Synthesis and Structures of Four Isomeric Binary Silver(I) - American

Feb 24, 2012 - Department of Chemistry and the Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan. 00931-...
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Relaying Isomerism from Ligands to Metal Complexes: Synthesis and Structures of Four Isomeric Binary Silver(I) 3,5-Dibutyl-1,2,4triazolates Guang Yang,*,† Peng-Cheng Duan,† Kai-Ge Shi,† and Raphael G. Raptis‡ †

Department of Chemistry, Zhengzhou University, Zhengzhou 450001, China Department of Chemistry and the Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan 00931-3346, Puerto Rico

‡

S Supporting Information *

ABSTRACT: Four isomers of 3,5-dibutyl-1H-1,2,4-triazole (3,5-R2tzH, R = butyl (nBu), isobutyl (iBu), secondary butyl (sBu), and tertiary butyl (tBu)) have been synthesized by condensation of the corresponding organic acids and hydrazine, followed by a deamination reaction. Their binary silver complexes have been prepared and structurally characterized by X-ray diffraction. [Ag(3,5-nBu2tz)]n and [Ag(3,5-iBu2tz)]n exhibit 3D structures of cds and dia topologies, respectively, while [Ag(3,5-sBu2tz)]n displays a 2D sql net and [Ag(3,5-tBu2tz)]4 is a 0D tetranuclear complex. Having the same empirical formula, these four binary silver triazolates are true isomers, differing in their linkage and triazole coordination modes. This work represents an interesting case in which ligand isomerism is manifested in the structures of the corresponding metal complexes.

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INTRODUCTION 1,2,4-Triazole and its derivatives are versatile ligands that have attracted much attention of inorganic chemists in recent years, partially for their rich structural chemistry upon binding to metals.1−4 Some binary Group 11 metal−1,2,4-triazolates have been structurally characterized by us and others. Therefore, for the general formula MI(3-R1-5-R2-tz), we are aware of only one binary gold(I) triazolate (M = Au; R1 = R2 = iPr),5 12 binary silver(I) triazolates (M = Ag, R1 = R2 = H, Me, Et, Pr, iPr, Ph, CF3, C3F7; R1 = H, R2 = Me, Et, Pr; R1 = Me, R2 = Ph),6−11 and six copper(I) triazolates (R1 = R2 = H, Me, Et, Pr, Bu, Ph),6,12 where tz− is 1,2,4-triazolate, R1 and R2 denote dangling 3,5substituents, regardless of possible isomers for each formula.13 Only two of the above compounds, [Au(3,5-iPr2-tz)]3 and [Ag(3,5-(C3F7)2-tz)]3, exhibit cyclic triangular structure,5,9 two more, [Cu(3,5-Ph2tz)]n and [Ag(3-Me-5-Ph-tz)]n, display a 1D infinite structure,6,11 while two further compounds, [Ag(tz)]n and [Cu(tz)]n, have similar 2D network.6 The remaining compounds have 3D framework structures of diverse topologies. It has become evident that the shape/size of 3,5-substituents is an important structural determinant for metal triazolates, which can be rationalized on the base of relief of the steric repulsions between approaching substituents when triazolato anions are brought to close proximity by coordination to a metal ion. In spite of the efforts devoted to understanding the structural role of substituents, however, even a rough prediction on the structures of metal triazolates remains far from reach. Pursuing further this goal, we now turn our attention to even © 2012 American Chemical Society

bulkier 3,5-substituents of triazole ring, such as the four isomers of butyl groups, namely, butyl (nBu), isobutyl (iBu), secondary butyl (sBu), and tertiary butyl (tBu) (Scheme 1). The four isomeric substituents have quite different shapes, from linear to approximately spherical. The structures of metal triazolates, especially binary ones, employing these new ligands will provide a good opportunity to demonstrate the role of the 3,5-substituent bulk on dictating their topology. In this article, we report the synthesis and structures of four isomeric binary silver(I) 3,5-dibutyl-1,2,4-triazolates, namely, 3D Ag(3,5nBu2tz) and Ag(3,5-iBu2tz), 2D Ag(3,5-sBu2tz), and 0D tetranuclear [Ag(3,5-tBu2tz)]4.

