Three Coordination Polymers Based on 1H-Tetrazole (HTz

Nov 24, 2009 - Shaoguan University, Shaoguan 512005, P. R. China. Received September 15, 2009; Revised Manuscript Received November 5, 2009...
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DOI: 10.1021/cg901128k

Three Coordination Polymers Based on 1H-Tetrazole (HTz) Generated via in Situ Decarboxylation: Synthesis, Structures, and Selective Gas Adsorption Properties

2010, Vol. 10 739–746

Di-Chang Zhong,† Wen-Guan Lu,‡ Long Jiang,† Xiao-Long Feng,† and Tong-Bu Lu*,† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, and School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China and ‡Department of Chemistry, Shaoguan University, Shaoguan 512005, P. R. China Received September 15, 2009; Revised Manuscript Received November 5, 2009

ABSTRACT: Three tetrazole-based coordination polymers, [Zn3(Tz)6(H2O)2]n (1), [Cu2(Tz)3(OH)]n (2), and {[Cu5(Tz)9](NO3) 3 8H2O}n (3) (Tz = tetrazolate) have been prepared under hydro(solvo)thermal conditions using ethyl tetrazolate5-carboxylate and corresponding metal salts as building blocks, in which tetrazolate was generated via in situ decarboxylation. The results of X-ray single diffraction analyses indicate that 1 possesses a 2D layer structure with μ2-Tz- bridged six- and fourcoordinated Zn(II), forming a rare kagom e dual topological layer. Compound 2 is a 3D layer-pillar structure, in which the Cu(II) ions are bridged by μ3-Tz- anions to generate a 2D sheet, and the 2D sheets are further pillared by μ2-Tz- and μ2-OH- to form a 3D framework with a fsc topology net. Compound 3 is a 3D porous framework with an acs topology net. Compound 1 exhibits photoluminescent properties with an emission peak at 467 nm, and compound 3 shows selective H2 and CO2 over N2 adsorption.

Introduction In the past decade, coordination polymers based on very small polydentate linkers such as polynitrogen heterocycles and their derivatives have attracted much attention due to their potential applications for gas storage and separation,1 molecular magnets,2 and photoluminescent materials.3 The representative polynitrogen heterocycles such as triazolates,4 tetrazolates,5 triazines,6 and tetrazines7 have been used for the construction of coordination polymers. Among them, 1Htetrazole (HTz) and tetrazolate (Tz-) draw particular attention due to their rich coordination modes (Scheme 1) and interesting structures.5 However, up to now, only Cu(I)/Cu(II), Ag(I), Zn(II), and Cd(II) coordination polymers with tetrazolate have been directly obtained by the reactions of HTz with corresponding metal salts under hydro(solvo)thermal conditions.8 In situ ligand synthesis, first proposed by Champness and Schr€ oder in 1997,9 is regarded as a new and efficient approach for the synthesis of both organic compounds and coordination compounds, because of its advantage in simplicity, effectiveness, and environmental friendliness. Recently, in situ ligand synthesis has been developed rapidly in coordination polymers, as it usually generates unexpected structures, which cannot be obtained by the direct reactions of the ligands with metal salts.10 Among the methods of in situ ligand synthesis, decarboxylation is a powerful technique for the constructions of coordination polymers. Several coordination polymers with aromatic carboxylate and azamacrocyclic carboxylate prepared by the in situ decarboxylation ligand synthesis have been reported.11 Recently, we used ethyl tetrazolate-5carboxylate (ETzc) as a ligand to react with Cd(NO3) 3 4H2O to get a 3D microporous metal-organic framework of

{[Cd5(Tz)9](NO3) 3 8H2O}n through in situ decarboxylation of ETzc, which exhibits a strong hydrogen binding property.12 It is interesting to note that the direct reaction of HTz with Cd(NO3) 3 4H2O under similar reaction condition gave different 3D coordination polymers of {[Cd5(Tz)9](OH)(H2O)} 3 5H2O}n rather than {[Cd5(Tz)9](NO3) 3 8H2O}n.8b Continuing our research on this in situ decarboxylation reaction of ETzc, three new coordination polymers of [Zn3(Tz)6(H2O)2]n (1), [Cu2(Tz)3(OH)]n (2), and {[Cu5(Tz)9](NO3) 3 8H2O}n (3) were obtained. In this paper, we reported the synthesis, structures, and luminescent and selective gas adsorption properties of 1-3, which were generated through in situ decarboxylation reactions of ETzc under hydro(solvo)thermal conditions. Experimental Section

*To whom correspondence should be addressed. Fax: þ86-20-84112921. E-mail: [email protected].

