Syntheses, Structures, and Properties of Honeycomb and Square Grid

DOI: 10.1021/cg9013398

Syntheses, Structures, and Properties of Honeycomb and Square Grid Coordination Polymers with In Situ Formed 5-(20 -Pyrimidyl)tetrazolate

2010, Vol. 10 2908–2915

Jian-Yong Zhang, Ai-Ling Cheng, Qian Sun, Qi Yue, and En-Qing Gao* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China Received October 27, 2009; Revised Manuscript Received May 23, 2010

ABSTRACT: Solvothermal reactions of Zn(II), Cd(II), Mn(II), Co(II), or Ni(II) salts and 2-pyrimidinecarbonitrile (pymCN) in the presence of NaN3 in DMF yielded seven metal-organic coordination polymers formulated as [Zn(pymtz)2]n (1), [NaMII(pymtz)3]n 3 G (M = Zn 2, Cd 3, Mn 4, Co for 5, and Ni for 6; G = DMF and/or H2O) and [Cd(pymtz)2]n (7) [pymtz = 5-(20 -pyrimidyl)tetrazolato]. Compounds 2-6 are isomorphous and all contain neutral honeycomb layers in which pymtz serves as a bis(chelating) bridge between M(II) (M = Zn for 2, Cd for 3, Mn for 4, Co for 5, and Ni for 6) and Naþ, and the ellipse stacking of the layers generates one-dimensional hexagonal channels. Compounds 1 and 7 exhibit different (4,4) square grid networks. In 1, the Zn ions are octahedrally coordinated and linked by pymtz ligands in the μ2 tridentate bridging mode, while in 7, the Cd(II) ions are eight-coordinated with a square prismatic geometry and bridged by bis(chelating) pymtz ligands. Compounds 2 and 3 can transform into 1 and 7, respectively, by immersing the sample in appropriate solvents. Interestingly, photoluminescence measurements on Zn(II) and Cd(II) compounds show that the emission bands can be correlated to the coordination modes of the pymtz ligands.

Introduction Immense current interest in the field of chemistry and material science has revolved around multidimensional inorganic-organic polymeric materials,1 which are not only excellent candidates to study interesting structural and topological novelties but also have potential applications in fields such as gas absorption and storage,2 magnetism,3 optical properties,4 etc. The selection of metal ions and bridging ligands with specific coordination preferences is crucial to the net topology and properties of the polymers obtained.5 In this context, the honeycomb (6,3) network, one of the most simple two-dimensional nets, has been pursued for a long time for the design of functional molecular materials. The dianionic oxalate ion, as the simplest bis(chelating) ligands, has been widely used to synthesize anionic honeycomb networks of the general formula [MIIMIII(ox)3]-.6 The neutral N-donor bis(chelating) ligand 2,20 -bipymidine (bpym) has also been used in the construction of neutral honeycomb networks with alternating bridging ligands (ox/bpym, azide/bpym, dca/ bpym, and SCN/bpym).7,8 Recently, a type of cationic honeycomb network has been constructed from divalent metal ions and a monoanionic bis(chelating) ligand, 2-pyrimidylcarboxylate.9 On the other hand, 5-(20 -pyrimidyl) tetrazolate (pymtz), which is also a potential bis(chelating) ligand with a mononegative charge, remains largely unexplored for the construction of coordination polymers. Only recently, a few one- and two-dimensional (1D and 2D) coordination compounds have been synthesized.10 However, no honeycomb networks have been reported for this ligand. In the past decades, the in situ [2 þ 3] cycloaddition reactions of nitriles with sodium azide in the presence of metal ions have been well established as convenient methods to synthesize coordination polymers with tetrazolate derivatives.11,12 *To whom correspondence should be addressed. Fax: þ86-10-62233424; e-mail: [email protected]. pubs.acs.org/crystal

