Increasing Structural Dimensionality in Ag(I) Coordination Polymers

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Increasing Structural Dimensionality in Ag(I) Coordination Polymers Formed from the Same Flexible Multimodal Thioether Pyrazine Ligand Jarrod J. M. Amoore,† Cory A. Black,† Lyall R. Hanton,*,† and Mark D. Spicer‡

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 1255-1261

Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand, and Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, Scotland Received January 10, 2005;

Revised Manuscript Received February 24, 2005

ABSTRACT: Three coordination polymers were prepared by the reaction of a new flexible multimodal ligand, bis(2-pyrazylmethyl)sulfide (L), with AgX salts (X ) BF4- or PF6-). The polymers showed an increasing dimensionality from 1-D to 2-D to 3-D, and the influence of solvent and anion on the formation and dimensionality of these coordination polymers was explored. A 1-D twisted ladder-like polymer with η1-bound benzene and a 2:1 AgPF6to-L ratio, 1‚MeCN, was formed when benzene was present in the solvent mix. A 2-D undulating sheet with 4.82 topology and a 1:1 AgPF6-to-L ratio, 2, was formed when CH2Cl2 was present in the solvent mix. A 3-D network, 3, with two different 4.82 nets linked together by 4.85 bridging nodes was formed when a 1:1 AgBF4-to-L ratio was used and MeNO2 was present in the solvent mix. Coordination arrays 2 and 3 were found to be related topologically. Introduction One of the interesting challenges in supramolecular chemistry and crystal engineering is the development of methods for the design and synthesis of topologically interesting architectures with increasing dimensionality. Rigid multimodal ligands have been found to generate various combinations of 1-D, 2-D, and 3-D structures depending on the metal and counterion employed and the crystallization conditions.1,2 However, the hierarchical assembly of architectures with systematically increasing dimensionality typically has been accomplished by using moieties capable of forming weak and extensive interactions such as H-bonding synthons,3 π-π interactions,4 coordinating cations5 and anions,6 and d10-d10 interactions.7 In contrast, we have found through the use of a flexible multimodal ligand (Scheme 1) that we can increase dimensionality using covalent metalligand bonds. The difficulty of using such ligands is that they have chelating pockets, which can lead to systems with low dimensionality.8,9 We have found that by using a flexible ligand, which has a number of different conformations available (Figure 1), we are able to circumvent this difficulty and build architectures of increasing dimensionality. Our ligand design incorporates both pyrazine and thioether moieties thereby providing bridging modes to expand the structure and flexibility to provide diversity of structure (Scheme 2). The use of flexible ligands in coordination-polymer synthesis appears to run contrary to the current paradigms of systematically engineering coordination arrays. While the predictability of polymeric networks based on flexible ligands is less reliable and depends on factors such as ligand conformation, the possibility of constructing unprecedented networks makes this approach an attractive one. Such studies using flexible ligands to * To whom correspondence should be addressed. Fax: (+64) 3-4797906. Phone: (+64) 3-479-7918. E-mail: [email protected]. † University of Otago. ‡ University of Strathclyde.

Figure 1. View of the conformations of ligand L found in the structures of coordination polymers 1-3.

Scheme 1. Diagram Showing the Conventional Numbering Scheme for Ligand L

Scheme 2. Coordination Modes of Ligand L Observed in 1-3 [Mode (a) 1, 3; (b) 2; (c) 3]

rationally design well-defined coordination polymer architectures are still at an inchoate stage of development. At this early stage, an important tool for understanding and comparing these often complex structures is topological analysis. This is because structures that

10.1021/cg050011+ CCC: $30.25 © 2005 American Chemical Society Published on Web 03/24/2005

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Table 1. Crystallographic Data for Complexes 1, 2, and 3

empirical formula M crystal system space group a, Å b, Å c, Å β, deg V, Å3 Z T, K µ, mm-1 reflns collected unique reflns (Rint) R1 indices [I > 2σ(I)] wR2 (all data)