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EXPERIMENTAL SECTION

The CHN microanalyses were obtained with a Flash EA 1112 elemental analyzer. IR spectra (KBr pellets) were recorded on a Nicolet Impact 420 FTIR spectrometer. 1H NMR spectra were recorded on a Bruker DPX-400 spectrometer. Thermogravimetry and differential scanning calorimetry were carried out on a NETZSCH STA 409 PC system in air at a scanning rate of 10 °C min−1. The preparation of 3,5-dibutyl-1H-1,2,4-triazole has been reported by us recently,12 based on a modified literature method.14 Other triazoles were synthesized similarly. Racemic 2-methylbutanoic acid was used for the synthesis of the sec-Bu isomer, resulting in a statistical mixture of 50% (R,S), 25% (R,R), and 25% (S,S) 3,5-di-sec-butyl-1HReceived: November 20, 2011 Revised: February 19, 2012 Published: February 24, 2012 1882

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Scheme 1. Four Isomers of 3,5-Di-butyl-1H-1,2,4-triazole

Table 1. Crystallographic Data and Structure Refinement 3,5-tBu2tzH·1/2H2O formula mol. wt. crystal system Space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) ρcalcd (Mg/m3) μ (mm−1) Flack parameter reflns collected reflns unique final R indices [I > 2σ(I)] S Δρmax (eÅ−3)

C20H40N6O 380.58 monoclinic Pc (No. 7) 11.244(3) 21.760(7) 11.660(2) 90 119.976(18) 90 2471.2(11) 4 298 1.023 0.066 0.0 (19) 18 151 8538 (Rint = 0.0278) R1 = 0.0558 wR2 = 0.1443 0.989 0.328

[Ag(3,5-nBu2tz)]n C10H18AgN3 288.14 tetragonal P42/mmc (No. 131) 11.9330(10) 11.9330(10) 8.5486(14) 90 90 90 1217.3(2) 4 298 1.572 1.625 6175 887 (Rint = 0.0362) R1 = 0.0414 wR2 = 0.1230 1.185 0.618

[Ag(3,5-iBu2tz)]n C30H54Ag3N9 864.43 orthorhombic Fdd2 (No. 43) 16.077(3) 58.089(12) 16.088(3) 90 90 90 15025(5) 16 298 1.529 1.580 −0.02 (3) 39 289 7364 (Rint = 0.0375) R1 = 0.0342 wR2 = 0.0768 1.078 0.451

1,2,4-triazole. All other reagents were obtained commercially and used as received. Synthesis of 3,5-Di-tert-butyl-1H-1,2,4-triazole (3,5-tBu2tzH). Pivalic acid (0.02 mol, 0.20 g) and 80% hydrazine (0.02 mol, 1.5 mL) were sealed in a Teflon-lined autoclave and heated in an oven at 190 °C for 72 h. The resulting viscous liquid was neutralized with dilute HCl solution to pH ≈ 7 and white precipitate formed. The 1H NMR spectrum suggests that this crude product is a mixture of 4-amino-3,5di-tert-butyl-4H-1,2,4-triazole and 3,5-di-tert-butyl-1H-1,2,4-triazole with the former prevailing. This crude product was used directly in the following deamination process without purification. So, 4.3 g of the above-mentioned solid was dissolved in aqueous HCl (6M, 10 mL) and 30 mL of aqueous NaNO2 solution (1.5 g, 22 mmol) was added slowly with stirring while the temperature was maintained at 20 − 30 °C. After the addition was completed, the mixture was stirred for another 6 h at the same temperature until the gas evolution ceased. Then, the pH of the obtained solution was adjusted to ca. 7 by 10% NaOH, and the turbid solution was extracted with CH2Cl2 (3 × 50 mL). After removal of CH2Cl2 under reduced pressure, a white residue was obtained, which was recrystallized from EtOH−H2O (v/v = 1:1) to afford colorless crystals of 3,5-di-tert-butyl-1H-1,2,4-triazole. The overall yield of this procedure is 8−10%. Mp 142−144 °C. C10H19N3·1/2H2O (190.3), C 63.12, H 10.59, N 22.08; found, C 62.96, H 11.14, N 22.29. 1H NMR (ppm, CDCl3): δ = 1.38 (s, 18H, tBu). 13C NMR (ppm, CDCl3): δ = 29.33 (Me), 32.39 (quaternary C of tBu), 168.28 (3,5-C of tz ring). Synthesis of the Complexes. [Ag(3,5-nBu2tz)]n. A solution of AgNO3 (8.5 mg, 0.05 mmol) in 2 mL of 25% aqueous ammonia was mixed with a 2 mL of methanol solution of 3,5-dibutyl1,2,4-triazole (9.1 mg, 0.05 mmol). The resulting solution was

