One-Dimensional Supramolecular Isomerism of Copper(I) and Silver

A mixture of AgNO3 (0.170 g, 1.0 mmol), Him (0.068 g, 1.0 mmol), aqueous ammonia (25%, 5.0 mL), and methanol (2 mL) was stirred for 15 min in air and ...
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CRYSTAL GROWTH & DESIGN

One-Dimensional Supramolecular Isomerism of Copper(I) and Silver(I) Imidazolates Based on the Ligand Orientations Xiao-Chun

Huang,†

Jie-Peng

Zhang,‡

and Xiao-Ming

Chen*,‡

State Key Laboratory of Optoelectronic Materials and Technologies/School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, People’s Rebublic of China, and Department of Chemistry, Shantou UniVersity, Shantou, Guangdong 515063, People’s Rebublic of China

2006 VOL. 6, NO. 5 1194-1198

ReceiVed January 25, 2006; ReVised Manuscript ReceiVed March 12, 2006

ABSTRACT: Supramolecular isomers of binary copper(I) and silver(I) imidazolates have been hydro(solvo)thermally prepared via variations of the reaction conditions such as solvent and additive. Among them, two new supramolecular isomers of Ag(im) [1c, P21/c, a ) 11.460(1) Å, b ) 16.882(2) Å, c ) 9.303(1) Å, β ) 106.61(1)°; 1d, Pbca, a ) 10.175(2) Å, b ) 6.8415(9) Å, c ) 23.881(2) Å; Him ) imidazole] exhibiting two-dimensional (6,3) and (4,4) networks, respectively, through short interchain AgI‚‚ ‚AgI contacts [3.1595(5) Å for 1c and 3.4445(5) Å for 1d] have been structurally established to be different from two known superstructures, whereas two supramolecular isomers of Cu(im) [2a′, C2/c, a ) 16.1207(17) Å, b ) 16.1229(17) Å, c ) 13.1021(14) Å, β ) 99.850(2)°; 2b, P21/c, a ) 5.410(2) Å, b ) 6.943(3) Å, c ) 33.058(12) Å, β ) 97.647(8)°] exhibit a two-dimensional 8210 network through short interchain CuI‚‚‚CuI contacts and zigzag chains, respectively. Introduction Supramolecular isomerism is currently an interesting subject in crystal engineering.1-5 However, the rational design and controlled synthesis of supramolecular isomers are still challenging topics. For example, although metal-organic architectures constructed by flexible, multifunctional ligands often exhibit structural diversity, many of them are not true supramolecular isomers for their different chemical compositions given rise by the coexistence of different guest components.2,3 Moreover, the maximum number of true supramolecular isomers with structural characterization found for metal-organic polymers is, to our knowledge, up to four, which are documented for copper(II) imidazolate Cu(im)24a (Him ) imidazole) and Cu(2-pytz)4b (2-Hpytz ) 3,5-di-2-pyridyl-1,2,4-triazolate). The four isomers of Cu(im)2 exhibit two-dimensional (2D) and 3D superstructures attributable to the variable coordination environments (flattened tetrahedral and distorted square-planar) of CuII ions, while those of Cu(2-pytz) exhibit 0D and 1D superstructures mainly due to the bidentate chelating sites of the ligand along with the tetrahedral coordination geometry of CuI ions. In contrast, only sporadic true supramolecular isomers of 1D metal-organic compound have been reported5 and only a system of three 1D isomers (along with a 0D isomer) has been known very recently.4b In our ongoing investigations on construction of new coordination polymers and in-situ metal/ligand reactions, we have recently paid our attention to metal azolates with the aim of rational design and controlled synthesis of the final product with desired supramolecular networks and definite chemical composition.3,4b,6,7 Imidazolates, as a kind of the simplest µ-bridging bidentate ligands, could construct the simplest system with supramolecular isomerism of two-coordinate univalent metal ions, typically CuI and AgI ions, which could exhibit 0D rings and 1D zigzag and helical chains, as demonstrated in our recent reports.3 For example, through a template synthetic strategy, * Corresponding author. Fax: Int. code +86 20 8411-2245. E-mail: [email protected]. † Shantou University. ‡ Sun Yat-Sen University.