Published on Web 06/08/2010

In this paper, we described the synthesis and characterization of seven compounds containing the pymtz ligand, which is formed in situ from the reaction of 2-pyrimidinecarbonitrile (pymCN) and sodium azide. The compounds are of the formula [Zn(pymtz)2]n (1), [NaMII(pymtz)3]n 3 G (M = Zn 2, Cd 3, Mn 4, Co for 5, and Ni for 6; G = DMF and/or H2O), and [Cd(pymtz)2]n (7). Compounds 2-6 are isomorphous and all contain neutral honeycomb layers in which pymtz serves as a bis(chelating) bridge between M(II) and Naþ, and the eclipsed stacking of the layers generates 1D hexagonal channels. Compounds 1 and 7 exhibit different (4,4) square grid networks. In 1, the metal ions are octahedral coordinated and linked by pymtz ligands in the μ2 tridentate bridging mode, while in 7, the Cd(II) ions are eight-coordinated with a square prismatic geometry and bridged by bis(chelating) pymtz ligands. Compounds 2, 3, 5, and 6 can transform into other compounds 1, 7, 11, and 12, respectively, with the common molecular formula [M(pmtz)2]n, which all exhibit the (4,4) square grid networks, by immersing the sample in appropriate solvents. The photoluminescence properties of the Zn(II) and Cd(II) compounds were investigated, and the results suggest that the emission bands may be correlated to the coordination modes of the pymtz ligands. Experimental Section Materials and Synthesis. All reagents were of A. R. grade and used without further purification. CAUTION! Although not encountered in our experiments, azido and tetrazolate compounds are potentially explosive. Only a small amount of compounds should be prepared and should be handled with care. [Zn(pymtz)2]n (1). A mixture of Zn(ClO4)2 (0.25 mmol, 0.067 g), pymCN (0.25 mmol, 0.02628 g), NaN3(0.375 mmol, 0.0244 g), and DMF (1 mL) was stirred for 30 min at room temperature and then was heated in a 23 mL Teflon-lined autoclave at 115 °C for 4 days. After cooling to room temperature slowly, rhombic crystals with only one pure phase were collected and dried in the air. Yield, ca. r 2010 American Chemical Society

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Crystal Growth & Design, Vol. 10, No. 7, 2010

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Table 1. Crystal Data and Structure Refinement Parameters for Compounds 1, 2, 3, 4, 5, 6, and 7 compound

1

2

3

4

empirical formula formula weight crystal system space group a, A˚ b, A˚ c, A˚ γ, ° V, A˚3 Z Fcalcd, g cm-3 μ, mm-1 θ range data/unique Rinta S on F2a R1, wR2 [I > 2σ(I)]a R1, wR2 (all data)a

C10H16N12Zn 359.64 orthorhombic Pbca 8.3400(8) 9.4598(9) 17.8056(17) 90 1404.8(2) 2 1.700 1.769 2.29-27.48 8097/1614 0.0336 1.026 0.0284, 0.0711 0.0453, 0.0800

C17H15.67N18.67O1.67NaZn 596.53 trigonal P31c 10.4655(6) 10.4655(4) 14.020(2) 120.000(10) 1329.8 (2) 2 1.490 0.993 2.25-26.99 7688/983 0.0280 1.258 0.0367, 0.1146 0.0399, 0.1158

C18H17N19O1.5NaCd 658.90 trigonal P31c 10.8014(5) 10.8014(5) 13.6319(12) 120 1377.36(15) 2 1.589 0.862 2.18-27.48 8205/1061 0.0222 1.188 0.0550, 0.1899 0.2090 0.0570, 0.1918 0.2106

C18H16N19ONaMn 592.43 trigonal P31c 10.6400(12) 10.6400(12) 13.795(3) 120 1352.5(4) 2 1.455 0.556 2.66-25.99 5897/872 0.0713 1.172 0.0874, 0.2544 0.2245 0.0934, 0.2609 0.2301

compound

5

6

7

empirical formula formula weight crystal system space group a, A˚ b, A˚ c, A˚ γ, ° V, A˚3 Z Fcalcd, g cm-3 μ, mm-1 θ range collected data/unique Rinta S on F2a R1, wR2 [I > 2σ(I)]a R1, wR2 (all data)

C18H16N19ONaCo 596.42 trigonal P3h1c 10.4089(15) 10.4089(15) 14.028(3) 120 1316.2(4) 2 1.505 0.708 2.26-25.99 5921/793 0.072 1.050 0.0453, 0.1189 0.0562, 0.1242

C18H17N19O1.5NaNi 605.21 trigonal P3h1c 10.3189(6) 10.3189(6) 14.0839(16) 120 1298.73(18) 2 1.548 0.820 2.28-26.97 7460/949 0.0268 1.110 0.0348, 0.0916 0.0376, 0.0926

C10H6N12Cd 406.67 tetragonal I41/amd 6.4690(3) 6.4690(3) 32.806(3) 90 1372.86(16) 4 1.968 1.612 2.48-27.47 3995/470 0.0155 1.161 0.0143, 0.0345 0.0146, 0.0347

a

The values for 2-6 are for the refinements after the SQUEEZE routine.