{[Ag(3,5-sBu2tz)]·1/3 H2O}n

[Ag(3,5-tBu2tz)]4

C15H28Ag1.5N4.5O0.5 441.22 orthorhombic Cmce (No. 64) 21.407(4) 18.825(4) 19.954(4) 90 90 90 8041(3) 16 93 1.458 1.479

C40H72Ag4N12 1152.58 orthorhombic Fddd (No. 70) 15.637(5) 26.005(8) 50.557(16) 90 90 90 20558(11) 16 298 1.490 1.539

31 244 4725 (Rint = 0.0285) R1 = 0.0826 wR2 = 0.2283 1.128 1.502

26 931 4532 (Rint = 0.0607) R1 = 0.0411 wR2 = 0.0906 0.998 0.740

allowed to evaporate slowly to produce colorless blocks of [Ag(3,5-nBu2-tz)]n in 65% yield within a couple of days. AgC10H18N3 (288.1), calcd. C 41.68, H 6.30, N 14.58; found, C 41.58, H 6.41, N 14.10. IR (KBr pellet, cm−1): 2959 (s), 2873 (m), 1763 (s), 1562 (s), 1505 (m), 1390 (m), 1068 (s), 826 (s), 732 (w). [Ag(3,5-iBu2tz)]n. This complex was prepared in a similar way to the method for [Ag(3,5-nBu2tz)]n, using 3,5-diisobutyl-triazole instead of 3,5-dibutyl-triazole. Yield: 55%. AgC10H18N3 (288.1), calcd. C 41.68, H 6.30, N 14.58; found, C 41.59, H 6.36, N 14.35. IR (KBr pellet, cm−1): 2959 (s), 2927 (w), 1710 (m), 1629 (s), 1383 (s), 1300 (m), 1219 (m), 1167 (s), 872 (w). [Ag(3,5-sBu2tz)]n. This complex was prepared in a similar way to the method for [Ag(3,5-nBu2tz)]n, using 3,5-di-sec-butyl-triazole instead of 3,5-dibutyl-triazole. Yield: 90%. AgC10H18N3 (288.1), calcd. C 41.68, H 6.30, N 14.58; found, C 41.47, H 6.44, N 14.17. IR (KBr pellet, cm−1): 3478 (m), 2962 (s), 2925 (m), 2873 (m), 1499 (w), 1460 (m), 1372 (w), 1348 (w), 1093 (w), 807 (w), 480 (w). [Ag(3,5-tBu2tz)]4. This complex was prepared in a similar way to the method for [Ag(3,5-nBu2tz)]n, using 3,5-di-tert-butyl-triazole instead of 3,5-dibutyl-triazole. Acetonitrile was used to dissolve triazole in this case. Yield: 42%. Ag4C40H72N12 (1152.4), calcd. C 41.68, H 6.30, N 14.58; found, C 41.69, H 6.58, N 14.67. IR (KBr pellet, cm−1): 2962 (s), 2925 (m), 2873 (w), 1499 (m), 1460 (m), 1372 (m), 1348 (m), 1093 (m), 807 (w), 480 (w). X-ray Crystallography. Diffraction intensities were collected on a Bruker SMART 1K CCD diffractometer (3,5-tBu2tzH and [Ag(3,5tBu2tz)]4), Bruker SMART Apex II CCD diffractometer ([Ag(3,5nBu2tz)]n and [Ag(3,5-iBu2tz)]n), or Rigaku Saturn 724+ CCD diffractometer ([Ag(3,5-sBu2tz)]n), with graphite-monochromated Mo−Kα radiation (0.71073 Å). Absorption corrections were applied 1883