polygons of [Cun(mim)n]‚(guest) (n ) 8, 10; Hmim ) 2-methylimidazole) were obtained,3 while through a solvent-polarityinduced strategy, both triple helical chain and simple zigzag chain structures of Cu(eim) (Heim ) 2-ethylimidazole) were produced.5 However, for the prototype or unsubstituted imidazolates, only a few supramolecular structures/isomers have been documented,8,9 wherein the first silver(I) imidazolate Ag(im) 1a was structurally characterized by powder X-ray diffraction (PXRD),8a while Ag(im) 1b,8b an isomer of 1a, and Cu(im) 2a9 were characterized by single-crystal structural analysis recently. All of these compounds exhibit zigzag chainlike structures. By considering that possible 1D isomers should have differences in the crystal packing patterns and both copper(I) and silver(I) atoms may have interchain metal-metal interactions, we anticipated that more 1D isomers could be isolated through variation of the reaction conditions. Herein we report four other supramolecular isomers of these silver(I) and copper(I) imidazolates, namely Ag(im) 1c and 1d and Cu(im) 2a′ and 2b. Experimental Section Materials and Physical Measurements. Commercially available reagents are used as received without further purification. Infrared spectra were obtained from KBr pellets on a Nicolet 5DX Infrared spectrometer in the 400-4000 cm-1 region. Elemental analyses (C, H, N) were performed on an Elementar Vario EL elemental analyzer. Steady-state photoluminescence spectra were measured on a SHIMADZU RF-5301PC spectrofluorophotometer. In all cases single crystalline samples were used for the photoluminescence measurements. Synthesis of Ag(im) 1c. A mixture of AgNO3 (0.170 g, 1.0 mmol), Him (0.068 g, 1.0 mmol), aqueous ammonia (25%, 5.0 mL), and 2,2′bipyridine (0.136 g, 1.0 mmol) was stirred for 15 min in air and then transferred and sealed in a 15-mL Teflon-lined reactor and heated at 120 °C for 72 h. Upon cooling to room temperature at a rate of 5 °C h-1, the resulting colorless needle crystals of 1c were mechanically isolated (yield ca. 20%, based on Ag). FT-IR (KBr, cm-1): 3130m, 3108m, 2586w, 2509w, 1651m, 1615m, 1488s, 1463s, 1316s, 1239s, 1170w, 1087vs, 951s, 827s, 775s, 664s. Anal. Calcd (%) (C3H3AgN2): C, 20.60; H, 1.73; N, 16.01. Found: C, 20.53; H, 1.81; N, 15.92. Synthesis of Ag(im) 1d. A mixture of AgNO3 (0.170 g, 1.0 mmol), Him (0.068 g, 1.0 mmol), aqueous ammonia (25%, 5.0 mL), and methanol (2 mL) was stirred for 15 min in air and then transferred and sealed in a 15-mL Teflon-lined reactor and heated at 140 °C for

10.1021/cg060048+ CCC: $33.50 © 2006 American Chemical Society Published on Web 04/04/2006

Supramolecular Isomerism of Imidazolates

Crystal Growth & Design, Vol. 6, No. 5, 2006 1195

Table 1. Crystallographic Data and Structure Refinement Details for Four Ag(im) and Cu(im) Polymorphs (esd’s in parentheses)

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dc (Mg/m3) µ (Mo KR, mm-1) F(000) temperature (K) reflctns collected reflctns unique goodness-of-fit R1 [I g 2σ(I)] wR2 (all data)

1c

1d

2a′

2b

C3H3AgN2 174.94 monoclinic P21/c 11.460(1) 16.882(2) 9.303(1) 106.61(1) 1724.9(4) 16 2.695 4.485 1312 294(2) 13507 4117 1.044 0.0252 0.0596

C3H3AgN2 174.94 orthorhombic Pbca 10.175(2) 6.8415(9) 23.881(2) 90 1662.3(4) 16 2.796 4.654 1312 294(2) 12100 2353 1.064 0.0272 0.0670

