catena-Poly­[[di­aqua­cadmium(II)]-μ-5-carboxyimidazole-4-carboxylato-κ⁴N¹,O⁵:O⁴,N³]

A new cadmium coordination polymer, [Cd(C₅H₂N₂O₄)(H₂O)₂]_n, possesses a one-dimensional zigzag chain structure built from Cd^II centers bridged sequentially by pairs of O and N atoms of the 5-carboxyimidazole-4-carboxylate ligand. The Cd^II center is in a distorted octahedral geometry, being coordinated by two O atoms from two coordinated water mol­ecules [Cd—O = 2.322 (7) and 2.364 (7) Å], and by two N atoms [Cd—N = 2.222 (6) and 2.232 (6) Å] and two carboxyl O atoms [Cd—O = 2.383 (6) and 2.414 (6) Å] from two 5-carboxyimidazole-4-carboxylate ligands.

S1. Comment

Coordination polymers built from d¹⁰ metals, such as cadmium(II), and rigid aromatic carboxylates have attracted considerable attention in recent years because they not only exhibit interesting structures but also possess photoluminescence properties (Evans et al., 1999; Evans & Lin, 2000; Liu et al., 2002; Tong et al., 1999). A series of cadmium(II) carboxylates has been described recently (Dai et al., 2002; Ganesan & Natarajan, 2004; Wang et al.,2004). Although a discrete dimer, trans-diaquabis(1H-imidazole-4,5-dicarboxylato)cadmium(II) [Cd(C₅H₃N₂O₄)₂(H₂O)₂], with the Cd atom on an inversion centre, has been reported (Zhang, Fang et al., 2004), to our knowledge there is no reported polymer constructed from Cd^II and the 4,5-imidazoledicarboxylic acid ligand. This ligand has not been well documented in the design of functional coordination polymers (Caudle et al., 1997; Huang et al., 2001; Ma et al., 2003; Rajendiran et al., 2003; Wang et al., 2004; Zhang, Hunag et al., 2004). However, it has two structural features that led to our research interest. The ligand has potential coordination sites involving both the N atoms of the imidazole ring and all the carboxyl O atoms. The flexible multifunctional coordination sites give a high likelihood for generation of structures with different dimensions. In addition, this ligand possesses three removable H atoms and can be successively deprotonated to generate (C₅H₃N₂O₄)⁻, (C₅H₂N₂O₄)²⁻ and (C₅HN₂O₄)³⁻ ions depending on the pH level, resulting in various acidity-dependant coordination modes. We report here the synthesis and structure of a new cadmium complex, [Cd(C₅H₂N₂O₄)(H₂O)₂]_n, (I). This is the first reported one-dimensional cadmium complex containing the 4,5-imidazoledicarboxylic acid ligand as linkage units.

Our X-ray diffraction study shows (Fig. 1) that the asymmetric unit contains one Cd²⁺ cation, one 4,5-imidazoledicarboxylate anion and two water molecules. The Cd^II centre is in a distorted octahedral geometry, being coordinated by two O atoms from two coordinated water molecules, and by two N and two O atoms from two b-glide-related 4,5-imidazoledicarboxylate ligands; the principal dimensions are given in Table 1. Each 4,5-imidazoledicarboxylate anion is quasi-planar and connects successive Cd^II ions in a bis(bidentate) mode, with Cd/N/C/C/O five-membered rings. One N atom and one carboxylic group in the ligand are deprotonated, and there is an intramolecular O—H···O hydrogen bond (Fig. 1 and Table 2). The 4,5-imidazoledicarboxylate ligands link neighboring Cd^II centres via a b-glide operation to form a one-dimensional zigzag chain composed of fused five-membered rings running along the b axis direction, as shown in Fig. 2.

In the crystal structure, adjacent chains are held together by hydrogen bonds (Table 2) between water molecules and all four unique carboxylate O atoms to generate a three-dimensional network. Fig. 3 shows a series of chains linked by hydrogen bonds involving one O—H bond from each of the water molecules [O1W—H11 and O2W—H22] to generate a sheet in the bc plane. The remaining two water OH groups then serve to link these sheets to give a three-dimensional network.