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Figure 1. (a) Ball-and-stick diagram of 1D chain of (Ag2tz2)n in the structure of [Ag(3,5-nBu2tz)]n. Selected distances (Å) and angles (deg): Ag1−N1 (N2) = 2.307(5); Ag2−N4 = 2.057 (7); N−Ag1−N = 102.7(2), 112.94(13); N−Ag2−N = 180. (b) Top view (along c axis) of 1D chain of (Ag2tz2)n in the structure of [Ag(3,5-nBu2tz)]n. (c) Packing diagram of [Ag(3,5-nBu2tz)]n viewed down the c axis. The n-butyls have been omitted for clarity. Key: Ag, red; N, blue; C, gray spheres. (d) Simplification of the structure of [Ag(3,5-nBu2tz)]n to illustrate its cds net. Key: square planar 4connected node, yellow sphere. by using the multiscan program. The structures were solved by direct methods and refined by least-squares techniques using the SHELXS97 and SHELXL-97 programs.15 All non-hydrogen atoms were refined with anisotropic displacement parameters; hydrogen atoms were generated geometrically (see Table 1). CCDC 854448-854452 contains the supplementary crystallographic data for this article. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif.

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yield of only less than 10%. This low yield is not surprising considering the formerly observed substituent effect on the yield. However, the easy availability and low cost of the starting materials make the method a practical one for the preparation of the hitherto unknown and probably the sterically most crowded 3,5-disubstitued 1,2,4-triazole. Crystal Structure of [Ag(3,5-nBu2tz)]n. The asymmetric unit of [Ag(3,5-nBu2tz)]n consists of two one-eighth Ag atoms and one-fourth of a 3,5-nBu2tz− (Figure S5, Supporting Information). The Ag atoms are either tetrahedrally coordinated (for Ag1) or linear two-coordinated (for Ag2) with the former located at the site of -4m2 and the latter of mmm symmetry (Figure 1a). The triazolato moiety, in μ3-N1,N2,N4 bridging mode, is located on a crystallographically imposed mirror plane and bisected by a 2-fold axis along the Ag2−N4tz bond, and therefore only 1/4 of it is present in the asymmetric unit. The butyl group is half occupied in the asymmetric unit and disordered over two positions in the crystal structure, as required by the aforementioned mirror. In order to understand the 3D structure of [Ag(3,5nBu2tz)]n, we can first consider the nearly planar Ag2tz2 subunit formed by two Ag1 atoms and two triazolates via N1 and N2 atoms of triazole. This six-membered M2(N−N)2

RESULTS AND DISCUSSION

Synthesis of 3,5-Dialkyl-1,2,4-triazole. 3,5-Di-tert-butyl1H-1,2,4-triazole was prepared from the condensation of pivalic acid and hydrazine, followed by a deamination process. The method to prepare 4-amino-3,5-dialkyl-4H-1,2,4-triazole from organic carboxylic acid and hydrazine was first reported by Herbst and Garrison.14 It has been noted that larger alkyl group of the acid would lead to a lower yield of the corresponding 4amino triazole. An attempt to prepare 4-NH2-3,5-tBu2tz by this method was once reported to be unsuccessful, which was ascribed to the strong steric hindrance caused by tert-butyl groups.16 We carried out the synthetic procedure in a autoclave at 190 °C and isolated a white product, which proved to be a mixture of 4-NH2-3,5-tBu2tz and 3,5-tBu2tzH with an overall 1884

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structural motif is frequently encountered in metal−diazole chemistry, which can be regarded as the basic unit for many such complexes. Second, the [Ag2tz2]n chain is formed along the c axis by sharing the Ag atoms of Ag2tz2 subunits, with each Ag2tz2 subunit being twisted at an angle of 90° to its neighboring one (Figure 1b). Finally, the Ag2 atoms link the [Ag2tz2]n chains to form a 3D structure by coordinating to the external N4 atoms of the triazolates (Figure 1c). If the Ag2tz2 subunit is taken to be a rectangular fourconnected node, then the topology of [Ag(3,5-nBu2tz)]n is cds, namely, the CdSO4 topology (Figure 1d).17 Crystal Structure of [Ag(3,5-iBu2tz)]n. In the asymmetric unit of [Ag(3,5-iBu2tz)]n, there are three crystallographically independent Ag atoms and three 3,5-iBu2tz− anions (Figure S6, Supporting Information). All the Ag atoms are coordinated by three triazolato N atoms in a nearly T-shape, while all the triazolato ligands adopt μ3-N1,N2,N4 bridging mode (Figure 2a).