C3H3CuN2 130.61 monoclinic C2/c 16.1207(17) 16.1229(17) 13.1021(14) 99.850(2) 3355.2(6) 32 2.069 5.012 2048 123(2) 10099 3913 1.002 0.0554 0.1463

C3H3CuN2 130.61 monoclinic P21/c 5.410(2) 6.943(3) 33.058(12) 97.647(8) 1230.7(8) 12 2.115 5.125 768 294(2) 6613 2656 1.082 0.0567 0.1282

48 h. Upon cooling to room temperature at a rate of 5 °C h-1, the resulting colorless claviform crystals of 1d were mechanically isolated (yield ca.10%, based on Ag). FT-IR (KBr, cm-1): 3129m, 3111m, 2578w, 2500w, 1650m, 1621m, 1478s, 1462s, 1307s, 1237s, 1168w, 1087vs, 945s, 828s, 775s, 665s. Anal. Calcd (%) (C3H3AgN2): C, 20.60; H, 1.73; N, 16.01. Found: C, 20.65; H, 1.85; N, 15.97. Synthesis of Cu(im) 2a′. A mixture of Cu2(OH)2CO3 (0.110 g, 0.5 mmol), Him (0.068 g, 1.0 mmol), aqueous ammonia (25%, 5.0 mL), and methanol (2 mL) was stirred for 15 min in air and then transferred and sealed in a 15-mL Teflon-lined reactor and heated at 160 °C for 72 h. Upon cooling to room temperature at a rate of 5 °C h-1, the resulting orange block crystals of 2a′ were mechanically isolated (yield ca.15%, based on Cu). FT-IR (KBr, cm-1): 3115m, 2578w, 2509w, 1575m, 1472s, 1306s, 1234s, 1181w, 1089vs, 962m, 851s, 754s, 666s. Anal. Calcd (%) (C3H3CuN2): C, 27.59; H, 2.32; N, 21.45. Found: C, 27.48; H, 2.78; N, 21.42. Synthesis of Cu(im) 2b. A mixture of Cu2(OH)2CO3 (0.110 g, 0.5 mmol), Him (0.068 g, 1.0 mmol), aqueous ammonia (25%, 5.0 mL), and toluene (2 mL) was stirred for 15 min in air and then transferred and sealed in a 15-mL Teflon-lined reactor and heated at 160 °C for 48 h. Upon cooling to room temperature at a rate of 5 °C h-1, the resulting yellow needle crystals of 2b were mechanically isolated (yield ca. 40%, based on Cu). FT-IR (KBr, cm-1): 3123m, 2581w, 2506w, 1573m, 1478s, 1306s, 1236s, 1176w, 1085vs, 968m, 849s, 756s, 663s. Anal. Calcd (%) (C3H3CuN2): C, 27.59; H, 2.32; N, 21.45. Found: C, 27.53; H, 2.46; N, 21.39. X-ray Crystallography. Data collection of 1c, 1d, 2a′, 2b were performed with Mo KR radiation (λ ) 0.710 73 Å) on a Bruker Apex CCD diffractometer at T ) 293(2) K (1c, 1d, and 2b) and 123(2) K (2a′). Absorption corrections were applied by using the multiscan program SADABS.10 The structures were solved by direct methods and all non-hydrogen atoms were refined anisotropically by least-squares on F2 using the SHELXTL program.11 Hydrogen atoms on organic ligands were generated by riding mode (C-H 0.95 Å). Crystallographic data for the four compounds are summarized in Table 1. Selected bond lengths and angles are summarized in Table S1 of the Supporting Information.