1‚MeCN

2

3

C22H25Ag2F12N7P2S 925.23 monoclinic P2/c 16.309(3) 14.636(3) 13.941(4) 93.669(3) 3320.9(13) 4 163(2) 1.433 39910 6907 (0.0497) 0.1027 0.2704

C10H10AgF6N4PS 471.12 monoclinic P21/c 9.9188(2) 12.8858(5) 11.7449(4) 100.646(2) 1475.30(8) 4 150(2) 1.684 8258 4291 (0.0201) 0.0276 0.0620

C21H23Ag2B2F8N9O2S2 886.96 monoclinic P21/a 10.7108(1) 13.2616(2) 20.7926(3) 92.5034(4) 2950.61(7) 4 123(2) 1.56 12595 6725 (0.0179) 0.0235 0.0524

empirically appear to be quite disparate may be shown to be closely related by topological analysis. Finally, the accommodating coordination behavior of the Ag(I) cation has proved useful in allowing the construction of diverse coordination polymers, often with interesting topological,2,10 luminescent,11 or chiral properties12 or a combination of these. We now report the synthesis of a new flexible multimodal ligand, bis(2-pyrazylmethyl)sulfide (L), and reactions with the AgX salts (X ) BF4- or PF6-). The series of coordination polymers formed show an increasing dimensionality from 1-D to 2-D to 3-D and suggest a degree of control of patterning at the nanoscale level is possible. In addition, we demonstrate that it is possible to prepare 4.82 nets and subsequently extend the array into three dimensions using unusual 4.85 bridging nodes. Experimental Section General. 2-Methylpyrazine was purchased from Aldrich and used without further purification. The precursor 2-(chloromethyl)pyrazine was prepared via literature methods,19 and the carbon tetrachloride used was dried over CaCl2 and distilled immediately before use. The 1H, 13C NMR spectra were recorded on a 300 or 500 MHz Varian UNITYINOVA spectrometer at 298 K. Elemental analyses were performed by the Campbell Microanalytical Laboratory at the University of Otago. Samples were predried under vacuum to remove volatile solvent residues. Bis(2-pyrazylmethyl)sulfide (L). KOH (3.5 g, 62 mmol) dissolved in distilled H2O (10 mL) together with 2-(chloromethyl)pyrazine (7.8 g, 61 mmol) and thioacetamide (2.3 g, 30 mmol) was refluxed for 18 h in a 250 mL benzene/ethanol (1:1 v/v) solution. The resulting black solution was filtered, and the solvent was removed in vacuo to give a black oil. This oil was dissolved in CH2Cl2 (200 mL) and washed with distilled water (7 × 100 mL), and the solvent was removed to give a red-black oil (4.8 g). Purification on a silica gel column (5% hydrated v/v) eluted with CH2Cl2/CHCl3 (30:70 v/v) gave L as a yellow-brown oil, which crystallized on standing (2.2 g, 33%). NMR: δH (CDCl3, 300 MHz) 8.57 (2H, s, H3), 8.44 (2H, d, H5), 8.39 (2H, d, H6), 3.79 (4H, s, H7); δC (CDCl3, 500 MHz) 153.99 (C2), 144.79 (C3), 143.89 (C5), 142.97 (C6), 34.54 (C7). Found: C, 55.08; H, 4.89; N, 25.94; S, 14.90. Calcd for C10H10N4S: C, 55.03; H, 4.62; N, 25.67; S, 14.69%. {[Ag2(L)(C6H6)(MeCN)2](PF6)2‚MeCN}∞ (1‚MeCN). AgPF6 (111 mg, 0.440 mmol) dissolved in 25 mL of degassed MeCN/benzene (1:4 v/v) was added via cannula to L (43 mg, 0.20 mmol) dissolved in 25 mL of degassed MeCN/benzene (1:4 v/v). The solution was stirred for 3 h, after which it was concentrated to a volume of 10 mL, causing the formation of a precipitate. The tan powder was then filtered and dried in