Although there are two crystallographically independent Ag2tz2 subunits, their topological role is identical, that is to say, both function as indistinguishable tetrahedral four-connected nodes in the dia net. Crystal Structure of {[Ag(3,5-sBu2tz)]·1/3H2O}n. While each [Ag(3,5-sBu2tz)] unit contains two chiral carbon centers, [Ag(3,5-sBu2tz)]n is crystallized in a nonchiral space group. The asymmetric unit of [Ag(3,5-sBu2tz)]n consists of two Ag atoms, Ag1 in a general position and Ag2 at a position of m site symmetry, and three half-triazolato moieties: one (N7, C4, N6) with a C2 axis passing through N4tz and the midpoint of N1tz and N2tz, containing homochiral sBu-groups, (R,R) or (S,S). Another (N4, C3, N5) with a vertical mirror plane containing N4tz and the midpoint of N1tz and N2tz to halve the meso-(R,S)sBu2-tz-ring. The third (N1, N2, N3, C1, C2) lies on a crystallographically imposed mirror plane with respect to which the sBu-groups are disordered; the handedness of these sBugroups therefore cannot be determined (Figure S7, Supporting Information). The Ag1 atom is tetrahedrally coordinated by four triazole N1 (or N2) atoms, while the linearly twocoordinated Ag2 atom links two triazole N4 atoms. The triazolate (N1, N2, N3, C1 and C2) acts as a μ 5 N1,N1′,N2,N2′,N4 bridging ligand, while two more triazolates behave in a common μ2-N1,N2 or μ3-N1,N2,N4 fashion (Figure 3a). This structure features an Ag4tz6 cluster, which can be regarded as formed by vertical insertion of an Ag2tz2 subunit to an approximately planar Ag4tz4 metallacycle (Figure 3a). It is different from what we have recognized as Ag4tz6-a and Ag4tz6b18 and is given here the symbol Ag4tz6-c in order to distinguish it from the former two. This cluster has also been observed in the structure of [Ag(3,5-Pr2tz)]n reported previously by us.7 Further connection of the Ag4tz6-c clusters from four of its six N4 positions by linearly two-coordinated Ag2 atoms results in a 2D structure, which can be simplified to the well-known 2D net −44·62 or sql, if the Ag4tz6-c clusters are regarded as the topological square planar four-connected nodes (Figure 3b).17 It is interesting to note that only four N4 atoms of Ag4tz6-c cluster are utilized to construct the 2D metal−organic framework in [Ag(3,5-sBu2tz)]n, while in [Ag(3,5-Pr2tz)]n, all six N4 atoms of the Ag4tz6-c cluster are engaged to form a 3D structure, which reflects the flexibility of binding of metals to triazole and also represents a strategy to build-up the frameworks of metal triazolates: the number of N4 atoms available for further connection in a polynuclear M-tz cluster (namely, SBU), formed by utilizing N1 and N2 atoms of tz−, is variable and dependent on the structural requirement of the system. Crystal Structure of [Ag(3,5-tBu2tz)]4. In the asymmetric unit of [Ag(3,5-tBu2tz)]4, there are four Ag atoms located at sites of 2 symmetry and two 3,5-tBu2tz− anions (Figure S8, Supporting Information). All the Ag atoms are approximately linearly two-coordinated by two N1 (or N2) atoms of triazole and the ligands adopt a μ2-N1,N2 coordination mode. The complexes thus formed are two kinds of crystallographically independent and structurally almost identical discrete tetranuclear complex, [Ag(3,5-tBu2tz)]4, of D2 symmetry (Figure 4). The four Ag atoms define a rhombus and four bridging triazolato ligands are located alternatively above and below the Ag4 plane; this arrangement is similar to that of [Ag(3,5tBu2pz)]4 (3,5-tBu2pz− = 3,5-di-tert-butylpyrazolate).19 All the N4 atoms of tz− remain uncoordinated, and therefore, a 0D