General Remarks. All of the chemicals are commercially available and used without further purification. Ethyl tetrazolate-5-carboxylate (ETzc) was purchased from Nanjing Tianzun chemical Co., Ltd. Elemental analyses were determined using an Elementar Vario EL elemental analyzer. The IR spectra were recorded in the 4000-400 cm-1 region using KBr pellets and a Bruker EQUINOX 55 spectrometer. The thermogravimetric analysis (TGA) was carried out on a NetzschTG-209 instrument in air atmosphere. The powder X-ray diffraction patterns (XRD) were recorded on a D8 ADVANCE X-ray diffractometer. Photoluminescent measurements were conducted on a RF-5301PC spectrometer. [Zn3(Tz)6(H2O)2]n (1). A mixture of Zn(Ac)2 3 2H2O (0.5 mmol, 0.110 g), ethyl tetrazolate-5-carboxylate (ETzc) (0.5 mmol, 0.071 g), NaOH aqueous solution (1 mL, 1M), and methanol (4 mL) was heated at 160 °C for 72 h in a 20 mL sealed Teflon-lined stainless steel vessel. After the autoclave was cooled over a period of 26 h at a rate of 5 °C 3 h-1, block-shaped colorless crystals of 1 were isolated by filtration, washed with methanol, and dried in air. Yield: 0.028 g, 52% based on ETzc. Anal. Calcd for C6H10N24O2Zn3: C, 11.15; H, 1.56; N, 52.00%. Found: C, 11.09; H, 1.60; N, 52.12%. IR data (KBr, cm-1): 3326vs, 3152m, 1624m, 1553s, 1383w, 1327m,

r 2009 American Chemical Society

Published on Web 11/24/2009

pubs.acs.org/crystal

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Scheme 1. Known Coordination Modes of HTz and Tetrazolate

Table 1. Crystal Data and Structure Refinements for 1-3a compound 1 2 3 C3H4N12OCu2 C9H25N37O11Cu5 formula C6H10N24O2Zn3 fw 646.50 351.26 1145.36 temp/K 173(2) 173(2) 173(2) 3 0.28  0.16  0.11 0.24  0.18  0.16 0.32  0.30  0.23 crystal size/mm crystal system trigonal orthorhombic hexagonal Pnma P6(3)/mmc space group P3 a (A˚) 9.4502(9) 9.5513(1) 12.3634(19) b (A˚) 9.4502(9) 14.0864(2) 12.3634(19) c (A˚) 6.3642(11) 7.5528(1) 12.539(4) 492.22(11) 1016.18(2) 1659.9(6) V (A˚3) Z 1 4 2 2.181 2.296 2.292 Dc (g cm-3) 3.696 5.350 3.266 μ (mm-1) 2403/720 (0.0237) 3606/840 (0.0206) 7185/706 (0.1426) data collected/uniq (Rint) 0.0278, 0.0694 0.0420, 0.1255 0.0800, 0.2140 R1, wR2 [I > 2σ(I)] 0.0354, 0.0742 0.0466, 0.1289 0.1489, 0.2720 R1, wR2 (all data) 1.023 1.034 1.012 GOF on F2 0.563/-0.717 0.469/-0.629 2.128/-1.426 residues (e A˚-3) P P P P a R1 = || Fo| - | Fc||/ | Fo|. wR2 = [ [w(Fo2 - Fc2)2]/ w(Fo2)2]1/2, w = 1/[σ2(Fo)2 þ (aP)2 þ bP], where P = [(Fo 2) þ2Fc2]/3.