S2. Experimental

The title compound was synthesized by reacting Cd(NO₃)₂·6H₂O (1 mmol), aqueous ammonia solution (0.1 ml, 25 wt%) and 4,5-imidazoledicarboxylic acid (1 mmol) in water (10.0 ml). The autoclave was heated at 443 K for 7 d. Upon cooling to room temperature, the desired product appeared as long yellow prisms in 43% yield.

S3. Refinement

H atoms were located in difference Fourier maps and were subsequently allowed for as riding atoms, with a C—H distance of 0.93 Å, a carboxyl O—H ditsance of 0.82 Å and water O—H distances of 0.95 Å, and with U_iso(H) values of 1.2U_eq(C,carboxy O) and 0.08 Ų for water H atoms.

Computing details

Data collection: SMART (Bruker, 1999); cell refinement: SMART; data reduction: SAINT and XPREP (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1997); software used to prepare material for publication: SHELXTL and PLATON (Spek, 2003).

Figures

	Fig. 1. A view of the coordination environment of the Cd atom. Anisotropic displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (a) 0.5 − x,-0.5 + y,z; (b) 0.5 − x,0.5 + y,z.]
	Fig. 2. A view showing the one-dimensional chain stucture extending in the b direction.
	Fig. 3. A view along the a axis direction, showing part of the three-dimensional network with chains linked into sheets by O—H···O hydrogen bonds.

catena-Poly[[diaquacadmium(II)]-µ-5-carboxy-1H-imidazole-4-carboxylato- κ⁴N³,O⁴:O⁵,N¹]

Crystal data

[Cd(C5H2N2O4)(H2O)2]	F(000) = 1168
Mr = 302.52	Dx = 2.233 Mg m^−3
Orthorhombic, Pbcn	Mo Kα radiation, λ = 0.7107 Å
Hall symbol: -P 2n 2ab	Cell parameters from 165 reflections
a = 10.3280 (12) Å	θ = 2.5–25.1°
b = 13.2177 (14) Å	µ = 2.43 mm^−1
c = 13.1813 (16) Å	T = 293 K
V = 1799.4 (4) Å^3	Prism, yellow
Z = 8	0.18 × 0.12 × 0.10 mm

Data collection

Siemens SMART CCD diffractometer	1587 independent reflections
Radiation source: fine-focus sealed tube	1149 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.053
ϕ and ω scans	θmax = 25.1°, θmin = 2.5°
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996)	h = −8→12
Tmin = 0.711, Tmax = 0.784	k = −13→15
4833 measured reflections	l = −15→11

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.051	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.120	H-atom parameters constrained
S = 1.18	w = 1/[σ^2(Fo^2) + (0.0428P)^2 + 9.3728P] where P = (Fo^2 + 2Fc^2)/3
1587 reflections	(Δ/σ)max < 0.001
128 parameters	Δρmax = 1.06 e Å^−3
0 restraints	Δρmin = −0.75 e Å^−3

Special details

Experimental. IR (KBr pellet, cm⁻¹): 3362 (s), 3133 (m), 2979 (w), 1746 (w), 1572 (s), 1504 (s), 1443 (m), 1399 (m), 1246 (m), 1109 (m), 778 (m), 652 (m), 517 (w), 449 (w).

The IR spectrum of the title compound shows broad strong bonds at the region of 3530–3100 cm⁻¹, revealing the presence of strong hydrogen bonding. The peak at 1746 cm⁻¹ indicates that not all carboxylic groups are deprotonated. This is consistent with the results of structural analysis.

The initial thermogravimetric analysis, performed under a flowing N₂ atmosphere in the range 313–873 K with a heating rate of 10 K min⁻¹, indicates that the structure of the title compound remains stable up to 373 K; on further heating, a two-step weight loss was observed. The first weight loss of 11.52% in the temperature range 373–541 K corresponds to the removal of two coordinated water molecules per formula unit (calculated: 11.91%). From 541 to 873 K, the title compound starts to decompose and the second weight loss of 52.75% due to the loss of the organic species (calculated: 45.64%) and the slow evaporation of ca 0.16 CdO per formula (calculated: 6.79%) [Vaidhyanathan, Natarajan & Rao (2002). Inorg. Chem. 41, 5226–5234.]