Figure 2. (a) Ball-and-stick diagram of a fragment of the 3D structure of [Ag(3,5-iBu2tz)]n, showing the connection between the subunits of Ag2tz2. Selected distances (Å) and angles (deg): Ag1−N1 (N2) = 2.157(3), 2.472(3); Ag1−N4 = 2.134 (3); Ag2−N1 (N2) = 2.178 (3), 2.414 (3); Ag2−N4 = 2.147 (3); Ag3−N1 (N2) = 2.177 (3), 2.419 (3); Ag3−N4 = 2.129 (3); N−Ag1−N = 89.34 (10), 110.85 (9), 159.73 (10); N−Ag2−N = 94.22 (10), 110.70 (10), 155.08 (10); N−Ag3−N = 93.51(10), 111.48(10), 154.77(10). Symmetry codes: (a) 1.25 − x, −0.25 + y, −0.25 + z; (b) 1.25 − x, 0.25 + y, 0.25 + z; (c) 1.5 − x, 0.5 − y, z; (d) x, 0.5 + y, −0.5 + z; (e) 1.5 − x, 1− y, −0.5 + z. Key: Ag, red; N, blue; C, gray spheres. The iso-butyls are omitted for clarity. (b) Simplification of the structure of [Ag(3,5-iBu2tz)]n to illustrate its dia net. Key: tetrahedral 4-connected node, yellow sphere.

The 3D structure of [Ag(3,5-iBu2tz)]n can be understood on the base of the Ag2tz2 subunit. However, unlike in [Ag(3,5nBu2tz)]n, the Ag2tz2 subunit here is folded at the two Ag atoms with the dihedral angle between two triazole rings being 80.7° and 92.4°, respectively. Each Ag2tz2 subunit is further connected to its neighboring four Ag2tz2 subunits from its two N4 atoms and two Ag atoms to form the 3D structure of dia (diamond) topology (Figure 2b).17 1885

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Figure 4. Ball-and-stick diagram of one of the two crystallographically independent molecules of [Ag(3,5-tBu2tz)]4. Selected distances (Å) and angles (deg): Ag1···Ag2 = 3.1137(10), Ag2···Ag2a = 3.2800(17), Ag1−N1 (N2) = 2.051(4); Ag2−N1 (N2) = 2.122 (4); N−Ag1−N = 163.9(2); N−Ag2−N = 179.9(3). Symmetry codes: (a) 1.25 − x, 0.25 − y, z. Key: Ag, red; N, blue; C, gray spheres. The tert-butyls are omitted for clarity.

This tetranuclear complex, [Ag(3,5-tBu2tz)]4, can be regarded as the molecular counterpart of Ag4tz4 SBU found in some silver triazolates.23 As far as we know, [Ag(3,5tBu2tz)]4 is the first discrete, neutral, tetranuclear silver triazolate, although the cationic [Ag(3,5-Me2tzH)]44+ was reported several years ago.24 Role of Substituents. So far there are 18 binary silver(I) 1,2,4-triazolates of known structure: two 0D complexes ([Ag(3,5-(C3F7)2-tz)]3 and [Ag(3,5-tBu2tz)]4), one 1D complex ([Ag(3-Me-5-Ph-tz)]n), two 2D complexes ([Ag(tz)]n and [Ag(3,5-sBu2tz)]n), and the remaining thirteen 3D complexes, of which only two, [Ag(3,5-(CF3)2tz)]n and {[Ag(3-Ettz)]·(H2O)}n, show porosity, reflecting a trend to form thermodynamically favored, compact, binary silver(I) triazolate structures. As depicted in Figure 5, the channels formed by Agtz skeletons of the 3D structures are actually occupied by the 3,5-substituents, leaving no voids for guest molecules. With the formation of Ag−Ntz bonds, the 3,5-substituents on triazoles are concomitantly brought to close proximity, for which weak hydrophobic attraction may exist. In the context of the materials described here, nestling these spectator pendant groups within the rather flexible Ag-tz skeleton is the key structure-dictating factor, resulting in the formation of the least porous possible structure. The comparison of the structure of [Ag(3,5-nBu2tz)]n with that of [Ag(3,5-iPr 2 tz)] n (qzd topology) provides an opportunity to comment on the role of substituents.7 The strategy to construct the 3D structures for both are similar: connecting (Ag2tz2)n chains from the N4 position by additional linear two-coordinated Ag atoms. The lengths of the a edges of two unit cells are in fact identical (11.932(3) Å for [Ag(3,5iPr2tz)]n and 11.933(1) Å for [Ag(3,5-nBu2tz)]n) because this length corresponds to the same tz-Ag-tz linker connecting (Ag2tz2)n chains in the structures of these two complexes. The difference lies in the relative orientation (or dihedral angle) for a pair of neighboring Ag2tz2 subunits in the chain of (Ag2tz2)n. This dihedral angle in [Ag(3,5-iPr2tz)]n is 60°, and as a result, a qzd net is formed. However, in [Ag(3,5-nBu2tz)]n, two adjacent