1248s, 1236s, 1152s, 1115s, 1079s, 1044s, 1002s, 901m, 736s, 727m, 690m. [Cu2(Tz)3(OH)]n (2). A mixture of CuCl2 3 2H2O (0.5 mmol, 0.085 g), ETzc (0.5 mmol, 0.071 g), NaOH aqueous solution (1 mL, 1 M), and distilled water (8 mL) was heated at 160 °C for 72 h in a 20 mL sealed Teflon-lined stainless steel vessel. After the autoclave was cooled over a period of 26 h at a rate of 5 °C 3 h-1, dark blue block-shaped crystals of 2 were isolated by filtration, washed with water, and dried in air. Yield: 0.039 g, 67% based on ETzc. Anal. Calcd for C3H4N12OCu2: C, 10.26; H, 1.15; N, 47.85%. Found: C, 10.31; H, 1.11; N, 47.91%. IR data (KBr, cm-1): 3392m, 3144m, 3122s, 1636w, 1478m, 1462m, 1444m, 1384m, 1345w, 1327w, 1233m, 1209m, 1177s, 1151s, 1141s, 1130s, 1111m, 1044m, 1027m, 1009w, 938w, 875w, 689m, 584m. {[Cu5(Tz)9](NO3) 3 8H2O}n (3). This green compound was obtained by a similar procedure to that for 2 except using Cu(NO3)2 3 4H2O instead of CuCl2 3 2H2O. Yield: 0.046 g, 73% based on ETzc. Anal. Calcd for C9H25N37O11Cu5: C, 9.44; H, 2.20; N, 45.25%. Found: C, 9.38; H, 2.13; N, 45.35%. IR data (KBr, cm-1): 3453vs, 3149w, 1655w, 1625m, 1452m, 1384vs, 1317w, 1230m, 1164s, 1056m, 1020m, 890w, 694s. X-ray Crystallography. Single-crystal data for 1 and 3 were collected at 173 K on a Bruker Smart 1000 CCD diffractometer, with Mo KR radiation (λ = 0.71073 A˚), and the empirical absorption corrections were applied using the SADABS program.13 Singlecrystal data for 2 were collected at 153 K on a Oxford Gemini S Ultra diffractometer, with Cu KR radiation (λ = 1.54178 A˚), and the empirical absorption corrections were applied using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. All the structures were solved using direct methods, which yielded the positions of all non-hydrogen atoms. These were refined first isotropically and then anisotropically. The disorder in the rings of the μ3-tetrazole ligands in 3 was treated by performing halfoccupancies with the C and N atoms. All the hydrogen atoms of the ligands were placed in calculated positions with fixed isotropic thermal parameters and included in the structure factor calculations in the final stage of full-matrix least-squares refinement. The hydrogen atoms of water molecules in 1 and the hydroxyl group in 2 were assigned in the difference Fourier maps and refined isotropically, and the hydrogen atoms of water molecules in 3 were not assigned. All calculations were performed using the SHELXTL

system of computer programs.14 The crystallographic data are summarized in Table 1, and the selected bond lengths and angles are listed in Table 2. Gas Sorption Measurements. The gas sorption experiments were measured with a BELSORP-max gas adsorption instrument. Both the N2, CO2, and H2 adsorption isotherms for 3 were collected in a relative pressure range from 10-4 to 1.0 atom. The cryogenic temperatures of 77 K required for N2 and H2 and 87 K for H2 sorption measurements were controlled by liquid nitrogen and liquid argon, respectively, and the 195 K required for CO2 sorption measurements was controlled using a dry ice-acetone bath. The initial outgassing process for the sample was carried out under a high vacuum (less than 10-6 mbar) at 120 °C for 10 h. The degassed sample and sample tube were weighed precisely and transferred to the analyzer.

Results and Discussion Preparation Chemistry. Under hydro(solvo)thermal conditions, the reaction of Cu(II) or Zn(II) salt with ethyl tetrazolate-5-carboxylate, which is easy to hydrolyze to yield tetrazole-5-carboxylate (TZc), afforded three coordination polymers of [Zn3(Tz)6(H2O)2]n (1), [Cu2(Tz)3(OH)]n (2), and {[Cu5(Tz)9](NO3) 3 8H2O}n (3), respectively. Tetrazole based coordination polymers are usually synthesized by direct reactions of metal salts with tetrazole ligand8b or by in situ metal/ligand reaction through [2 þ 3] cycloaddition reactions of nitriles and azide.5d,15 We found that coordination polymer based on tetrazole can also be obtained through in situ decarboxylation of ETzc under hydro(solvo)thermal conditions.12 It is difficult to understand the reaction mechanism of in situ ligand synthesis due to the complexity occurring in the hydro(solvo)thermal reaction at high temperature and pressure.16 Crystal Structures. As shown in Figure 1a, there are two crystallographically independent Zn(II) ions in 1. Zn1 is sixcoordinated with six N atoms from six individual Tzanions, forming a slightly distorted octahedral geometry,

