. 2002, 1, 118-121. Shmilovits, M.; Vinodu, M.; Goldberg, I. Cryst. Growth Des. 2003, 3, 855-863. Hargman, P. J.; Hargman, D.; Zubieta, J. Angew. Chem. Int. Ed. 1999, 38, 2639-2684. (a) Abrahams, B. F.; Hoskins, B. F.; Michail, D. M.; Robson, R. Nature (London) 1994, 369, 727-729. (b) Abrahams, B. F.; Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1991, 113, 3606-3609.

Shmilovits et al. (12) Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas, J. D. Angew. Chem. Int. Ed. 1998, 37, 2344-2347. (13) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.; Polidori, G. SIR-97. J. Appl. Crystallogr. 1994, 27, 435-436. (14) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The DIRDIF-96 Program System; Technical Report of the Crystallography Laboratory; University of Nijmegen: The Netherlands, 1996. (15) Sheldrick, G. M. SHELXL-97. Program for the Refinement of Crystal Structures from Diffraction Data; University of Go¨ttingen: Germany, 1997. (16) (a) Van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, 194-201. (b) Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, C34. (17) See, for example, Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley & Sons: New York, 1980. (18) See in Cambridge Structural Database. Allen, F. H. Acta Crystallogr. Sect. B 2002, 58, 380-388. (19) Batten, S. R.; Robson, R. Angew. Chem. Int. Ed. 1998, 37, 1460-1494. (20) Ermer, O. J. Am. Chem. Soc. 1988, 110, 3747-3754.

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