vacuo. Yield: 117 mg (82%). Orange X-ray quality crystals were grown from the slow diffusion of a CHCl3 solution of L (19 mg, 0.087 mmol) layered with benzene into a MeCN solution of AgPF6 (26 mg, 0.10 mmol). Found: C, 27.79; H, 2.63; N, 9.83; S, 4.08. Calcd for C20H22N6SAg2P2F12‚1/2CH3CN: C, 27.88; H, 2.62; N, 10.06; S, 3.54%. {[Ag(L)](PF6)}∞ (2). AgPF6 (76 mg, 0.30 mmol) dissolved in degassed MeCN (25 mL) was added via cannula to L (58 mg, 0.27 mmol) dissolved in degassed MeCN (25 mL). The solution was left to stir overnight, after which it was concentrated to a volume of 5 mL, and 1-butanol (10 mL) was added. The solution was then concentrated further to 10 mL, causing the formation of a precipitate. The tan powder was then filtered and dried in vacuo. Yield: 100 mg (79%). Tan X-ray quality crystals were grown from the slow diffusion of a CHCl3 solution of L (20 mg, 0.093 mmol) layered with CH2Cl2 and 2-propanol into a MeCN solution of AgPF6 (24 mg, 0.093 mmol). Found: C, 25.54; H, 2.04; N, 11.81; S, 6.70. Calcd for C10H10N4SAgPF6: C, 25.49; H, 2.14; N, 11.89; S, 6.81%. {[Ag(L)](BF4)}∞ (3). AgBF4 (47 mg, 0.24 mmol) dissolved in degassed MeCN (25 mL) was added via cannula to L (52 mg, 0.24 mmol) dissolved in degassed MeCN (25 mL), and the mixture was allowed to stir for 1 h. The resultant solution was concentrated to 5 mL, and 1-butanol (10 mL) was added. The solution was then concentrated in vacuo to 10 mL, which caused the formation of a precipitate. This precipitate was filtered off and dried in vacuo to give a fawn powder (yield 90 mg, 91%). Colorless crystals of X-ray quality were grown by the slow diffusion of a MeNO2 solution of L (22 mg, 0.099 mmol) layered with benzene and an EtOH solution of AgBF4 (32 mg, 0.16 mmol). Found: C, 28.64; H, 2.45; N, 14.11; S, 7.29. Calcd for C10H10BN4F4SAg: C, 29.08; H, 2.44; N, 13.57; S, 7.77%. X-ray Crystallography. Diffraction data for 1‚MeCN were collected on a Bruker SMART CCD diffractometer, and data for 2 and 3 were collected on a Nonius Kappa-CCD diffractometer; both had graphite monochromated Mo KR (λ ) 0.71073 Å) radiation. Intensities were corrected for Lorentz-polarization effects,13 and a multiscan absorption correction14 was applied to 1‚MeCN. The structures were solved by direct methods (SHELXS)15 and refined on F2 using all data by fullmatrix least-squares procedures (SHELXL 97).16 All calculations were performed using the WinGX interface.17 Despite many attempts, crystals of 1‚MeCN always grew as thin plates of poor quality. Two data sets on separate batches of crystals were collected and gave similar structural solutions. The structure reported herein represented the better result for 1‚ MeCN although a number of large peaks, possibly Fourier ripples, were located between Ag(1) and other atoms at distances that made no chemical sense. Topological analyses were conducted using OLEX (version 2.55).18 Crystal data and refinement details for the three structures are summarized in Table 1.