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Table 2. Selected Bond Lengths (A˚) and Angles (deg) for 1-3a

Zn(1)-N(4) N(4)#1-Zn(1)-N(4) N(4)#4-Zn(1)-N(4) O(1)-Zn(2)-N(1)#6 N(1)-Zn(2)-N(1)#7 O1-H1B O1-H1A O1-H1A O1-H1B 3 3 3 N3#8

2.172(2) 88.30(8) 91.70(8) 114.19(7) 104.37(8) 0.85 0.85 0.85 122.6

Zn(2)-N(1) N(4)#2-Zn(1)-N(4) N(4)-Zn(1)-N(4)#5 N(1)-Zn(2)-N(1)#6 H1B 3 3 3 N3#8 H1A 3 3 3 N3#9 H1A 3 3 3 N2#9 O1-H1A 3 3 3 N3#9

Cu(1)-O(1) Cu(1)-N(5) O(1)-Cu(1)-N(4)#2 O(1)-Cu(1)-N(5) O(1)-Cu(1)-N(2)

1.896(2) 2.029(4) 91.28(19) 84.74(18) 94.31(18)

Cu(1)-N(1)#1 Cu(1)-N(2) O(1)-Cu(1)-N(1)#1 N(1)#1-Cu(1)-N(5) N(1)#1-Cu(1)-N(2)

Cu(1)-N(1) Cu(2)-N(5) N(4)#1-Cu(1)-N(4) N(4)#1-Cu(1)-N(1) N(1)#1-Cu(1)-N(1) N(4)-Cu(1)-N(1)#2 N(1)-Cu(1)-N(1)#2 N(2)-Cu(2)-N(5)

2.134(10) 2.149(12) 90.1(4) 179.9(4) 90.3(4) 179.9(5) 90.3(4) 90.5(4)

1

2.015(2) 91.70(8) 88.30(8) 104.37(8)

Zn(2)-O(1) N(4)#3-Zn(1)-N(4) O(1)-Zn(2)-N(1) O(1)-Zn(2)-N(1)#7

1.987(4) 180.0 114.19(7) 114.19(7)

2.45 2.36 2.50 131.2

O1 3 3 3 N3#8 O1 3 3 3 N3#9 O1 3 3 3 N2#9 O1-H1A 3 3 3 N2#9

2.994(2) 2.994(2) 3.102(2) 128.7

1.999(4) 2.233(4) 169.75(18) 90.97(17) 95.90(17)

Cu(1)-N(4)#2

2.013(4)

N(4)#2-Cu(1)-N(5) N(5)-Cu(1)-N(2) N(4)#2-Cu(1)-N(2)

154.16(18) 109.59(18) 96.14(17)

Cu(1)-N(4)

2.092(13)

Cu(2)-N(2)

2.115(9)

N(4)-Cu(1)-N(4)#2 N(4)-Cu(1)-N(1) N(2)#3-Cu(2)-N(2) N(2)#4-Cu(2)-N(2) N(2)-Cu(2)-N(2)#5 N(2)#3-Cu(2)-N(5)

90.1(4) 89.8(3) 90.2(5) 89.8(4) 180.00(1) 90.5(4)

N(4)-Cu(1)-N(1)#1 N(4)#2-Cu(1)-N(1) N(2)-Cu(2)-N(5)#5 N(2)#4-Cu(2)-N(5) N(5)#5-Cu(2)-N(5) N(2)#5-Cu(2)-N(5)

89.8(3) 89.8(3) 89.5(4) 89.5(4) 180.0(6) 89.5(4)

2

3

a Symmetry transformations used to generate equivalent atoms for 1: #1: y - 1, -x þ y, -z; #2: -y þ 1, x - y þ 2, z; #3: -x, -y þ 2, -z; #4: -x þ y - 1, -x þ 1, z; #5: x - y þ 1, x þ 1, -z; #6: -y þ 1, x - y þ 1, z; #7: -x þ y, -x þ 1, z; #8: -1 - x þ y, -x, z þ 1; #9: 1 þ x -y, x, -z þ 1; 2: #1 -x þ 1, -y, -z þ 1; #2: x - 1/2, y, -z þ 1/2; 3: #1: -x þ y, -x þ 1, z; #2: -y þ 1, x - y þ 1, z; #3: x, x - y þ 1, z; #4: -x, -x þ y, -z; #5: -x, -y þ 1, -z.