0.037 g (82% on pymCN). CHN element analysis: Calcd(%) for C10H6N12Zn%: C, 33.40; H, 1.68; N, 46.74. Found: C, 33.79; H, 1.91; N, 47.02. The main IR band (KBr, cm-1): 1570(vs), 1452(s), 1400(vs), 1262(w), 1187(w), 1125(w), 1089(w), 1043(w), 820(m), 737(m), 639(w). {NaZn(pymtz)3 3 2/3DMF 3 H2O}n (2). The preparation of compound 2 is similar to that of compound 1 except that Zn(ClO4)2 was replaced by ZnCl2. Colorless prism shaped crystals were obtained with only one pure phase. Yield, ca. 0.041 g (83% on pymCN). CHN element analysis: Calcd.(%) for (C17H15.67N18.67O1.67ZnNa%: C, 34.23; H, 2.65; N, 43.83. Found: C, 33.94; H, 2.78; N, 43.60. The main IR band (KBr, cm-1): 3448(br), 1682(s), 1580(vs), 1536(m), 1453(m), 1390(vs), 1261(w), 1190(w), 1135(w), 1088(m), 1057(w), 827(m), 738(s), 649(m). {NaCd(pymtz)3 3 DMF 3 1/2H2O}n (3) was prepared using a procedure similar to that adopted for 2 except with CdCl2 (0.1 mmol, 0.023 g), pymCN (0.3 mmol, 0.032 g), NaN3(0.3 mmol, 0.02 g) and DMF (1 mL) yield pale-yellow prism shaped crystals with only one phase. The yield (ca. 0.057 g) was about 87% on pymCN. Anal. Calcd. for C18H17N19O1.5CdNa%: C, 32.81; H, 2.60; N, 40.39. Found: C, 32.74; H, 2.68; N, 40.11. The main IR band (KBr, cm-1): 3442(br), 3063(w), 1683(vs), 1574(vs), 1528(s), 1452(m), 1391(vs), 1260(w), 1189(w), 1132(w), 1087(m), 1058(w), 824(m), 737(s), 647(s). {NaMn(pymtz)3 3 DMF}n (4) was prepared using a procedure similar to that of 3 except using MnCl2 (0.10 mmol, 0.02 g) in place of CdCl2 yield pale yellow needle-shaped crystals, which was contaminated by some brown powder, and the crystals can be collected manually, washed with water and dried in air. The yield (ca. 0.021 g) was about 36% on pymCN. Anal. Calcd. for C18H16N19OMnNa%: C, 36.50; H, 2.72; N, 44.93. Found: C, 36.94; H, 2.28; N, 45.10. The main IR band (KBr, cm-1): 3440(br),

3065(m), 1683(vs), 1580(vs), 1568(s), 1529(s), 1450(s), 1391(vs), 1264(m), 1234(w), 1186(m), 1135(w), 1087(m), 1060(m), 824(m), 739(s), 649(s). {NaCo(pymtz)3 3 DMF}n (5) was prepared using a procedure similar to that of 3 except using CoCl2 (0.10 mmol, 0.02 g) in place of CdCl2 yield brown prism-shaped crystals with only one phase. The yield (ca. 0.041 g) was about 68% on pymCN. Anal. Calcd. for C18H16N19OCoNa%: C, 36.25; H, 2.70; N, 44.62. Found: C, 35.83; H, 3.05; N, 43.99. The main IR band (KBr, cm-1): 3438(br), 3062(m), 1682(vs), 1583(vs), 1537(s), 1456(s), 1389(vs), 1262(m), 1235(w), 1189(m), 1134(w), 1090(m), 1019(w), 827(m), 737(s), 650(s). {NaNi(pymtz)3 3 DMF 3 1/2H2O}n (6) was prepared using a procedure similar to that of 3 except using NiCl2 (0.10 mmol, 0.02 g) in place of CdCl2 yield purple prism-shaped crystals with only one phase. The yield (ca. 0.032 g) was about 52% on pymCN. Anal. Calcd. for C18H17N19O1.5NiNa%: C, 35.73; H, 2.83; N, 43.98. Found: C, 35.83; H, 2.95; N, 44.01. The main IR band (KBr, cm-1): 3425(br), 3062(m), 1682(vs), 1585(vs), 1541(s), 1460(s), 1390(vs), 1262(m), 1232(w), 1192(m), 1138(w), 1089(m), 1053(m), 827(m), 739(s), 654(s). [Cd(pymtz)2]n (7) was prepared using a procedure similar to that of compound 3, except with CdCl2 (0.1 mmol, 0.023 g), pymCN (0.25 mmol, 0.026 g), NaN3 (0.3 mmol, 0.02 g) and DMF (8 mL) yield colorless bulk crystals with only one phase. The yield (ca. 0.024 g) was about 47% on pymCN. Anal. Calcd. for C10H6N12Cd%: C, 29.54; H, 1.49; N, 41.33. Found: C, 29.64; H, 1.79; N, 41.79. The main IR band (KBr, cm-1): 3074 (w), 1644 (w), 1568 (vs), 1450(s), 1402 (vs), 1259 (m), 1190 (m), 1125 (m), 1092 (m), 1041 (w), 819 (s), 736(s), 637(s). Physical Measurements. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. FT-IR