Results and Discussion Synthesis. It is noteworthy that to predict and control the final superstructures of supramolecular isomers is still a great challenge. To obtain possible supramolecular isomers, in our trials for the preparation of silver(I) imidazolates, a hydro(solvo)thermal method was used, in contrast to the conventional solution methods previously reported for the syntheses of two other polymorphs as either microcrystals (1a)8a or single crystals (1b).8b Keeping the same molar ratio of AgNO3:Him:NH3 (1: 1:74), the single crystals of 1c and 1d were formed as two different polymorphs in the presence of aqueous ammonia, being

different from the preparation of microcrystals of 1a8a and single crystals of 1b.8b This fact strongly suggest that ammonia molecules can effectively influence the supramolecular structures of the crystal seeds, leading to the formation of different superstructures from those crystallized in the absence of ammonia molecules. Obviously, ammonia is a kind of strong ligating agent for silver(I) and other metal ions, and it should be involved in the coordination to the silver(I) ion before crystallization. However, the anionic imidazolate group is a stronger ligating agent and can form a neutral metal complex with Ag(I) ions in a 1:1 molar ratio, hence excluding the presence of ammonia molecules in the crystalline product. This deduction is not only in accord with our previous observation that the presence of pyridyl-like ligands (such as 2,2′-bipyridine, 4,4′-bipyridine and pyrazine) could promote the formation of a new 3D supramolecular isomer of Cu(I) triazolate7c but also accounts for the crystallization of 1c and 1d in two different polymorphs in the presence of 2,2′-bipyridine and methanol, respectively. We can conclude here that some organic ligands may act as buffering additives to the metal ions and influence the superstructures of the crystal seeds, thus leading to the formation of supramolecular isomers. Other reaction conditions were also tried, wherein molar ratios of Ag/Him other than 1:1 (such as 1:2) could not give the single crystals of silver(I) imidazolates or gave the single crystals in such a poor yield that they were hard to isolate. Meanwhile, although common organic solvents (such as alcohols, benzene, toluene, chloroform, cyclohexane) were also added into the reaction system, nonpolar solvent always led to non-singlecrystalline products at arbitrary temperatures; only ethanol could give single crystals of 1d in a very low yield. It is also interesting to note that the products were sensitive to the hydro(solvo)thermal reaction temperature. The reaction at the range of 120140 °C and below 120 °C usually led to the formation of the silver(I) imidazolates as larger single crystals and microcrystals, respectively, whereas the method usually led to the formation of metallic silver above 140 °C. On the other hand, cuprous imidazolates in powder form are relatively unstable because of facile oxidation by oxygen gas in moist air, while large single crystals have higher stability. In fact, the preparation of cuprous imidazolate powders was previously reported, but their structures could not be determined because of bad PXRD data due to the unstability of the fine powder.8a Single crystals of 2a were later obtained under solvothermal condition using cupric acetate as the copper source at 140 °C, and the structure was reported.9 At the same time, in the course of our investigation on fine-tuning the valence of copper in copper imidazolates, we reported that CuII could be reduced to CuI to different degrees under basic hydrothermal conditions at higher temperatures (120-160 °C),12 whereas 2a′ was prepared under hydrothermal condition using cupric carbonate basic as the copper source at 160 °C. Our preliminary X-ray single-crystal structural analysis at room temperature revealed that 2a′ has the same structure as 2a, showing “disordered” im ligands. However, we later found that such “disordered ligands” are ordered at low-temperature, giving rise to a new crystal unit-cell volume four times than that of 2a. Meanwhile, introduction of the nonpolar solvent toluene into the reaction system for 2a′ resulted in the formation of 2b as a true supramolecular isomer of 2a′. This fact implies that the polarity of the solvent has an important role in the formation of the superstructures, in accordance with our previous observation for the formation of two supramolecular isomers of triplestranded helical and zigzag chainlike structures of copper(I)

1196 Crystal Growth & Design, Vol. 6, No. 5, 2006

Huang et al.

Figure 1. (a) ORTEP plot [50% probability ellipsoids, (A) x - 1, y, z - 1; (B) x + 1, -y + 1/2, z + 1/2; (C) x - 2, -y + 1/2, z - 3/2; hydrogen atoms are omitted for clarity] and (b) the simplified 2D (6,3) network of 1c (purple balls, Ag atoms; purple sticks, short Ag‚‚‚Ag contacts; golden sticks, bridged im ligands).