Figure 1. (a) Coordination environments of Zn(II) and μ2-Tz; (b) μ2-Tz bridged 2D layer; (c) kagom e dual topology of the 2D layer (the atoms in orange and purple represent Zn1 and Zn2, respectively); (d) hydrogen bonding interactions between adjacent layers.

while Zn2 is four-coordinated with three N atoms from three individual Tz anions and one water molecule, forming a distorted tetrahedral geometry. All the Tz- anions in 1 act as

the same μ2-bridging ligand (mode II) connecting Zn1 and Zn2. The Zn1-N4 distance (2.172(2) A˚) is slightly longer than the Zn2-N1 distance (2.015(2) A˚), and these distances

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Figure 2. (a) Coordination environments of copper(II), μ2-Tz-, and μ3-Tz- in 2; (b) μ3-Tz- bridged 2D sheet along the ac plane; (c) 3D MOF formed through the 2D sheets pillared by μ2-Tz- and μ2-OH-; (d) fsc topological structure (light blue and dark blue nodes stand for Cu(II) and μ3-Tz-, respectively, and light blue lines stand for μ2-Tz- connections).

are in good agreement with the literature values in sixcoordintated8c,10k,17 and four-coordinated18 Zn-tetrazole complexes. Through the bridging of μ2-Tz- ligands, a 2D layer structure is formed (Figure 1b), in which each six-coordinated Zn1 ion is connected with six four-coordinated Zn2 ions, and each four-coordinated Zn2 ion is surrounded by three six-coordinated Zn1 ions. Consequently, a kagom e dual (kgd) topological layer with the Schl€ afli symbol e (43)2(46 3 66 3 83) is generated (Figure 1c). Though the kagom topology is very common in the reported layer structures,19 the kgd topological structure has not been reported so far.20 The 2D layers are further connected through interlayer hydrogen bonds between the coordinated O1 atom in one layer and the uncoordinated N atoms of μ2-Tz- in the adjacent layer (see Table 2) to form a 3D structure (Figure 1d). In 2, the Cu(II) is five-coordinated with four N atoms from four individual Tz- anions and one O atom of hydroxyl anion, resulting in a distorted square pyramid geometry, in which O1, N5, N1A, and N4A atoms locate at the bottom plane, and the axial position is occupied by an N2 atom (Figure 2a). The axial Cu1-N2 distance (2.233(4) A˚) is

longer than those of the bottom plane (1.896(2)-2.029(4) A˚). The Tz- anions in 2 exhibit μ3-Tz- (mode V) and μ2-Tz(mode III) coordination modes, in which the μ2-Tz- coordination mode in 2 is different from that in 1 (mode II). The Cu(II) are connected via μ3-Tz- bridges to form a 2D sheet (Figure 2b), and the 2D sheets are further pillared by μ2-Tzand hydroxyl anions to construct a 3D layer-pillar type of metal-organic framework (Figure 2c). According to the regulation of the topology, each μ2-Tz- and μ2-OH- can be considered as a connection, each μ3-Tz- is regarded as a three-connected node, and the Cu(II) is viewed as a fourconnected node. Thus, the 3D metal-organic frameworks of 2 can be regarded as a fsc type of topology net with the Schl€ afli symbol of {4 3 6 3 8}{4 3 62 3 83} (Figure 2d),20 which was also found in other reported coordination polymers.21 The structure of 3 is isomorphous to that of the previously reported complex of {[Cd5(Tz)9](NO3) 3 8H2O}n.12 As illustrated in Figure 3a, each asymmetric unit in 3 contains two crystallographically independent Cu(II) ions, in which Cu1 is coordinated with six nitrogen atoms from three individual μ3-Tz- and three individual μ4-Tz-, respectively, forming a slightly distorted octahedral geometry, and Cu2 is also sixcoordinated with six nitrogen atoms from two individual

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Figure 3. (a) Coordination environments of Cu(II) and μ3- and μ4-Tz- in 3. (b) A trigonal prism with an NO3- occupied cage. (c) 3D framework with 1D channels of 3 (The water molecules in channels are omitted for clarity). (d) 1D hexagonal channel with a pore diameter of 4.3 A˚. (e) acs topology net of 3.