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spectra were recorded in the range 500-4000 cm-1 on a Nicolet NEXUS 670 spectrophotometer using KBr pellets. TG analyses were carried out on a Mettler Toledo TGA/SDTA851 instrument under flowing air at a heating rate of 5 °C min-1. Powder X-ray diffraction data were collected on a Bruker D8-ADVANCE diffractometer equipped with Cu KR at a scan speed of 1°/min. Excitation and emission spectra were recorded with an F-4500 instruments luminescence spectrometer. The energy dispersive X-ray spectroscopy (EDS) analyses were carried out with a S-4800 HITACHI scanning electron microscope. Crystallographic Studies. Diffraction intensity data were collected on a Bruker APEX II diffractometer equipped with a CCD area detector and graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). Empirical absorption corrections were applied using the SADABS program.13 The structures were solved by the direct method and refined by the full-matrix least-squares method on F2,14 with all non-hydrogen atoms refined with anisotropic displacement parameters. All the hydrogen atoms attached to carbon atoms were placed in calculated positions and refined using the riding model. The structures have a large volume fraction of solventaccessible voids [26.6 (2-Zn), 24.0 (3-Cd), 24.4 (4-Mn), 23.2 (5-Co), and 22.8% (6-Ni) of the unit cell volume] containing a number of residual electron density peaks, which may be attributed to solvent molecules. But the solvents could not be crystallographically defined satisfactorily, perhaps due to the heavy disorder problem. To improve the refinement, the SQUEEZE routine within the PLATON software package15 was applied to treat the diffuse electron density peaks. According to elemental analysis (EA), IR spectra and TGA analyses, the solvent molecules were proposed to be DMF and/ or H2O. The estimated numbers of the solvent molecules were in approximate agreement with the number of the electrons in the void given by SQUEEZE analysis. All the pymtz ligands in 2-6 display orientational disorder imposed by the crystallographic C2 axes bisecting the Ctz-Cpym bonds. The metal sites in 3 cannot be refined to have satisfactory displacement parameters by assigning fully occupied Cd and Na atoms, so it was assumed that the sites are occupied by a mixture of Cd and Na. The final refinement suggested that the Cd and Na ions are distributed over the two independent metal sites in the occupancy ratios of 0.9/0.1 and 0.1/0.9. Selected crystallographic data are summarized in Table 1.

Results and Discussion Synthesis. We have synthesized seven coordination polymers of the pymtz ligand by the hydro-/solvothermal reactions involving the in situ [2 þ 3] cycloaddition of pymCN with sodium azide in the presence of appropriate transition metal ions (Scheme 1). The synthetic reactions in DMF using Zn(ClO4)2, Zn(NO3)2, and ZnSO4 as the metal source all led to compound 1, which exhibits a 2D square grid coordination network, while the use of ZnCl2, ZnBr2, or Zn(OAc)2 in the same procedure led to compound 2, which exhibits a 2D heterometallic (Na-Zn) honeycomb network. This suggests that the anions, although not present in the final structures, play an important structuredirecting role. The products were also influenced by the solvent. When the reaction was performed in water using ZnCl2, Zn(ClO4)2, or Zn(OAc)2, a mononuclear complex [Zn(pymtz)2(H2O)2] (9) was obtained.16 The phase purity of the products has been confirmed by powder X-ray diffraction (PXRD) experiments (Figure S1, Supporting Information). For Cd(II), the synthetic factors influencing the products include the nature of the solvent, the ratio, and the concentration of the reactants. The reaction of CdCl2, pymCN, and NaN3 in a concentrated solution in DMF (0.1 mmol of Cd(II) in 1 mL of DMF) with a Cd-to-pymCN ratio of 1:1, a mixture containing prism shaped (3) and block (7) crystals was obtained. While compound 3 could be synthesized as pure phase by reducing the Cd-to-pymCN ratio in the

Zhang et al. Scheme 1. Syntheses of the Complexes Involving in Situ Ligand Reactions