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Results and Discussion Ligand Synthesis. The ligand bis(2-pyrazylmethyl)sulfide (L) was prepared in moderate yield by the reaction of 2-(chloromethyl)pyrazine19 with thioacetamide under basic conditions.9 The chlorination of the precursor 2-methylpyrazine was accomplished in good yield using N-chlorosuccinimide.19 Use of the less common chlorinating agent trichloroisocyanuric acid20 was investigated in an alternative preparation. However this approach was abandoned because it gave a mixture of products in lower yield and the products were not easy to separate by column chromatography. Synthesis of AgPF6 and AgBF4 Coordination Polymers Using L. It was found that bulk reactions in MeCN of AgPF6 with L in either 2:1 or 1:1 molar ratios gave tan powders for which the microanalyses were consistent with the solids always having a 1:1 metal-to-ligand ratio. Interestingly, attempts to grow X-ray quality crystals of this material by the slow diffusion through a benzene layer of L in CHCl3 and AgPF6 in MeCN led to the isolation of 1‚MeCN as orange crystals with a 2:1 metal-to-ligand ratio. Subsequently, 1‚MeCN was prepared in bulk in high yield by the 2:1 molar reaction of AgPF6 with L using a MeCN/benzene (1:4 v/v) solvent mix. Tan X-ray quality crystals of the 1:1 product, 2, were grown in a manner similar to 1‚MeCN except that the reactants were diffused through a layer of CH2Cl2, rather than benzene. The 1:1 molar reaction of L and AgBF4 in MeCN on workup gave a tan powder in high yield. Microanalysis showed the complex was consistent with a 1:1 metalto-ligand ratio. However, if the reaction was carried out using a 2:1 metal-to-ligand ratio, the tan solid isolated in moderate yield gave a microanalysis consistent with the 1:1 product. Despite a number of attempts and the careful workup of reaction solids and filtrates, the 2:1 product could not be isolated. Structure of {[Ag2(L)(C6H6)(MeCN)2](PF6)2‚ MeCN}∞ (1‚MeCN). X-ray structural analysis of 1‚ MeCN revealed a one-dimensional twisted ladder-like arrangement of Ag(I) ions held together by the ligands (Figure 2). The asymmetric unit contained one benzene and three MeCN molecules, two Ag(I) ions, two half ligands and one and two half PF6- counterions. Interestingly, the two independent ligands were related as enantiomers. The ladder had alternating enantiomeric forms directed in a similar fashion along the ladder, which consequently gave rise to the ladder’s twist. The ladder showed 2-fold symmetry about the two crystallographically distinct S donors, which both had long bond distances to the Ag(I) ions. The shorter of the two distances, Ag(1)-S(1), was just above the upper quartile of reported Ag-S(thioether) values in a search of the Cambridge Crystallographic Database (CSD, version 5.25).21 The Ag(1) center can be considered to adopt a distorted square-pyramidal arrangement [trigonality index22 τ ) 0.18] with one chelating ligand, one monodentate ligand, and two bound MeCN solvent molecules. The long Ag(1)-N(6) distance of 2.752(15) Å suggested that one of the MeCN solvent molecules only weakly interacted with the Ag(I) ion. This was borne out by the observation that microanalyses typically showed only half a MeCN molecule was lost on drying. The longer of the two distances, Ag(2)-S(2), was at the boundary

Figure 2. Perspective view (crystallographic numbering) of the polymeric chain of 1‚MeCN illustrating the η1-bound benzene and endo-anti ligand conformation. Thermal ellipsoids are drawn at the 50% probability level, and PF6anions are omitted for clarity. Selected bond length (Å) and angles (deg) are as follows: Ag(1)-N(1) 2.420(10); Ag(1)-N(3) 2.294(10); Ag(1)-S(1) 2.7903(16); Ag(1)-N(5) 2.461(14); Ag(1)-N(6) 2.752(15); Ag(2)-N(2) 2.310(10); Ag(2)-N(4) 2.345(10); Ag(2)-S(2) 2.9688(16); Ag(2)-N(7) 2.506(16); Ag(2)-C(15) 2.654(18); N(3)-Ag(1)-N(1) 147.1(4); N(3)-Ag(1)-N(5) 119.1(4); N(1)-Ag(1)-N(5) 90.0(4); N(3)-Ag(1)-S(1) 107.6(3); N(1)Ag(1)-S(1) 75.2(2); N(5)-Ag(1)-S(1) 106.2(4); N(6)-Ag(1)N(5) 79.5(4); N(3)-Ag(1)-N(6) 86.7(4); N(6)-Ag(1)-N(1) 83.8(4); N(6)-Ag(1)-S(1) 158.1(2); N(2)-Ag(2)-N(4) 166.6(4); N(2)Ag(2)-N(7) 92.2(4); N(4)-Ag(2)-N(7) 85.8(4); N(2)-Ag(2)C(15) 88.0(4); N(4)-Ag(2)-C(15) 105.2(4); N(7)-Ag(2)-C(15) 108.4(5); N(2)-Ag(2)-S(2) 104.2(3); N(4)-Ag(2)-S(2) 73.4(3); N(7)-Ag(2)-S(2) 153.2(4); C(15)-Ag(2)-S(2) 93.4(3).

