Journal of Molecular Structure 933 (2009) 98–103

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

In situ hydrothermal synthesis and structural characterizationof two novel CdII–tetrazole coordination polymers

Jie Xiao, Wen-Xiang Wang, Jin-Rui Lin, Hong Zhao *

Ordered Matter Science Research Center, School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, People’s Republic of China

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 March 2009Received in revised form 2 June 2009Accepted 2 June 2009Available online 9 June 2009

Keywords:CadmiumTetrazoleMetal–organic frameworksHydrothermal synthesis

0022-2860/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.molstruc.2009.06.002

* Corresponding author. Tel./fax: +86 025 5209062E-mail address: zhaohong@seu.edu.cn (H. Zhao).

The in situ hydrothermal reactions of 4-nitrobenzonitrile with Cd(ClO4)2 and CdCl2 afforded two novelCdII–tetrazole coordination polymers, 2D network {[Cd(H2O)2(4-nptz)2](H2O)2}n (1) and 1D double chains[Cd(H2O)2(4-nptz)2]n (2). Their synthesis, solid-state structure, and XRPD patterns are reported.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

A main objective of crystal engineering is the design of solid-state structures of molecular solids with specific attributes suchas topological features, chemical functions, and physical properties[1–4]. Generally, structural motifs of these metal–organic frame-works are closely related to the geometry around the metal centersand the number of coordination sites provided by the organic li-gands. In recent years, the ‘‘node-and-spacer” synthetic approach,which was proposed by Wells and O’Keeffe [5,6] for controllingthe topology of complex bonding networks, is widely used in crys-tal engineering for it is an effective way to new networks that mayserve as ‘‘templates” for new functional materials. Therefore, judi-cious combination of a metal ‘‘node” and a ligand ‘‘spacer” isimportant for the construction of novel metal–organic frameworks.

Putting aside the mechanism of the [2 + 3] cycloaddition [7–9]and in situ hydrothermal method [8–15], the deprotonate tetrazoleligand anion has been shown to be able to participate in at leastnine distinct types of coordination modes with metal ions in theconstruction of metal-organic frameworks (MOFs). As shown inScheme 1, the nitrogen-containing heterocycle can either coordi-nate in a l1-tetrazolyl (Modes I and II); l2-tetrazolyl mode, whichin itself has three different modes of coordination (Modes III–VI);adopt two different l3-tetrazolyl modes (Modes VII–VIII); or actin a l4-tetrazolyl mode (Mode IX).

Tetrazoles have attracted a growing amount of attention theseyears in coordination chemistry due to excellent coordination abil-

ll rights reserved.

6.

ity of the four nitrogen atoms of the functional group to act aseither a multidentate or a bridging building block in supramolecu-lar assemblies. Thus, it is not surprising to find in the literature thedevelopment of numerous synthetic routes to this ubiquitouslyuseful functional group [16].

In this context and as part of an ongoing program in our labora-tory to explore the scope of MOFs with tetrazole ligands underhydrothermal conditions, we report herein the synthesis and struc-tural characterization of two novel CdII–tetrazole coordinationpolymers, compounds 1 and 2 (Scheme 2). The MOFs 1 and 2 weresynthesized by the hydrothermal self-assembly. Such hydrother-mal preparation is not very sensitive to reaction temperature andMOFs 1 and 2 can be obtained between 110 and 140 �C. Onceformed, single crystals of 1 and 2 are air-stable and insoluble incommon solvents such as water, methanol, ethanol, acetonitrile,and DMF.

2. Experimental

2.1. Materials and physical measurements

All other reagents were commercially available. Elemental anal-yses for carbon, hydrogen, and nitrogen were performed on a Per-kin-Elmer 240 �C elemental analyzer. IR spectra were obtainedwith KBr pellets in the 4000–400 cm�1 region, using a ShimadzuIRprestige-21 spectrophotometer. The crystal structures weredetermined by Rigaku SCX mini diffractometer. The X-ray powderdiffraction data was obtained on a Rigaku X-ray powder diffrac-tometer D/MAX 2000/PC.