Figure 2. (a) ORTEP plot [50% probability ellipsoids, (A) -x + 1/2, -y, z - 1/2; (B) -x + 1/2, y - 1/2, z; (C) x, -y - 1/2, z - 1/2; hydrogen atoms are omitted for clarity] and (b) the simplified 2D (4,4) network of 1d (the colors are same as in Figure 1).

2-ethylimidazolate.6 Similar to the preparation of silver(I) imidazolates, various reaction conditions other than those described in the Experimental Section were also tried for growing single crystals of cuprous imidazolates. Unfortunately, other reaction temperatures and molar ratios of the reactants basically could not produce single crystals in good yield or could not give good-quality single crystals. For common organic solvents, only the polar solvent ethanol could give single crystals of 2a′ in a very low yield, whereas when toluene was replaced by benzene or cyclohexane, the single crystals of 2b could also be isolated, but in a lower yield. Crystal Structures. Among silver(I) imidazolates, the structure of 1a determined by PXRD8a exhibits a 3D (10,3)-a network based on infinite Ag(im) chains and short interchain Ag‚‚‚Ag contacts [3.161(4) Å] when regarding Ag(I) atoms associating with Ag‚‚‚Ag contacts as three-connected nodes and im ligands as well as Ag‚‚‚Ag contacts as linkers.13 The structure of 1b determined by single-crystal X-ray diffraction also possesses argentophilic interactions [3.120(1), 3.226(1), and 3.266(2) Å] that link the chains into a complex 3D net.8b In 1c and 1d, the silver(I) atoms show similar coordination geometries [Ag-N ) 2.064(3)-2.087(2) Å, N-Ag-N ) 169.9(1)-178.2(1)° for 1c, Ag-N ) 2.070(2)-2.119(2) Å, N-Ag-N ) 173.5(1) and 173.8(1)° for 1d], which are also similar to those for the known isomers [Ag-N ) 2.05(1)2.12(1) Å, N-Ag-N ) 167.6(1) and 171.8(8)° for 1a,8a Ag-N ) 2.063(8)-2.088(7) Å, N-Ag-N ) 170.1(3)-176.5(4)° for 1b8b]. These observations illustrate that the linear local coordination environments are maintained for all the isomers of silver(I) imidazolates within minor geometric variations. In contrast, as imidazolates are V-shaped bridging ligands (Mim-M at ca. 139-145° for the polymers in this work), the variation in the dihedral angles (or the relative orientations) between each pair of adjacent imidazolato rings (in the range of ca. 20-80° for 1c, ca. 2-5° for 1d) can cause drastic changes in the shapes and overall configurations of the chains, as illustrated in the structures of 1c and 1d. In fact, attributable to the different relative orientations between adjacent im ligands, the crystal structure of 1c contains

four crystallographically unique Ag(im) units (Figure 1a), whereas that of 1d contains two crystallographically unique Ag(im) units. Such different relative orientations are also associated with different Ag‚‚‚Ag contacts. In 1c, two crystallographically unique Ag(I) atoms have short Ag‚‚‚Ag contacts between adjacent chains [3.1595(5) Å], and the chains extended along [201] directions are interlinked into a 2D network through the short AgI‚‚‚AgI contacts, which can be simplified to be a uninodal (6,3) net (Figure 1b) with the two unique silver atoms having short Ag‚‚‚Ag contacts acting as three-connected nodes and two other silver(I) atoms and the im ligands as well as short AgI‚‚‚AgI contacts act as linkers. In contrast, only one crystallographically unique silver(I) atom in 1d has short Ag‚‚‚Ag contacts [3.4445(5) Å, a value slightly longer than the sum of van der Waals radii of silver atoms, 3.40 Å14] between adjacent chains (Figure 2a). These chains are extended along the c-axis and are linked to each other through the AgI‚‚‚AgI contacts to construct a 2D (4,4) network (Figure 2b). For cuprous imidazolates, the previously reported singlecrystal structure of 2a exhibits a 2D bilayer net constructed by zigzag chains in two perpendicular directions ([110] and [11h0]) linked through interchain CuI‚‚‚CuI interactions (ca. 2.78 Å), where the im rings were disordered in two orientations.9 For our 2a′ obtained under similar hydrothermal condition of 2a, the crystal cell parameters measured by X-ray single-crystal diffraction at room temperature are similar to those of 2a. However, the cell parameters measured at low temperature of 123 K are different than those of 2a, and the unit-cell volume is increased by four times. Meanwhile, the crystal structure at the low temperature exhibits no disorder for im ligands. Therefore, we can conclude that the disorder in 2a is due to the superstructure rather than the rotation of the ligand or packing sequence of the layers. There are four crystallographically unique Cu(im) units, and the coordination geometries of copper(I) atoms in 2a′ (Figure 3) [Cu-N ) 1.849(4)-1.858(4) Å, N-Cu-N ) 174.3(2)175.5(2)°] are similar to those observed in 2a [Cu-N ) 1.833(3)-1.873(3) Å, N-Cu-N ) 173.9(2) and 174.2(2)°].9 Interestingly, there are three types of short CuI‚‚‚CuI contacts in 2a′