of reported values,23 but at 2.969(2) Å it was less than the value (3.16 Å) considered to be noninteracting.24 The Ag(2) center had one chelating and one monodentate ligand, one MeCN, and a η1-benzene coordinated to it and can also be considered to adopt a distorted squarepyramidal geometry [trigonality index22 τ ) 0.22]. The Ag-C distance of the η1-benzene interaction was short,25 and this relatively strong interaction may have been responsible for the corresponding lengthening of the Ag-S bond as required by the electroneutrality principle.26 The ligands used all available lone pairs on each donor atom, including both on the S donor, and coordinated to four Ag(I) cations (Scheme 2a). The two enantiomeric ligands adopted almost identical endoanti conformations (Figure 1). The pyrazine rings of each ligand were almost parallel to each other with tilts of 2.3° and 3.8° and centroid-to-centroid distances of 5.73 and 5.57 Å for the ligands containing S(1) and S(2), respectively. The mean planes of the enantiomeric ligands were twisted with respect to each other by 41°. This conformation was also found in copper(I) iodide complexes of the closely related pyridine-based ligands.27 The ligands in each ladder were oriented in the same direction. The ladders formed two-dimensional sheets in the bc plane, with adjacent ladders having ligands oriented in opposite directions (Figure 3). Adjacent ladders were held together by strong π-π interactions (centroid-to-centroid distance 3.54 Å) between an η1bound benzene and a pyrazine ring with donors N(3) and N(4). Unusually, there was a near face-to-face alignment of the rings with an angle of 4.6° between the centroid-to-centroid vector and the ring normal to the plane of the pyrazine.28 This π stacking was supplemented by three different C-H‚‚‚F-P interactions [C‚‚‚F separations range from 3.26 to 3.39 Å] and three different Ag‚‚‚F-P interactions [Ag‚‚‚F separations range from 3.16 to 3.57 Å] arising from the PF6anion situated on a general crystallographic position.

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Figure 3. A view of the sheet structure of 1‚MeCN in the bc plane illustrating the interladder π stacking between η1benzene and pyrazine rings from L and the twisting of the ladders. Alternate ladders are colored differently, and hydrogen atoms are omitted for clarity.

Figure 5. Perspective view (crystallographic numbering) of the coordination environment of 2 illustrating the twisted endo-syn ligand conformation. All Ag(I) atoms are labeled to show connectivity between ligands. Thermal ellipsoids are drawn at the 50% probability level, and PF6- anions are omitted for clarity. Selected bond lengths (Å) and angles (deg) are as follows: Ag(1)-N(2B) 2.2951(17); Ag(1)-N(3A) 2.3393(17); Ag(1)-N(4) 2.4405(18); Ag(1)-S(1) 2.5453(5); N(2B)-Ag(1)-N(3A) 95.18(6); N(2B)-Ag(1)-N(4) 133.84(6); N(3A)Ag(1)-N(4) 101.06(6); N(2B)-Ag(1)-S(1) 120.05(5); N(3A)Ag(1)-S(1) 130.70(4); N(4)-Ag(1)-S(1) 79.49(4) (symmetry code (A) x, 1/2 - y, z - 1/2 and (B) -x, -y, 1 - z). Figure 4. View (bc plane, left) of the two-dimensional sheet of 2 with an overlay of the 4.82 topological net (hydrogen atoms and anions omitted for clarity) and view (ab plane, right) showing the undulations of the sheets and the placement of anions in sheet clefts (hydrogen atoms omitted for clarity).