The emission spectra of the 4,5-imidazoledicarboxylic acid ligand and the title compound were measured with an Edinburgh FLS920 analytical instrument. The strongest emission peak for the ligand is located at 490 nm (λ_ex = 296 nm), which is assigned to the π^* → n transition. Interestingly, the emission spectrum for the title compound shows a main peak at 491 nm with a weak peak at about 456 nm (λ_ex = 330 nm). Therefore, the emission may be related to the π^* → n transition of the ligand. In addition, the weak peak of the emission spectrum at 456 nm might be attributable to the ligand-to-metal charge transfer.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cd1	0.20862 (6)	0.31873 (4)	0.08417 (5)	0.0319 (2)
O1	0.0604 (5)	−0.1701 (4)	0.1098 (4)	0.0304 (13)
O2	−0.0927 (5)	−0.0586 (4)	0.1471 (4)	0.0289 (13)
O3	−0.1231 (5)	0.1261 (4)	0.1464 (5)	0.0357 (15)
H3	−0.1113	0.0649	0.1509	0.043*
O4	−0.0054 (6)	0.2628 (4)	0.1218 (5)	0.0341 (14)
O1W	0.1179 (6)	0.3276 (4)	−0.0802 (5)	0.0436 (15)
O2W	0.2054 (8)	0.3402 (6)	0.2590 (6)	0.071 (2)
N1	0.2144 (6)	0.1500 (5)	0.0791 (6)	0.0308 (16)
C2	0.2953 (8)	0.0726 (6)	0.0628 (7)	0.033 (2)
H2	0.3818	0.0811	0.0451	0.039*
N3	0.2399 (6)	−0.0184 (5)	0.0746 (5)	0.0262 (15)
C4	0.1140 (7)	0.0021 (6)	0.1011 (5)	0.0209 (17)
C5	0.0988 (7)	0.1063 (6)	0.1032 (6)	0.0229 (18)
C6	0.0230 (8)	−0.0816 (6)	0.1200 (6)	0.0269 (18)
C7	−0.0147 (8)	0.1704 (6)	0.1247 (6)	0.0278 (18)
H11	0.0640	0.2696	−0.0792	0.080*
H12	0.1891	0.3162	−0.1244	0.080*
H21	0.2535	0.3929	0.2906	0.080*
H22	0.1378	0.3192	0.3030	0.080*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.0295 (4)	0.0186 (3)	0.0476 (4)	−0.0041 (3)	−0.0007 (3)	−0.0010 (3)
O1	0.032 (3)	0.017 (3)	0.043 (4)	0.000 (2)	−0.002 (3)	−0.001 (2)
O2	0.021 (3)	0.028 (3)	0.038 (3)	−0.002 (3)	0.003 (2)	0.004 (3)
O3	0.025 (3)	0.024 (3)	0.057 (4)	0.004 (3)	0.005 (3)	0.003 (3)
O4	0.034 (3)	0.021 (3)	0.047 (4)	0.005 (3)	0.003 (3)	−0.008 (3)
O1W	0.043 (4)	0.037 (3)	0.050 (4)	−0.008 (3)	0.002 (3)	−0.001 (3)
O2W	0.073 (5)	0.096 (6)	0.046 (4)	−0.059 (5)	0.011 (4)	−0.009 (4)
N1	0.025 (4)	0.020 (3)	0.047 (4)	0.000 (3)	−0.003 (4)	0.000 (3)
C2	0.022 (4)	0.022 (4)	0.055 (6)	−0.007 (4)	0.007 (4)	0.002 (4)
N3	0.019 (3)	0.021 (4)	0.039 (4)	0.000 (3)	−0.001 (3)	0.000 (3)
C4	0.024 (4)	0.016 (3)	0.023 (4)	−0.004 (3)	−0.003 (3)	0.004 (3)
C5	0.022 (4)	0.020 (4)	0.026 (5)	0.002 (3)	0.004 (4)	−0.002 (3)
C6	0.029 (5)	0.032 (5)	0.020 (4)	−0.001 (4)	−0.007 (4)	0.001 (4)
C7	0.028 (5)	0.028 (5)	0.027 (4)	−0.003 (4)	−0.003 (4)	−0.003 (4)