Figure 3. (a) Ball-and-stick diagram of an Ag4tz6-c cluster of the 2D structure of {[Ag(3,5-sBu2tz)]·1/3H2O}n; the four peripheral Ag2 atoms are also shown. Selected distances (Å): Ag1−N1 (N2) = 2.1348(14), 2.1920(10), 2.5557(10), 2.6309(11); Ag2−N 4 = 2.0803(14), 2.0875(13). (b) Perspective view of a layer of {[Ag(3,5sBu2tz)]·1/3H2O}n. Key: Ag, red; N, blue; C, gray spheres. The secbutyls are omitted for clarity.

molecular isomer was generated, which is in contrast with its other three polymeric isomers. The steric repulsion between the bulky tert-butyls may become prominent when two 3,5-tBu2tz ligands are approaching during the formation of Ag−N bonds. For a structurally similar ligand, 3,5-di-tert-butyl-pyrazole (3,5-tBu2pzH), it has been suggested, based on a theoretical calculation, that the closest allowed distances between N-atoms of a pair of approaching pyrazoles are approximately 3, 4, and 5.5 Å for the potential H-bonded dimer (C2h), saddle-like tetramer (S4), and trimer (C3h), respectively, in order to minimize the steric repulsions between bulky tBu-groups.20 We have rationalized the saddle-like structures of [MI(3,5-tBu2pz)]4 (M = Cu, Ag, Au) based on aforementioned arguments since the M−N distances for coinage metals are approximately 2 Å and N−M− N angles are close to 180°.19,21,22 Similarly, we may explain why in the solid state the present tetranuclear molecule is formed, rather than the alternative cyclic trinuclear arrangement observed in [Au(3,5-iPr2-tz)]3 and [Ag(3,5-(C3F7)2-tz)]3.5,9 1886

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Figure 5. (a) Fragment diagram of [Ag(3,5-nBu2tz)]n viewed down the c axis, showing the n-butyls in one channel. (b) Fragment diagram of [Ag(3,5-iPr2tz)]n viewed down the c axis, showing the iso-propyls in one channel. (c) Fragment of [Ag(3,5-iBu2tz)]n, showing the iso-butyls in the voids. H atoms have been omitted for clarity. Key: Ag, red; N, blue; C, gray spheres. 1887

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indicates that [Ag(3,5-tBu2tz)]4 may undergo a partial decomposition or rearrangement in CHCl3 (Figure S12, Supporting Information). Structurally similar pyrazolato analogues, [Au(3,5-tBu2pz)]4 and [Ag(3,5-tBu2pz)]4, were reported to have complicated solution chemistry. For example, the reaction of AuCl(SMe2) with 3,5-tBu2pzH in presence of a base resulted, based on Burini’s 1H NMR study, in a mixture of [Au(3,5-tBu2tz)]3 and [Au(3,5-tBu2tz)]4,27 the latter of which has been isolated by us using a similar reaction.21 [Ag(3,5tBu2pz)]4 shows two sets of 1H NMR resonances in CHCl3, the stronger one assigned to the Ag4 molecule, and the weaker tentatively assigned to the partial decomposition product [Ag(3,5-tBu2pzH)2]+.19 However, in a recent report, [Ag(3,5tBu2pz)]4 was demonstrated to show only one set of NMR signals.22 Considering these discrepancies and unresolved assignment of 1H NMR resonances, the solution chemistry of this type of tetranuclear complexes deserves a further systematic study.