μ4-Tz- and four individual μ3-Tz-. The Cu-N distances of 2.092(13)-2.149(12) A˚ are approximate to those reported in other Cu-tetrazole complexes,22 while they are shorter than the Cd-N distances (2.329(10)-2.361(10) A˚) in {[Cd5(Tz)9](NO3) 3 8H2O}n12 due to the smaller ion radii of Cu(II). In 3, two Cu1 and six Cu2 are connected through the bridging of three μ4-Tz- and six μ3-Tz-, forming a trigonal prism second building unit (SBU) (Figure 3e). The adjacent SBUs are further connected by an additional three μ4-Tz- to form a long trigonal prism with NO3- occupied cages (Figure 3b). Each prism connects with adjacent three prisms through sharing Cu2 ions to form a 3D metal-organic framework with 1D hexagonal channels (Figure 3c and d). The sizes of the channel are about 4.3  4.3 A˚2. It is interesting to note that though compound 3 and {[Cd5(Tz)9](NO3) 3 8H2O}n are isomorphous, the sizes of the channels in the former are smaller than that of 4.9  4.9 A˚ in the latter, due to the smaller ion radii of Cu(II) and

Figure 4. TG curves for 1-3.

subsequent shorter Cu-N distances. The channels are filled with water molecules. About 23% solvent-accessible volume is estimated by using PLATON software. It should be noted that the inner surface of the hexagonal channel is decorated with pairs of μ3-Tz rings directing toward the cavity interiors,

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resulting in an uncoordinated N atoms decorated surface (Figure 3c and d), which is similar to the inner surface of {[Cd5(Tz)9](NO3) 3 8H2O}n. To well understand the framework topological strucure, it is necessary to simplify the building units. The [Cu8(Tz)9] SBU can be regarded as one node. As each SBU is surrounded by six adjacent SBUs through sharing Cu2 ions

Figure 5. Variable temperature XRD patterns for 3.

Figure 6. Fluorescent emission spectra of 1 in the solid state at room temperature (λex = 292 nm).

Zhong et al.

(Figure 3e), thus each node is a six-connected node. Consequently, the framework of 3 belongs to a uninodal sixconnected acs type of topology net (Figure 3e), with Schl€ afli symbol of {49 3 66}.20 Thermal Analyses, X-ray Powder Diffraction, and Photoluminescent Properties. The results of thermogravimetric (TG) analyses indicate that compound 1 lost its coordinated water molecules in the 240-300 °C temperature range (Figure 4). The weight loss found of 5.7% is consistent with that calculated (5.6%). After the loss of the water molecules, the 3D framework began to decompose upon further heating. Compound 2 was stable up to about 220 °C and then began to decompose with a continuous weight loss up to 450 °C. The TG curve of 3 showed an initial weight loss of 12.3% from room temperature to 100 °C, corresponding to the removal of eight guest water molecules per formula unit (calcd 12.6%), and then the 3D framework was stable up to 230 °C, followed by another weight loss after that temperature. XRD was used to check the purity of 1-3. The results show that all the peaks displayed in the measured patterns at room temperature closely match those in the simulated patterns generated from single-crystal diffraction data, indicating single phases of 1-3 were formed. The variable temperature XRD patterns of 3 were measured, and the results indicate that the framework is stable up to 200 °C (Figure 5), which is consistent with the result of thermal analysis. The photoluminescent property for 1 was investigated in the solid state at room temperature. As shown in Figure 6, it can be observed that 1 exhibits photoluminescence upon excitation at 292 nm. The emission peak at 467 nm can be attributed to the LMCT transition, which has been observed in other tetrazole-based Zn(II) coordination polymers.8b,c,10k,23 Gas Adsorption Properties of 3. To examine the pore characteristic and storage capability, the gas adsorption properties of desolvated 3 were investigated. The results

Figure 7. (a) Gas sorption isotherms of N2 and CO2 for desolvated 3. (b) H2 sorption isotherms for desolvated 3 (H2 desorption isotherm at 77 K was not measured due to the extremely long equilibrium time caused by the small pore sizes). (c) Enthalpy of adsorption plots as a function of the amount of H2 uptake in desolvated 3.