reaction mixture to 1:3 or less, compound 7 has been synthesized as pure phase by performing the reaction in a much less concentrated solution (0.1 mmol of Cd(II) in 8 mL of DMF), with the Cd-to-pymCN ratio varying from 1:3 to 1:1. We have also performed the reactions in water, and the product is compound 8, which is a polymorph of 7 and has been reported elsewhere.9a,10c No dependence of the product upon the anion of the Cd(II) salt used was observed. The Mn(II) compound 4 could be synthesized from the solvothermal reaction of MnCl2, pymCN and NaN3 in DMF with a Mn-to-pymCN ratio of 1:3 or less. Increasing the Mnto-pymCN ratio or lowering the concentration did not lead to crystalline products. When the reaction was carried out in water, the product was a mononuclear complex, [Mn(pymtz)2(H2O)2] (10), which has been reported previously17 and is isomorphous with 9.16 When CoCl2 or NiCl2 was used in the synthetic procedure for compounds 2-4, brown and purple prism-shaped crystals of compound 5 and 6, respectively, were prepared as pure phases. When the reaction was performed in a much less concentrated solution (0.1 mmol of M(II) in 8 mL of DMF) or in water, the compounds of composition [M(pymtz)2]n (M = Co, 11 and Ni, 12) were obtained, which have been reported elsewhere10a,e and are isomorphous with compound 1. No dependence of the product upon the anion of the Co(II) or Ni(II) salt was observed. From these experiments, it can be seen that the final products of the reactions are dependent upon the metal salts, the solvents, the concentration, and ratio of the reactants. The Zn(II) products (1 and 2) in DMF are structurally directed by the anions of the Zn(II) salts used, although the mechanism is unclear. The Cd(II) products (3 and 7) in DMF

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Figure 1. The perspective view of the local coordination environment in 1; all hydrogen atoms are omitted for clarity.

are dependent upon the concentration and the Cd-topymCN ratio, with 3 preferred at a lower Cd-to-pymCN ratio and a higher concentration. This is consistent with the lower Cd-to-pymtz ratio in 3, and implies that the mixing of Na and Cd ions in 3 needs a higher concentration of the reaction mixture, which is supported by the observation that 3 can lose the sodium ions to give 7 when immersed in DMF (see below). The formation of compounds 4-6 with mixed Na and Mn, Co or Ni ions also need concentrated reaction mixures in DMF. Finally, the change of solvents from DMF to water led to different structures, all containing no Na ions. This is consistent with the observation that all the M-Na compounds (2-6) lose the Na ions in water (see below). Structural Description. Crystal Structure of [Zn(pymtz)2]n (1). X-ray analysis revealed that compound 1 crystallized in the Pbca space group and exhibits a 2D framework. The asymmetric unit consists of half of a Zn(II) atom and a pymtz ligand. As shown in Figure 1, located at an inversion center, the Zn atom assumes a slightly distorted octahedral coordination geometry, ligated by six nitrogen atoms from four pymtz ligands with the Zn-N distances in the range of 2.11-2.19 A˚. Two nearly coplanar equivalent pymtz ligands, each contributing a tetrazole nitrogen (N1) and a pyrimidyl nitrogen (N5), chelate the metal ion in the equatorial plane of the octahedron, and two tetrazole nitrogen atoms (N3 and N3B) belonging to another two pymtz ligands occupy the axial positions. The distortion of the metal polyhedron is mainly due to the small bite angle [N1-Zn1-N5, 77.895(2)o] in the five-membered chelating ring. The Zn-Ntz (equatorial) bond length (2.111(2) A˚) is shorter than Zn-Ntz(axial) [2.187 (2) A˚], while the Zn-Npym distance [2.167(2) A˚] lies between. As shown in Figure 1, the pymtz ligand in 1 serves as a μ2 tridentate bridging ligand using a pyrimidyl nitrogen (N5) and a tetrazole nitrogen (N1) to chelate a metal ion and another tetrazole nitrogen (N3) to bind another metal ion, and each metal ion is linked to four neighbors by four pymtz ligands, thus generating square-grid (4,4) sheets parallel to the ab plane (Figure 2a). Within the Zn4(pymtz)4 square units, the Zn 3 3 3 Zn edge distance linked by the tetrazole ring is 6.306(1) A˚, while the Zn 3 3 3 Zn distances through the diagonals along the a and b direction are 8.340(8) and

Figure 2. (a) View of the 2D sheet parallel to the ab plane in 1. (b) The (4,4) net topology of the 2D sheet showing the 2-fold alternately stacking fashion.

9.460(1) A˚, which correspond to the a and b unit cell parameters, respectively. As shown in Figure 2b, these layers are stacked alternately, so that the Zn centers of neighboring 2D layer are right above the mesh centers of another layer (i.e., an ABAB mode). There is some interdigitation between the pyrimidyl groups that protrude from different sheets, and each pyrimidyl ring from one sheet uses a CH group (C3) to form a weak hydrogen bond with an uncoordinated tetrazole N atom (N4) from another sheet (C 3 3 3 N, 3.494 A˚; C-H 3 3 3 N, 147.54o, and H 3 3 3 N, 2.674 A˚) (Figure S2, Supporting Information). Crystal Structures of Compounds 2-6. X-ray analysis revealed that compounds 2-6 are isomorphous, all crystallizing in the P3h1c space group and exhibiting 2D open honeycomb frameworks. As shown in Figure 3, there are two independent metal sites having the point symmetry of D3, and they all adopt the pseudo-octahedral MN6 coordination geometry completed by six nitrogen atoms from three pymtz ligands. The distortion of the geometry from ideal octahedron is mainly due to the significant deviations of the bite angles of pymtz from 90o. In 2 and 4-6, Na and M (Zn, Mn, Co, or Ni) reside on different sites, and the N-M-N bite angles (74.83-79.16°) are larger than N-Na-N (70.0-72.01°). Consistently, the Na-N bond lengths for (2.452-2.548 A˚) are significantly longer than M-N (2.089-2.275 A˚). Differently, the two