NN

NN

RN

N

NN

RN

N

NN

RN

N

NN

RN

N

NN

R

NN

NN

RN

N

NN

RN

N

NN

RN

N

NN

R

Mode I Mode IV Mode V

Mode VI Mode VII

Mode II Mode III

Mode VIII Mode IX

Scheme 1. Nine coordination modes of the deprotonate tetrazole ligand anion.

CN

NO2

+ Cd(ClO4)2 sealed tubeNaN3/H2O

Ar2Cd

nN N

NN

CN

NO2

+ CdCl2 sealed tube

NaN3/H2OAr Cd

nN N

NN

ArNN

N N

(1)

(2)

H2O22

H2O22

H2O22

120 oC

120 oC

Scheme 2. Synthesis of compounds 1 and 2.

Table 1Crystal data and structure refinement parameters for 1 and 2.

Structure parameters 1 2Empirical formula C14H16CdN10O8 C14H12CdN10O6

fw 564.77 528.74Crystal structure Monoclinic MonoclinicSpace group P21/c C2/cCrystal size (mm) 0.10 � 0.10 � 0.07 0.12 � 0.03 � 0.103a (Å) 12.408(3) 31.490(3)b (Å) 9.2082(18) 6.871(2)c (Å) 8.18092(18) 18.172(3)b (o) 102.32(3) 109.20 (3)V (Å3) 983.4(3) 3712.9(13)Z 2 8T(K) 293(2) 293(2)F(0 0 0) 564 2096qcalc (g cm�3) 1.907 1.892l (mm�1) 1.181 1.237h range (�) 3.24–27.48 3.04–25.00Total number of data collected 9942 14,462Number of unique data 2248 3252R indexesa [I > 2r(I)] R1 = 0.0213 R1 = 0.0964

wR2 = 0.0501 wR2 = 0.2332R (all data)a R1 = 0.0250 R1 = 0.1652

wR2 = 0.0516 wR2 = 0.3309Goodness-of-fit on F2 1.066 1.043Largest diff map hole and peak (e �3) 0.290 and �0.351 1.991 and �2.654

a R1,P

||Fo| � |Fc||/P

| Fo|; wR2, [P

w(Fo2 � Fc

2)2/P

w(Fo2)2]1/2.

Table 2Selected bond lengths (Å) and angles (o) for 1 and 2.

(a) Compound 1Cd(1)AO(3)i 2.3130(14) Cd(1)AO(3) 2.3130(14)Cd(1)AN(3)ii 2.3597(15) Cd(1)AN(3)iii 2.3597(15)Cd(1)AN(1) 2.3674(15) Cd(1)AN(1)i 2.3674(15)C(1)AN(4) 1.326(2) C(1)AN(1) 1.346(2)N(1)AN(2) 1.343(2) N(2)AN(3) 1.310(2)N(3)AN(4) 1.347(2) N(3)ACd(1)iv 2.3597(15)O(3)iACd(1)AO(3) 180.00(4) O(3)iACd(1)AN(3)ii 93.61(6)O(3)ACd(1)AN(3)ii 86.39(6) N(3)iiACd(1)AN(1)i 87.33(5)O(3)ACd(1)AN(3)iii 93.61(6) N(3)iiACd(1)AN(3)iii 180.0O(3)iACd(1)AN(1) 86.39(6) O(3)ACd(1)AN(1) 93.61(6)N(3)iiACd(1)AN(1) 92.67(5) N(3)iiiACd(1)AN(1) 87.33(5)O(3)iACd(1)AN(1)i 93.61(6)