The overall three-dimensional structure was formed by the parallel stacking of the sheets, which were held in place by a series of C-H‚‚‚F-P interactions arising from the PF6- anions situated on special crystallographic positions [C‚‚‚F separations range from 3.34 to 3.43 Å]. Structure of {[Ag(L)](PF6)}∞ (2). Compound 2 formed a two-dimensional undulating sheet with a relatively rare Schlafli 4.82 topology2,29 in which both the Ag(I) ion and L acted as three-connected nodes (Figure 4). No solvent molecules were incorporated in the structure. The asymmetric unit contained one ligand molecule, one Ag(I) ion, and one PF6- counterion. The ligand adopted a twisted endo-syn form (Figure 1) in which the pyrazine rings were tilted by 48.9° with respect to each other. The Ag(I) ion was coordinated in a severely distorted tetrahedral fashion by a NSN′N′′ donor set from one chelating and two monodentate ligands (Figure 5). One of the lone pairs of the S donor and one of the N donors remained uncoordinated (Scheme 2b). This remaining N donor pointed into the sheet and as such was prevented from undergoing any further interactions. The structure of the sheet consisted of two interlocking 12- and 30-membered metallomacrocyclic rings (Figure 4). The sheets undulated due to the pseudo-chair arrangement of the larger rings, which were fused in an offset cis fashion. The PF6- anions were situated in each cleft formed from the undulation of the sheet. There were C-H‚‚‚F-P interactions [C‚‚‚F separations range from 3.03 to 3.49 Å] between the anions

and the ligands of adjacently stacked sheets generating the overall structure. Structure of {[Ag(L)]BF4‚1/2MeNO2}∞ (3). X-ray structural analysis of 3 revealed a three-dimensional network based on two different pseudo-tetrahedral Ag(I) ions and two completely different ligand conformations. One of the arrangements adopted by the ligand was a folded endo-syn conformation (Figure 1) that was different from that found in 2. The pyrazine rings had intramolecular π-stacking interactions with centroidto-centroid distances of 3.57 Å (Figure 6) and were tilted by 20.4° with respect to each other. The other arrangement adopted by the ligand was a twisted endo-anti conformation (Figure 1), which was different from that found in 1. In the conformation found for 3, one of the pyrazine arms was rotated by approximately 90° compared to that for 1 and the pyrazine rings were twisted by 113.3° with respect to each other. Each ligand conformation was partitioned in one of the two major structural features of the three-dimensional array. Ligands in an endo-syn conformation constituted a zigzag sheet, while ligands in an endoanti conformation constituted a doubly braced wall between the zigzag sheets (Figure 7). The zigzag sheets consisted of alternatively linked 30- and 10-membered metallomacrocyclic rings. Intermolecular π-stacking interactions form the endo-syn ligands into pairs [centroid-to-centroid 3.56 Å], and these pairs of ligands define the length of each zigzag. Ag(I) ions within the sheet were bound to three individual endo-syn ligands through a 4-substituted N donor, a 1-substituted N donor, and a S donor (Figure 6). The remaining coordination site was occupied by a 4-substituted N donor from an endo-anti ligand. Differently positioned

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Figure 7. View (bc plane) of 3 showing the zigzag sheets and doubly braced walls (hydrogen atoms, solvent molecules, and anions omitted for clarity) with an overlay of the topological net (zigzag sheets in red and doubly braced walls in purple and black).