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Figure 4. (a) View of the 2D honeycomb network of compound 2-6, showing the cavity along the c direction; (b) view of the 1D channels produced by the packing of the honeycomb layers. Figure 3. Views showing the local coordination environments and disorder in 2-6; all hydrogen atoms omitted for clarity. (a) M = Zn for 2, Mn for 4, Co for 5, Ni for 6; (b) the Cd and Na ions are disordered in 3.

independent metal sites in 3 are occupied by a mixture of Cd and Na in the occupancy ratios 9/1 and 1/9, respectively, with Cd(Na)-N in the range of 2.257-2.514 A˚. The disordered distribution of the metal ions in 3 may be related to the similarity of Cd and Na in coordination bond distances. Each metal site surrounded by three pymtz groups defines a propeller-like chirality with Δ or Λ configurations.9c Differently from that in compound 1, the unique pymtz ligand in 2-6 serves as a bis(chelating) ligand to link the two different metal sites, with the metal-to-metal distance in the range of 5.95-6.24 A˚. The ligand resides on the C2 axis that passes through the bridged metal ions and defines the D3 symmetry of the metal sites. The C2 symmetry makes the pymtz ligand disordered over two opposite orientations. The pyrimidyl and tetrazole rings from the two disordered sets share the same N1-C1-N4 moiety (see Figure 3). Through the bridging ligands, each transition metal ion is connected with three Na ions, and vice versa, generating a (6,3) honeycomb network in the ab plane (Figure 4a), with a perfect alternation of Δ and Λ chiral sites. The 2D network possesses cavities with radii of about 4.6 A˚ (the shortest distances between the cavity centers and any framework atoms). The parallel sheets, with the equivalent metal sites in neighboring sheets having opposite chiral configurations, are stacked in a cavity-above-cavity (or framework-aboveframework) fashion to result in an infinite 1D hexagonal channel along the c direction (Figure 4b). The metal sites from different sheets alternate in the M-Na-M-Na sequence

along the c direction, with an intersheet M 3 3 3 Na distance being in the range of 6.81-7.04 A˚. PLATON calculations18 revealed that the solvent accessible void volumes comprise 22.8-26.6% of the crystal volumes. Although the electronic residues in the voids could not be crystallographically modeled, the voids are assumed to be occupied by heavily disordered DMF and water molecules, according to IR, thermogravimetric analysis (TGA) and element analysis data. Crystal Structure of [Cd(pymtz)2]n (7). This structure has been described in our previous communication,9a and for the convenience of comparison, here we describe it briefly. This compound crystallized in the tetragonal space group I41/amd. The asymmetric unit is only 1/8 of the formula, with the Cd(II) ion residing at the crystallographic D2d position and the pymtz ligands having the C2v symmetry. The MN8 coordination geometry can be described as a highly distorted square prism with an llll arrangement of the pymtz ligands:19 each ligand is parallel to the unique axis of the prism (along the c direction) and defines one of the four lateral (l) edges. The basal face of the prism consists of alternating Npym and Ntz atoms, and the distortion of the prism is mainly reflected by the folding of the base about the Npym 3 3 3 Npym diagonal, which is related to the relative slipping of the adjacent ligands along the unique axis of the prism. The prismatic llll arrangement of the ligands dictates the [Cd(N-C-C-N)4] unit as a square building block (SqBU), and hence 2D grid networks with the (4,4) net topology are the outcomes of selfassembly. The edges of the square meshes (Cd 3 3 3 Cd distances spanned by the ligands) are 6.469 A˚ (Figure 5). The noncentrosymmetric grid layers in 7 are parallel to the ab plane and stacked in an offset ABCDABCD fashion, with a interlayer separation of 8.201 A˚ (c/4). Interlayer

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Figure 5. View of the structure of 7, showing the environment of Cd(II) and pymtz and the resulting square grid network.