(b) Compound 2Cd(1)AO(6) 2.303(9) Cd(1)AN(2) 2.326(10)Cd(1)AN(9)i 2.346(13) Cd(1)AN(3)ii 2.376(12)Cd(1)AN(6) 2.381(13) Cd(1)AO(5) 2.381(11)C(1)AN(4) 1.342(17) C(1)AN(1) 1.358(17)N(1)AN(2) 1.362(16) N(2)AN(3) 1.295(16)N(3)AN(4) 1.344(16) N(6)AN(7) 1.364(16)N(7)AN(8) 1.313(17) N(8)AN(9) 1.344(16)O(6)ACd(1)AN(2) 172.5(4) O(6)ACd(1)AN(9)i 82.6(3)N(2)ACd(1)AN(9)i 93.2(4) O(6)ACd(1)AN(3)ii 92.7(4)N(2)ACd(1)AN(3)ii 93.6(4) N(9)iACd(1)AN(3)ii 91.4(4)O(6)ACd(1)AN(6) 88.2(3) N(2)ACd(1)AN(6) 95.7(4)N(9)iACd(1)AN(6) 170.5(4) N(3)iiACd(1)AN(6) 91.0(4)O(6)ACd(1)AO(5) 87.4(4) N(2)ACd(1)AO(5) 86.2(4)N(9)iACd(1)AO(5) 87.5(4) N(3)iiACd(1)AO(5) 178.9(4)N(6)ACd(1)AO(5) 90.1(4)

(a) i�x, �y + 1, �z + 2, ii�x, y + 1/2, �z + 5/2, iiix, �y + 1/2, z � 1/2, iv�x, y � 1/2,�z + 5/2.(b) ix, y � 1, z, ii�x, y, �z + 1/2.

J. Xiao et al. / Journal of Molecular Structure 933 (2009) 98–103 99

2.2. Synthesis of compound 1

Hydrothermal treatment of Cd(C1O4)2 (2.0 mmol), 4-nitrobenzo-nitrile (4.0 mmol), NaN3 (6.0 mmol), and water (5.0 ml) over 24 h at120 �C to give colorless prism crystals (pure phase) in 63% yield basedon Cd(C1O4)2. Anal. Calcd. for C14H16CdN10O8 (%): C, 29.77; H, 2.86; N,

24.80. Found (%): C, 29.74; H, 2.85; N, 24.79. IR (cm�1): 3349(m) ,3083.8(w), 3026.9(w), 1621.7(m), 1543.5(s), 1474.8(m), 1455.6(m),1425.4(m), 1354.0(s), 1338.3(m), 1278.9(w), 1217.6(w), 1125.4(w),1076.9(m), 869.5(m), 775.5(m), 743.0(m), 706.3(m), 510.1(w).

2.3. Synthesis of compound 2

Hydrothermal treatment of CdCl2 (2.0 mmol), 4-nitrobenzonitrile(4.0 mmol), NaN3 (6.0 mmol), and water (5.0 ml) over 24 h at 120 �Cto give colorless prism crystals (pure phase) in 52% yield based onCdCl2. Anal. Calcd for C14H12CdN10O6 (%): C, 31.80; H, 2.29; N, 26.49.Found (%): C, 31.77; H, 2.27; N, 26.48. IR (cm�1): 3354(m),3107.9(w), 3046.8(w), 1645.7(m), 1574.7(s), 1500.1(m), 1491.4(m),1430.4(m), 1374.4(s), 1347.3(m), 1301.1(w), 1276.4(w), 1211.3(w),1175.8(w), 1144.8(w), 1107.3(m), 871.4(m), 794.3(m), 764.5(m),719.5(m), 527.4(w).

Fig. 1. The coordination environments of the CdII centers in compound 1 (50% probability thermal ellipsoids). Hydrogen atoms and guest water molecules are omitted forclarity. Symmetry codes: (A) �x, 1 � y, 2 � z; (B) �x, 1/2 + y, 5/2 � z; (C) x, 1/2 � y, �1/2 + z; (D) �x, 1 � y, 2 � z.

Fig. 2. A view of the 2D sheet in compound 1, water molecules and part of theligands are omitted for clarity.

Fig. 3. A view of the 2D network of compound 1 without organic ligands, showingthe distances of the inorganic CdII centers along a-axis.