Supramolecular Isomerism of Imidazolates

Crystal Growth & Design, Vol. 6, No. 5, 2006 1197

Figure 3. (a) ORTEP plot [50% probability ellipsoids, (A) x - 1/2, y + 1/2, z; (B) x -1/2, y - 1/2, z; (C) -x + 1, y, -z + 3/2; (D) -x + 2, y, -z + 3/2; (E) -x + 2/3, y + 1/2, -z + 2/3; hydrogen atoms are omitted for clarity] and (b) the simplified 2D 8210 network of 2a′ (golden balls, Cu atoms; golden sticks, short Cu‚‚‚Cu contacts; green sticks, bridged im ligands).

Figure 4. ORTEP plot [50% probability ellipsoids, (A) x + 1/2, -y + 3/ , z - 1/ ] of 2b. 2 2

that are not supported by any obvious intermolecular metalligand (shortest Cu-N ) 3.30 Å) or ligand-ligand interactions. Among them the shorter Cu‚‚‚Cu separations correspond to less ligand overlaps [Cu1‚‚‚Cu1C ) 2.789(1) Å and Cu4‚‚‚Cu4D ) 2.782(1) Å, N-Cu‚‚‚Cu-N ≈ 90°], while the longer Cu‚‚ ‚Cu separations [Cu2‚‚‚Cu3 ) 2.687(1) Å, N-Cu‚‚‚Cu-N ≈ 50°] correspond to stronger ligand overlaps, implying that steric repulsion between the im ligands is likely obstructive for closer cuprophilic interactions. This phenomenon is covered up by the “disorder” in the structure of 2a. It is worthy of note that there are few known examples of ligand-unsupported cuprophilic separations shorter than the van der Waals radius sum of copper atom (2.80 Å);14 the shortest one of 2.651(4) Å was observed in a 3D pseudopolyrotaxane compound, [Cu2(4,4′-bpy)(CN)2]‚ [Cu(SCN)].15 In the new true supramolecular isomer of copper(I) imidazolate 2b, there are three crystallographically unique Cu(im) units, and the coordination geometries of Cu(I) atoms [Cu-N ) 1.849(5)-1.863(5) Å, N-Cu-N ) 171.8(2)-176.0(2)°] are similar to those found in 2a and 2a′ (Figure 4). The crystal structure of 2b also exhibits zigzag chains running along the c-axis, without any obvious CuI‚‚‚CuI interactions between chains [Cu‚‚‚Cu g 3.387(2) Å]. The dihedral angles (or the relative orientations) between each pair of adjacent im rings are ca. 4-56° in 2b, being significantly different from those observed in 2a′ (ca. 41 and 43° for two perpendicular chains, respectively), which result in the great difference between these two superstructures. Photoluminescence. Supramolecular isomers of 1c and 1d for silver(I) imidazolates are nonemissive under UV excitation, while the cuprous imidazolates of 2a′ and 2b are strong greenish-blue emitters (Figure 5). Photoexcitations of 2a′ and 2b with 280 nm UV light give strong emissions centered at 493 and 484 nm, respectively, which can be assigned to originate from the 3[MLCT] excited states, similar to other reported cuprous 2-alkylimidazolates.3,6 The very slight difference in the emission spectra of these two isomers implies that the properties of the excited state are mainly related to the local coordination