Figure 6. Perspective view (crystallographic numbering) of 3 showing the two different Ag(I) ion coordination spheres, the two different ligand conformations, the N‚‚‚H-C interactions, and intramolecular π stacking of the folded endo-syn ligands. Thermal ellipsoids are drawn at the 50% probability level with BF4- anions and MeNO2 solvent molecules omitted for clarity. Selected bond lengths (Å) and angles (deg) are as follows: Ag(1)-N(4C) 2.272(2); Ag(1)-N(3D) 2.305(2); Ag(1)-N(1) 2.406(2); Ag(1)-S(1) 2.7501(5); Ag(2)-N(2) 2.327(2); Ag(2)-N(5B) 2.338(2); Ag(2)-N(8A) 2.406(2); Ag(2)-S(2) 2.5831(5); N(6A)C(2) 3.459(3); N(7B)-C(15A) 3.392(3); N(4C)-Ag(1)-N(3D) 128.87(6); N(4C)-Ag(1)-N(1) 102.64(6); N(3D)-Ag(1)-N(1) 125.62(6); N(4C)-Ag(1)-S(1) 123.66(5); N(3D)-Ag(1)-S(1) 88.56(4); N(1)-Ag(1)-S(1) 73.79(4); N(2)-Ag(2)-N(5B) 95.81(6); N(2)-Ag(2)-N(8A) 103.36(6); N(5B)-Ag(2)-N(8A) 116.91(6); N(2)-Ag(2)-S(2) 115.27(5); N(5B)-Ag(2)-S(2) 122.08(4); N(8A)-Ag(2)-S(2) 102.40(4) (symmetry codes (A) 11/2 - x, 1/2 + y, 1 - z, (B) 1 - x, -y - 1, -z, (C) 1 - x, -1 - y, -z, and (D) 1 /2 + x, -y - 11/2, z).

N donors on each pyrazine ring of the endo-syn ligand remained uncoordinated to Ag(I) ions. Instead they were involved in N‚‚‚H-C interactions giving rise to a 1-substituted N‚‚‚H-C(methylene) distance of 2.51 Å and angle of 149° [N‚‚‚C distance 3.392(3) Å] within the zigzag sheet structure and a 4-substituted N‚‚‚HC(pyrazine) distance of 2.59 Å and angle of 153° [N‚‚‚C distance 3.459(3) Å] between the sheets and the braces. The doubly braced walls were made up from alternatively linked 20- and 12-membered metallomacrocyclic rings. Ag(I) ions were bound to three individual endoanti ligands through a 4-substituted N donor, a 1-substituted N donor, and chelating 1-substituted N and S donors (Scheme 2a). The remaining 4-substituted N donor of the endo-anti ligand was used to bind to a Ag(I) ion in the zigzag sheet thereby connecting the doubly braced walls to the zigzag sheets. Together these structural features gave rise to two different cavities. The larger cavities contained both BF4- anions and MeNO2 solvent molecules, while the smaller cavities contained only BF4- anions. The anions and solvent molecules were not disordered due to extensive interactions within the cavities. The BF4- anions were involved

in a series of C-H‚‚‚F-B interactions with the ligands [C‚‚‚F separations range from 2.96 to 3.07 Å]. The MeNO2 molecules were held in place by a series of N-O‚ ‚‚H-C interactions [O‚‚‚C separations range from 3.28 to 3.36 Å] and C-H‚‚‚F-B interactions [C‚‚‚F separations range from 3.05 to 3.54 Å] involving BF4- anions from both cavities. This structural description was mirrored in the topological analysis of the network. The network can be described by four distinct nodes representing the two silver ions and the two ligand conformations. The zigzag sheet and doubly braced wall were represented by two different three-connected nodes giving rise to two separate two-dimensional nets each with 4.82 topology. The two 4.82 nets were linked together through four-connected silver and ligand nodes of a most unusual 4.85 topology (Figure 7).30 Comparison. Comparison of 1‚MeCN and 2, containing the same PF6- anion, demonstrated the significant role that solvent had in their preparation and structure. The presence of benzene in the solvent mix was essential to the synthesis of the one-dimensional polymer because without it, the two-dimensional sheet was formed regardless of the metal-to-ligand ratio used. The fact that the structures were so solvent-dependent was a consequence of the tight η1-binding of benzene to Ag(I) in 1‚MeCN. Comparing polymers 2 and 3, with the same metalto-ligand ratio, showed that the counterions appeared to have a significant influence on the overall structure, although there may also have been a contribution due to solvent in 3. The larger PF6- anions (anion volume 109 ( 8 Å3)31 appeared to be more readily accommodated in the clefts of the undulating sheet in 2. The smaller BF4- anions (anion volume 73 ( 9 Å3)31 were possibly better suited than PF6- to occupy the different sized cavities present in the more complex structure of 3. Furthermore, the two-dimensional structure 2 had a pyrazine N atom pointing into the sheet, possibly preventing its extension into three dimensions. To form a three-dimensional array with the same metal-toligand ratio, it seemed that two different ligand confor-