100 J. Xiao et al. / Journal of Molecular Structure 933 (2009) 98–103

2.4. X-ray measurements

X-ray diffraction data of compounds 1 and 2 were collected on aRigaku SCX mini diffractometer using Mo Ka radiation(k = 0.71073 Å). The structure was solved by direct methods withSHELXS-97 and refined by full matrix least squares on F2 withSHELXL-97 [17]. All non-hydrogen atoms were refined with aniso-tropic thermal parameters. Hydrogen atoms were added theoreti-cally and refined with riding model and fixed isotropic thermalparameters. Detailed data collection and refinement of the com-pounds 1 and 2 are summarized in Table 1. Selected bond lengthsand angles are listed in Table 2. The CIF files of the structure s aredeposited to CCDC database and have the CCDC Nos. 711508 and711509.

3. Results and discussion

Compound 1, {[Cd(H2O)2(4-nptz)2](H2O)2}n [4-nptz = 5-(4-nitrophenyl)tetrazolate], was synthesized hydrothermally bytreating Cd(ClO4)2�6H2O, NaN3, and 4-nitrobenzonitrile at 120 �C,

while compound 2, [Cd(H2O)2(4-nptz)2]n, was obtained by treatingCdCl2, NaN3, and 4-nitrobenzonitrile at the same temperature. TheIR spectra of 1 and 2 both show the absence of a cyano peak in the2100 cm�1, which supports the proposed reaction between the ni-trile and the azide. The formation of a tetrazole group is supportedby the emergence of peaks at ca. 1440 cm�1.

3.1. Crystal structure of compound 1

The single-crystal X-ray structure reveals that compound 1 pos-sesses a two-dimensional coordination network. The asymmetricunit of compound 1 consists of one crystallographically indepen-dent CdII atom, one 4-nptz ligand, one aqua ligand, and one guestwater molecule (Fig. 1). Each CdII center displays slightly distortedoctahedral coordination geometry. The equatorial plane of theoctahedral geometry is provided by four tetrazolate nitrogenatoms from four 3-nptz ligands with the normal CdAN bondlengths [Cd(1)AN(1) = 2.367(2) Å and Cd(1)AN(3) = 2.360(2) Å].The axial position of the octahedron is occupied by two oxygen

Fig. 4. A view of the extended 3D supramolecular network in compound 1 linked by2D layers through hydrogen-bonding interactions along c-axis.

Table 3Geometrical parameters of hydrogen bonds in compounds 1 and 2.

D–H. . .A D–H/Å H. . .A/Å \DHA/o D. . .A/Å

(a) Compound 1O3AH3AAO4i 0.841 1.967 167.84 2.794O3AH3BAO4ii 0.862 1.939 164.17 2.778O4AH4BAO1iii 0.894 2.328 177.34 3.222O4AH4BAO2iii 0.894 2.485 125.28 3.089

(b) Compound 2O5AH11BAO1i 0.900 2.2779 127.47 2.910O6AH11DAO6ii 0.900 2.4833 133.21 3.169

(a) ix, y, z + 1, iix, �y + 3/2, z + 1/2, iii�x + 1, �y + 1, �z + 1.(b) i1/2 � x, 1/2 � y, 1 � z, ii�x, 1 � y, �z.

J. Xiao et al. / Journal of Molecular Structure 933 (2009) 98–103 101

atoms from two aqua ligands [Cd(1)AO(3) = 2.313(2)Å], with theO(3)ACd(1)AO(3D) bond angle of 180.00(4)o. As shown in Figs. 2and 3, 4-nptz ligand acts as a l2-tetrazolyl mode (Mode V) withtwo N atoms (1,3-position) bridging two Cd atoms that are sepa-rated by 6.372(1) Å. Moreover, each CdII center is connected withfour 4-nptz ligands. The organic ligands link inorganic CdII nodes

Fig. 5. The coordination environments of the CdII centers in compound 2 (30% probabilit�x, 1 � y, 2 � z; (B) �x, 1/2 + y, 5/2 � z.

to form 2D sheets which are extended into a 3D supramolecularframework by intermolecular hydrogen-bonding interactions(Fig. 4). The O. . .O distances range from 2.778(3) to 3.222(3) Å.The relevant hydrogen-bonding parameters are summarized in Ta-ble 3.