Figure 5. Solid-state emission spectra of 2a′ (solid line) and 2b (dashed line) at room temperature.

geometry and may be slightly perturbed by the intermolecular interactions such as weak cuprophilic interactions.7c,d Discussion As noted above, all the coordination geometries for AgI and CuI ions in their binary prototype imidazolates are basically linear within minor differences due to the very strong ligation ability of imidazolate; hence, such minor geometric differences are not sufficient in controlling the formations of different supramolecular isomers. In contrast, different relative orientations of the im groups can cause rather drastic differences in the conformation of the chains, which should be the key factor in the formation of different supramolecular isomers. On the other hand, as im ligand is rather small, it is difficult to offer strong intermolecular interactions (such as hydrogen bonding, π-π stacking, and van der Waals interactions) between the ligands. Therefore, it is theoretically possible to exhibit a large number of chainlike isomers with different relative orientations of the im groups in the infinite chains. However, as such differences in the relative orientation of the im groups should have no significant difference in the formation energy, the finetuning of the reaction condition to obtain specific isomer can be expected to be very difficult. Therefore, we are so far unable to control specific orientations of the ligands for the formation of peculiar helical or cyclic isomers for the binary prototype imidazolates of univalent metal ions. Instead, only the irregular zigzag chains have so far been isolated. This argument is supported by the fact that, upon using 2-alkyl-substituted imidazolates, which can enhance supramolecular hydrophobic

1198 Crystal Growth & Design, Vol. 6, No. 5, 2006

interactions, we could construct the regularly orientated, helical and cyclic structures through solvent polarity induction and guest templating, respectively.3,6 Furthermore, the orientations of the im groups may also affect the presence or absence of interchain metal-metal interactions, which may play a secondary role in the crystal packing and the final superstructures as well. Although the theoretical and experimental studies of metallophilicity for the closed-shell d10 metal ions have been widely documented,16 examples of ligandunsupported AgI‚‚‚AgI interactions17 and especially CuI‚‚‚CuI interactions3,6,15,18 are relatively few, as compared to those of AuI. This fact may be ascribed to the strength of argentophilicity/ cuprophilicity being relatively weaker than aurophilicity, which is comparable to the typical hydrogen bonding.19 Therefore, the examples of d10 metal complexes with ligand-unsupported argentophilicity/cuprophilicity are very important for the study of this weak interaction.20 The investigation of the imidazolate system just gives an opportunity to construct this intermolecular interaction for the following reasons: first, the ligand hindrance and interchain metal-ligand interactions are decreased by the linear coordination geometry of AgI/CuI ions and the very small im ligand; second, hydrogen-bonding and π-π interactions are avoided because of the small π system and weak proton donor and acceptor (N atom coordinated to MI) in the ligand. However, as argentophilicity/cuprophilicity is very weak, its presence seems to be controlled by the orientation of ligands, especially for cuprophilicity. This deduction may accounts for no obvious weak interactions observed in 2b. Conclusions Although not designable, four true supramolecular isomers for silver(I) imidazolate and two supramolecular isomers for copper(I) imidazolate have been synthesized under different conditions in the literature and our work. Our analysis based on the structural data suggests that the relative orientations of the im ligands are the key factor affecting the patterns of superstructures, which can be tuned by the reaction conditions, especially the addition of different buffering agents. Therefore, we can also expect that more supramolecular isomers are possible for these simple systems because of the high flexibility in the orientation of imidazolate groups in such 1D coordination polymers, even the widely interesting helical structures,21 though it will not be easy to find out the reaction or crystallization conditions. Acknowledgment. This work was supported by the NSFC (No. 20531070) and the Scientific and Technological Department of Guangdong Province (No. 04205405). Supporting Information Available: An additional table and plots (PDF) and X-ray data files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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