interdigitation occurs with the pyrimidyl CH groups of a layer protruding toward the mesh centers of the two neighboring layers. The offset and interdigitating stacking reduces and blocks the void spaces in the structure. Equivalent weak C-H 3 3 3 N hydrogen bonds are operative between neighboring layers (H 3 3 3 N = 2.72 A˚, C-H 3 3 3 N = 145.7°): each pyrimidyl C3-H group from a layer acts as a bifurcated donor to form two hydrogen bonds with two N2 atoms of a tetrazole ring from another layer, and each N2 atom acts as a bifurcated acceptor to form two hydrogen bonds with two C3-H groups from different pyrimidine rings (Figure 6). Structural Transformation. We have described some Zn(II), Cd(II), Mn(II), Co(II), and Ni(II) compounds with the pymtz ligand. Under appropriate conditions, all the five metal ions can give heterometallic honeycomb layers of general stoichiometry [NaM(pymtz)3], in which octahedral coordinated metal ions are linked by the bis(chelating) pymtz ligands. The incorporation of the Naþ ions plays an important role in generating the neutral honeycomb structures: it serves as the bridge between the [MII(pymtz)3]- units, with a perfect match in charge. However, perhaps due to the poor coordination ability of Naþ to nitrogen atoms, the formation of the honeycomb compounds is very sensitive to synthetic conditions. For example, the variation of the anions in the starting zinc salts may lead to a completely different monometallic compound (1), in which octahedral Zn(II) ions are linked into neutral (4,4) layers through tridentate ligands. In the case of Cd(II), lowering the concentration of the reaction system led to compound 7, in which the eight-coordinated Cd(II) ions are linked into another type of neutral (4,4) layer through bis(chelating) pymtz ligands. For all the five metal ions, the reactions in aqueous media also yielded different compounds, in which no Naþ ion is present. These observations may imply solvent-induced transformation of these compounds. The structural transformation for Cd(II) and Zn(II) compounds has been confirmed by IR, PXRD and EDS experiments. When the crystals of compound 2 were immersed in


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Figure 6. The ABCD stacking of the layers via C-H 3 3 3 N hydrogen bonds in 7.

Figure 7. PXRD patterns. (a) Calculated for 2; (b) as-synthesized 2; (c) compound 1 obtained by immersing 2 in methanol; (d) calculated for 1.

methanol or water at room temperature, they pulverized slowly. The IR band observed at 1683 cm-1 in 2, which is attributable to the ν(CdO) vibration of the DMF guest molecules, disappears in the pulverized sample. Actually, the IR spectrum (Figure S3, Supporting Information) and the PXRD pattern (Figure 7) of the pulverized sample coincides with those of 1, suggesting that 2 has transformed into 1. In addition, the Na and O peaks observed in the EDS spectra of 2 disappear in the spectra of the pulverized sample (see Figure S4, Supporting Information), further supporting the transformation from 2 to 1. According to the compositions of 1 and 2, the transformation corresponds to the release of an Naþ cation and a pymtz- anion per formula into the solvent. This may be completed through the dissolution and recrystallization processes. The release of Naþ and pymtz- has been confirmed by IR and EDS analysis on the solid residue of the filtrate obtained by filtering off the pulverized solid: the IR spectrum clearly suggested the presence of the pymtz ligand, while the EDS analysis suggested the presence of Na, C and N, with no evident signal of

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Figure 8. PXRD patterns (a) calculated for 3; (b) as-synthesized 3; (c) compound 7 obtained by immersing 3 in methanol; (d) calculated for 7.

Cd. It is noted that the transformation is solvent selective and does not occur in DMF or ethanol. Compound 3 can transform into 7 upon immersing it in water, methanol, ethanol, or DMF at room temperature, as confirmed by IR, PXRD, and EDS characterizations (see Figure 8, S5 and S6, Supporting Information). In addition, when compound 7 was heated in water under reflux for three days, it turned into compound 8 (see Figure S7, Supporting Information), which has been prepared from the aqueous solution.10c Compound 8 has the same stoichiometry as 7, and also exhibits 2D grid networks in which eight-coordinated Cd(II) ions are linked by bis(chelating) pymtz ligands. The difference mainly lies in the coordination geometry. The four pymtz ligands around each Cd in 8 define a square antiprism instead of square prism. Both 7 and 8 remained unchanged after being heated in DMF at 120 °C for several days. These preliminary observations suggest that compound 8 is thermodynamically more stable in the presence of water than compound 7. When the two samples were heated in the solid state, no evidence of transformation was observed, suggesting that the transformation from 7 to 8 in water may also involve dissolution and recrystallization. Compound 4 changes into an amorphous phase in common solvents such as DMF, methanol, ethanol, and water. It is noted that, although synthesized from DMF, compounds 3 and 4 are unstable in the same solvent, implying that the Naþ ions in the solvent are important in stabilizing the solid structures of 3 and 4. The structural transformation of compounds 5 and 6 are similar to that of 2. When 5 and 6 are immersed in water or methanol, the crystals pulverized slowly, and the IR and PXRD measurements (see Figure S8 -S11, Supporting Information) suggested that they had changed into the previously reported compounds 11 and 12,10a,e which are isomorphous with compound 1. The transformation is also solvent selective, and does not occur in ethanol. Compounds 5 and 6 are soluble in DMF. Thermal Gravimetric Analysis. TGA (see Figure S12, Supporting Information) of compound 1 in air atmosphere reveals that there is no weight loss from room temperature to 310 °C. For compound 2, the weight loss (10.8 wt %) in the range of 130-240 °C is attributable to the loss of the DMF and H2O guest molecules included in the channels (calcd. 11.19% for 2/3 DMF and H2O per formula). Compound 3 showed a weight loss of 12.0% (calcd. 12.45%) from room temperature to 165 °C corresponding to the removal of one DMF and 0.5 H2O molecules per formula. Compound 4 showed a weight loss of 12.1% (calcd. 12.34%) for the removal of DMF molecules in the range of 110-220 °C.