3.2. Crystal structure of compound 2

In contrast to compound 1, the structure of compound 2 fea-tures 1D CdII tetrazolate double chains. The asymmetric unit of 2contains two tetrazolate ligands, one CdII node, and two water li-gands. As shown in Fig. 5, the CdII center has a slightly distortedoctahedral geometry and is six-coordinated with four N atoms ofequivalent tetrazolates from four different ligands and two aqua li-gands. Unlike that in complex 1, the 4-nptz ligands adopts two dif-ferent coordination modes: one remains a l2-tetrazolyl mode(Mode V) with two N atoms (1,3-position) bridging two Cd atomsthat are separated by 6.871(10) Å which is longer than those incompound 1; another one acts as a l2-tetrazolyl mode (Mode IV)with two 4-nptz ligands (2,3-position) chelating two Cd atoms thatthe CdACd distance is 4.509(48) Å. As shown in Fig. 6, each pair ofCd atoms is chelated by a pair of 4-nptz ligands, forming a six-numbered ring, and each pair of such rings is bridged by two 4-nptz ligands to form 1D double chains. Moreover, the hydrogenbond occurring at O5AH11B. . .O1 and O6AH11D. . .O6 give riseto a three-dimensional framework (Fig. 7).

Herein, we present 1D compound 1 and 2D compound 2 com-pounds constructed from the same tetrazole ligands and Cd ionsunder the same hydrothermal condition. The successful synthesisof compounds 1 and 2 may be supported by the following rea-sons. First, tetrazole ligands possess the excellent coordinationability of the four nitrogen atoms of the functional group tobridge metal atoms in various coordination modes. Second, theCd ion is flexible in adopting different coordination geometriesand its coordination bonds are prone to cleavage and formation[18]. Finally, hydrothermal techniques provide a condition tomake compounds crystallize at high temperature and pressureand to deprotonate tetrazole ligands, which resulting in themcrystallize in different formations. The extension work to hydro-thermally synthesize 3D complexes based on 4-nptz ligand andCd ion is currently underway.

3.3. XRPD patterns of compounds 1 and 2

Compounds 1 and 2 were characterized by X-ray powder dif-fraction (XRPD) at room temperature. As shown in Fig. 8, the pat-

y thermal ellipsoids). Hydrogen atoms are omitted for clarity. Symmetry codes: (A)

Fig. 6. A view of the 1D double chains in compound 2, hydrogen atoms and part of the ligands are omitted for clarity.

Fig. 7. A view of the extended 3D supramolecular framework in compound 2 linkedby hydrogen-bonding interactions.

5 10 15 20 25 30

1 simulated

simulated

experimental

experimental

Inte

nsity

(a.u

.)

2 theta ( )o

2

Fig. 8. Experimental and simulated X-ray powder diffraction patterns of 1 and 2.

0 100 200 300 400 50050

60

70

80

90

100

2w

eigh

t /%

T/ oC

1

Fig. 9. TGA curves for compounds 1 and 2.

102 J. Xiao et al. / Journal of Molecular Structure 933 (2009) 98–103

terns calculated from the single-crystal X-ray data of 1 and 2 werein good agreement with the observed ones in almost identical peakposition but different peak intensities.

3.4. Thermogravimetric analyses

Thermogravimetric analyses (TGA) have been taken on com-pounds 1 and 2 by heating to 500 �C under flowing N2. As shownin Fig. 9, the TGA curve for 1 showed the first weight loss betweenroom temperature and 140 �C, corresponding to the loss of two-noncoordinated water molecules (5.98% weight loss observed;6.37% calculated). The second weight loss occurred between 140and 277 �C with the loss of two-coordinated water molecules(6.02% weight loss observed; 6.37% calculated). Then the continu-ous weight loses above 277 �C correspond to the decompositionof tetrazole ligands. For compound 2, the TGA data indicates6.51% weight loss between room temperature and 263 �C, in agree-ment with the corresponding calculated values of 6.80%, due to theloss of two water molecules.

Acknowledgements

We gratefully acknowledge the financial support of the NationalNatural Science Foundation of China (20801012) and the financialsupport for Young researcher from Southeast University(4007041027).

J. Xiao et al. / Journal of Molecular Structure 933 (2009) 98–103 103

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