Article

Crystal Growth & Design, Vol. 10, No. 2, 2010

indicate that no N2 uptake was observed at 77 K (Figure 7a). In contrast, it was found that significant amounts of CO2 (195 K) and H2 (77 K) were adsorbed (Figure 7a and b) and that both isotherms present typical type-I curves, which is the characteristic of a microporous material. The amounts of CO2 uptake increase abruptly at the beginning and reach a plateau of 49.7 cm3 (STP)/g at 1.0 atom, corresponding to 2.6 CO2 molecules per formula unit, indicating a uniform microporous structure. Using the BET method, a pore volume of 0.059 cm3/g is estimated. Fitting the BET and Langmuir equations to the CO2 adsorption isotherm gave estimated surface areas of 232 and 282 m2/g, respectively, which are smaller than the values of 310 and 338 m2/g observed in the desolvated compound of {[Cd5(Tz)9](NO3) 3 8H2O}n.12 The desolvated 3 can store up to 0.71 wt % (80.3 cm3 (STP)/g) of H2 at 760 Torr and 77 K, which is also slightly smaller than the value of 0.75 wt % (83.4 cm3 (STP)/g) observed in desolvated {[Cd5(Tz)9](NO3) 3 8H2O}n. The shape of the H2 adsorption curve is similar to that of the desolvated {[Cd5(Tz)9](NO3) 3 8H2O}n, with an extremely steep slope at initial pressure, indicating the pores in desolvated 3 can also strongly interact with H2 molecules. To quantitatively compare their affinities to H2 molecules, a second set of H2 isotherms for desolvated 3 was measured at 87 K (Figure 7b). The enthalpies of H2 adsorption were calculated using the modified Clausius-Clapeyron equation,24 resulting in an initial enthalpy of adsorption of 12.6 kJ/mol (Figure 7c); this value is larger than that of 11.3 kJ/mol for desolvated {[Cd5(Tz)9](NO3) 3 8H2O}n (the previous reported value12 of 13.3 kJ/mol is inaccurate; see the detail in the Supporting Information). The larger ΔHads value for desolvated 3 can be attributed to its relatively smaller pore sizes. Though the ΔHads values of 12.6 and 11.3 kJ/mol do not reach the record of 13.5 kJ/mol,25c they are still large among the known values of enthalpies of H2 adsorption observed in the reported MOFs.25 It is interesting to note that though compound 3 and {[Cd5(Tz)9](NO3) 3 8H2O}n12 are isomorphous, they show different adsorption abilities due to the presence of different metal ions. {[Cd5(Tz)9](NO3) 3 8H2O}n shows adsorptions to N2, CO2, and H2, while 3 exhibits selective adsorptions of CO2 and H2 over N2. The above different adsorption abilities can be ascribed to the different size dimensions of the pores caused by the different metal ion radii. The porous diameters for 3 and {[Cd5(Tz)9](NO3) 3 8H2O}n are 4.3 and 4.9 A˚, respectively. The smaller pore in 3 can let H2 and CO2 enter, while it cannot let the N2 enter, as the kinetic diameter of N2 (3.64 A˚) is larger than those of H2 and CO2 (H2 2.89 A˚, CO2 3.30 A˚). The metal ion radii induced different adsorption ability is quite novel and interesting. Conclusions Three coordination polymers based on 1H-tetrazole through in situ decarboxylation ligand synthesis, which cannot be obtained by the direct reactions of tetrazole with corresponding metal salts, have been presented. Compound 1 shows a 2D layer structure with a novel kagom e dual topology net and photoluminescent emission at 467 nm. Compound 2 is a 3D layer-pillar type of structure with a common fsc topology net. Compound 3 possesses a 3D porous framework, which exhibits selective adsorptions of CO2 and H2 over N2 after the removal of the guest water molecules within the pores.

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Acknowledgment. This work was supported by the NSFC (20625103, 20831005, 20821001) and the 973 Program of China (2007CB815305). Supporting Information Available: Description of the analysis of the gas adsorption isotherms and ΔHads vs H2 uptake plots in PDF format; and X-ray crystallographic files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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