Zhang et al.

Figure 9. The solid-state luminescence spectra for the three compounds: compound 1 (a, ex = 383 nm), compound 2 (b, ex = 335 nm), compound 3 (c, ex = 362 nm), and compound 7 (d, ex = 338 nm).

The included guest molecules lost from room temperature to 265 °C (found: 12.5%, calcd. 12.3%) for 5 and from room temperature to 260 °C for 6 (found: 13.2%, calcd. 13.6%). The desolvated samples of 2-6 exhibit PXRD patterns different from those for the as-synthesized samples and exhibit no appreciable N2 absorption, suggesting that the frameworks collapse into nonporous structures upon desolvation and that the solvent molecules play important template and stabilization roles in the honeycomb structures. Photoluminescence. Previous studies have shown that Cd(II) and Zn(II) coordination polymers with tetrazole ligands exhibit photoluminescence properties.20-22 Hence, we investigated the photoluminescence properties of 1, 2, 3, and 7. The emission spectra of these compounds in the solid state at room temperature are shown in Figure 9. Interestingly, there seems to be a correlation between the emission bands and the coordination modes of the pymtz ligands. The (6,3) and (4,4) layer compounds 2, 3 and 7, in which the ligand assumes the bis(chelating) bridging mode, exhibit maximum emissions at similar wavelengths (417, 418, and 411 nm for 2, 3, 7, respectively). The previous compound 8 with bis(chelating) pymtz was reported to display photoluminescence at a similar position (416 nm).10c However, compound 1, which contains μ2-tridentate pymtz ligands, shows an emission band around λmax = 483 nm, which is much red-shifted compared with the proceeding compounds. Tentatively, all these emissions are attributable to intraligand transitions, following previous reports on Zn(II) and Cd(II) compounds with bridging tetrazole ligands.21,22 The difference between 1 and others in emission positions reflects the strong influence of the coordination modes upon the intraligand energy levels. Conclusions In summary, we have isolated and characterized seven coordination compounds containing pymtz ligand which is formed in situ from pymCN, NaN3 and Zn(II), Cd(II), Mn(II), Co(II), and Ni(II) salts under solvothermal conditions. In compound 1, the octahedral coordinated Zn(II) ions are linked into a (4,4) square grid through the μ2 tridentate bridging pymtz ligand. Compound 2-6 are isomorphous in which the pymtz serves as bis(chelating) bridging ligands and links the M (M = Zn(II) for 2, Cd(II) for 3, Mn for 4, Co for 5, and Ni for 6) and Naþ into neutral honeycomb networks and displays 1D hexagonal channels. The incorporation of the Na(I) ion plays an important role in generating the structures: it serves as the bridge between the [MII(pymtz)3]- units, with a perfect match in charge. In compound 7, the eight coordinated Cd(II) ion

Article


assumes square prismatic coordination geometry and is bridged by four bis(chelating) pymtz ligands into 2D (4,4) square grid. We also investigated the stabilities of these compounds: Compounds 2 and 3 can transform into compounds 1 and 7, respectively. The transformation involves the dissociation of Naþ into the solvent followed by the destruction/rearrangement of the coordination interactions between Zn(II)/Cd(II) and the pymtz ligands. Compound 7 can changed into 8 when it is refluxed in water while 8 cannot transform into 7. These reactions are solvent selected and may be completed through the dissolution and recrystallization processes. The photoluminescence properties of these compounds in the solid state are also investigated, and interestingly, there seems to be a correlation between the emission bands and the coordination modes of the pymtz ligands. Acknowledgment. We are thankful for the financial support from NSFC (20571026 and 20771038), Shanghai Leading Academic Discipline Project (B409), and “PhD program Scholarship Fund of ECNU 2009” (2009015). Supporting Information Available: Crystallographic data in CIF format for all the crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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