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modes of the ligand thereby affecting the nature of the array. Thus in complexes with low metal-to-ligand ratios, it would be expected that there were donor atoms not involved in coordination to Ag(I) ions. However, our flexible multimodal ligand can allow such donors to be utilized in other ways, and we found that they can form supramolecular synthons as in 3 (Scheme 2c). In summary, each of the above factors was used to influence the way in which the ligand when coordinated was able to increase the dimensionality of the structures thereby enabling ladders, 4.82 sheets, and linked 4.82 sheets to be formed (Figure 8). Conclusion

Figure 8. Increasing the dimensionality of structures from 1-D (ladder), 2-D (sheet), and 3-D (bridged sheets).

mations were required or two different coordination modes needed to be present or both. The structures showed that the ligands were capable not only of adopting different conformations (Figure 2) but also of a wide variety of coordination modes (Scheme 2). One of the most common coordination modes for L used the N donor atoms in the 1-positions and the S donor atom to chelate to one Ag(I) ion, as in 2, or two Ag(I) ions, using both S lone pairs, as in 1 and 3. The N donor atom in the 4-position acted as a monodentate ligand often binding to the chelated Ag(I) ion as in 1 and 3. This tendency for Ag(I) to adopt both chelating and monodentate donors with multimodal ligands has been attributed to the drive toward a “homogeneous” metal coordination environment throughout the coordination polymer.2 While this appears to be the case for rigid multimodal ligands, it seems less so for flexible ligands. One of our examples, 2, was in accord with this notion of a “homogeneous” metal coordination environment. Our two remaining examples, 1 and 3, showed quite emphatically that other factors, such as solvent and ligand conformation, were also involved in influencing metal coordination environment. This was particularly clear in 1 where the Ag(I) ions could easily have adopted a “homogeneous” coordination environment but did not because of the effect of coordinated solvents. Also in 3, a “homogeneous” coordination environment was not adopted due to the partitioning of the Ag(I) ions into regions where one particular ligand conformation dominates, so different coordination environments were necessary to optimize the use of the donor atoms to arrive at a stable network structure. In addition, the metal-to-ligand ratios were an obvious factor affecting the overall structure. Not surprisingly, this multimodal ligand, with an enhanced number of donor atoms, was particularly sensitive to this ratio. Also, the metal ratios can limit the possible binding

The various effects influencing these systems were complimented by the versatility of this flexible multimodal ligand in being able to adopt four different conformations (Figure 1) and three different binding modes (Scheme 2), thereby enabling the efficient use of as many donor atoms as possible in forming a stable molecular architecture. Perhaps the most remarkable result from this set of structures was their increasing dimensionality with the same ligand generating architectures of one, two, and three dimensions. Included in this result was the striking topological relationship between the two-dimensional 4.82 nets of 2 and the 4.82 nets of 3 where bridging through 4.85 nodes produced a three-dimensional array. Acknowledgment. We thank Professor Ward T. Robinson and Dr. Jan Wikaira (University of Canterbury) for X-ray data collection and the University of Otago for financial support. Supporting Information Available: X-ray data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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