Geometric parameters (Å, º)

Cd1—N1	2.232 (6)	O1W—H12	0.95
Cd1—N3^i	2.222 (6)	O2W—H21	0.95
Cd1—O1W	2.364 (7)	O2W—H22	0.95
Cd1—O2W	2.322 (7)	N1—C2	1.339 (10)
Cd1—O1^i	2.414 (6)	N1—C5	1.364 (10)
Cd1—O4	2.383 (6)	C2—N3	1.341 (10)
O1—C6	1.239 (9)	C2—H2	0.93
O2—C6	1.284 (10)	N3—C4	1.373 (10)
O3—C7	1.296 (10)	C4—C5	1.387 (10)
O3—H3	0.82	C4—C6	1.473 (10)
O4—C7	1.225 (9)	C5—C7	1.474 (11)
O1W—H11	0.95
N1—Cd1—N3^i	163.8 (2)	H21—O2W—H22	109.4
N3^i—Cd1—O2W	86.6 (3)	C2—N1—C5	105.1 (6)
N1—Cd1—O2W	98.8 (3)	C2—N1—Cd1	141.7 (5)
N3^i—Cd1—O1W	89.7 (2)	C5—N1—Cd1	113.1 (5)
N1—Cd1—O1W	91.9 (2)	N1—C2—N3	113.6 (7)
O1W—Cd1—O2W	153.9 (3)	N1—C2—H2	123.2
N3^i—Cd1—O4	122.3 (2)	N3—C2—H2	123.2
N1—Cd1—O4	73.8 (2)	C2—N3—C4	104.9 (6)
O2W—Cd1—O4	79.5 (2)	C2—N3—Cd1^ii	140.6 (5)
O1W—Cd1—O4	80.7 (2)	C4—N3—Cd1^ii	113.8 (5)
N3^i—Cd1—O1^i	73.3 (2)	N3—C4—C5	107.9 (6)
N1—Cd1—O1^i	92.2 (2)	N3—C4—C6	120.0 (7)
O2W—Cd1—O1^i	82.4 (2)	C5—C4—C6	132.1 (7)
O1W—Cd1—O1^i	121.1 (2)	N1—C5—C4	108.5 (7)
O4—Cd1—O1^i	155.04 (19)	N1—C5—C7	119.8 (7)
C6—O1—Cd1^ii	112.4 (5)	C4—C5—C7	131.7 (7)
C7—O3—H3	109.5	O1—C6—O2	123.0 (7)
C7—O4—Cd1	112.9 (5)	O1—C6—C4	119.5 (7)
Cd1—O1W—H11	100.4	O2—C6—C4	117.6 (7)
Cd1—O1W—H12	104.3	O4—C7—O3	121.7 (8)
H11—O1W—H12	109.5	O4—C7—C5	120.3 (7)
Cd1—O2W—H21	121.1	O3—C7—C5	118.0 (7)
Cd1—O2W—H22	125.5

Symmetry codes: (i) −x+1/2, y+1/2, z; (ii) −x+1/2, y−1/2, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O2	0.82	1.64	2.461 (7)	174
O1W—H11···O1^iii	0.95	1.88	2.806 (8)	165
O1W—H12···O3^iv	0.95	2.10	2.879 (8)	138
O2W—H21···O2^v	0.95	1.90	2.769 (9)	151
O2W—H22···O4^vi	0.95	1.85	2.790 (9)	172

Symmetry codes: (iii) −x, −y, −z; (iv) x+1/2, −y+1/2, −z; (v) x+1/2, y+1/2, −z+1/2; (vi) −x, y, −z+1/2.