Ag2tz2 subunits are set perpendicular to each other, and therefore a cds net is generated. Comparing the c edges of the [Ag(3,5-iPr2tz)]n and [Ag(3,5-nBu2tz)]n unit cells shows a ratio of 3:2, consistent with the fact that an (Ag2tz2)3 repeating unit is needed to construct the qzd structure for [Ag(3,5-iPr2tz)]n, while (Ag2tz2)2 repeating unit is required for the cds structure of [Ag(3,5-nBu2tz)]n. Since all other factors, such as reaction conditions and anion of metal salt are similar (if not identical) for preparing these two triazolates, the difference in the size/ shape of 3,5-substituents turns out to be the main factor causing the dissimilarity in their structures and topologies. As shown in Figure 5, the voids and channels in the Ag-tz skeletons of [Ag(3,5-iPr2tz)]n and [Ag(3,5-nBu2tz)]n are occupied by 3,5-substituents; the porosity for both is zero.25 In order to form a less porous (or more compact) structure, the smaller 3,5-substituents require a smaller space to be accommodated in the 3D framework of AgI-tz and vice versa. In the case of [Ag(3,5-nBu2tz)]n, the larger space required by disposition of larger butyls (compared to isopropyl) is achieved by twisting two neighboring Ag2tz2 units in the chain of (Ag2tz2)n to make a dihedral angle of 90°, which results in tetragonal channels along c axis, if the butyls are ignored. Another example to show the role of substituents is provided by comparison of the structures of [Ag(3,5-Me2tz)]n and β[Cu(3,5-Me2tz)]n of lig topology with the structures of [Ag(3Me-tz)]n and [Ag(3-Et-tz)]n of utp topology.7,10,13 Zhang et al., who reported the latter two structures, commented that the two closely related nets (lig and utp) arise from the different offsets of adjacent 41 helices. It is not surprising that these complexes utilize a similar construction strategy because all of them belong to the category of binary Group 11 metal−1,2,4-triazolate. However, if [Ag(3-Me-tz)]n adopted lig topology, there would be vacant volume generated since the position of one of the two methyl groups now is replaced by a H atom. Changing the linkage mode between the 41 helices while other part remains almost the same as in the lig arrangement seems to be the correct strategy toward achieving the more compact structure of utp topology. The structural role of the 3,5-substituents can be summarized as follows: the arrays of closely piled substituents act as the template to organize the Ag-tz skeletons to form the most compact structures. This conclusion appears to be valid for some binary copper(I) 1,2,4-triazolates, as discussed in our two recent articles involving a pair of complexes, the 2-fold interpenetrated α-[Cu(3,5-Me2tz)]n and [Cu(3,5-Bu2tz)]n,12,13 as well as another pair of complexes, [Cu(3,5-Bu2tz)]n and [Ag(3,5-(C5H11)2tz)]n.26 Thermal Analysis. The TG/DSC measurements under air show that these complexes are stable up to 200, 280, and 260 °C for [Ag(3,5-nBu2tz)]n, [Ag(3,5-iBu2tz)]n, and [Ag(3,5sBu2tz)]n, respectively (Figure S9−S11, Supporting Information). From the onset points, all complexes experience successive weight losses, corresponding to the decomposition of the triazolato ligands. The residues at 600 °C account for 36.8%, 36.7%, and 37.7% of the total mass, which suggests that the residues seem to be just Ag rather than the more likely formed Ag2O, based on the calculated residual percentages (37.4% for Ag vs 40.2% for Ag2O). A similar situation was met for the TG analyses of other binary silver triazolates.7 Solution Chemistry of [Ag(3,5-tBu2tz)]4. [Ag(3,5tBu2tz)]4 dissolves easily in CHCl3. Its 1H NMR spectrum shows three signals at 1.65, 1.51, and 1.36 ppm with the relative integrals being ca. 1, 5 and 13, respectively. This observation

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CONCLUSIONS This work reports the synthesis and crystal structures of four isomeric binary silver(I) 3,5-dibutyl-1,2,4-triazolates, 3D [Ag(3,5-nBu2tz)]n and [Ag(3,5-iBu2tz)]n, 2D [Ag(3,5-sBu2tz)]n, and 0D tetranuclear [Ag(3,5-tBu2tz)]4, together representing an interesting case in which ligand isomerism is manifested in the structures of the corresponding metal complexes. The fact that the bulk of the 3,5-subsituents dictates the structures of metal triazolates has again been affirmed. It is worth noting that the dimensions of the complexes have been reduced from 3D to 0D with the substituent becoming more spherical, reflecting the flexibility of the AgI-tz skeleton to adapt the change of the size/ shape of 3,5-substituents on triazole ring.

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ASSOCIATED CONTENT

S Supporting Information *

NMR and IR spectra, TG analysis, figures of the asymmetric units, 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the NSF of China (21071126). Work in UPR has been supported by the NASA University Research Center, Center for Nanoscale Materials (NCC3-1034).

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REFERENCES

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