Crystal Structure and Characterization of the Dinuclear Cd(II) Complex [Cd(H2O)2(ο-HOC6H4COO)2]2

The structure of a new binuclear cadmium (II) complex, [Cd(H2O)2(Sal)2]2 (Sal= salicylate), has been determined by X-ray crystallography. It was also characterized by elemental analysis, its IR spectrum and thermogravimetric-differential scanning calorimetry (TG-DSC). It crystallizes in the monoclinic system, space group P2(1)/c, with lattice parameters a = 15.742(3) A, b = 12.451(3) A, c = 7.7225(15) A, beta = 96.07(3)degrees and Z = 4. Two cadmium (II) ions are bridged by two mu(2)-carboxy oxygen atoms. Each cadmium atom lies in a distorted capped octahedron coordination geometry. The thermal gravimetry (TG) data indicate that there are four discrete decomposition steps with two endothermic peaks and one exothermic peak. The final thermal decomposition product is CdO.


Introduction
Cd 2+ ions have been found to induce various pathological conditions, such as cardiovascular diseases [1], hypertension and cancer [2].Cadmium can replace the zinc of superoxide dismutase (SOD) and this results in a drop in the biological activity of SOD [3].In addition, cadmium has some stress-causing activity towards enzymes inolved in the tolerance mechanisms of crops [4].Cadmium(II), being a d 10 ion, provides few spectroscopic signatures for structure monitoring, however, the structure of the cadmium complex could be elucidated by X-ray crystallography.In this paper, we report the synthesis and crystal structure of the title compound.Its elemental analysis, FT-IR and thermal analysis have also been investigated.

IR Spectra
In the IR spectra, the bands around 3418~3548 cm -1 can be attributed to O-H stretching modes, which are consistent with the presence of water in the crystal [12].The characteristic absorption bands at 1617 and 1637 cm -1 are the salicylate antisymmetric carboxyl vibrations, while the symmetric carboxyl stretching frequency occurs at 1429 cm -1 [13,14].

Thermal Analysis
The TG/DSC curves of the title compound are presented in Figure 3.In this figure we can see that there are four discrete weight loss steps and that the decomposition events mainly take place at 66.9ºC, 172.3ºC and 512.5ºC, with two heat-absorption peaks and one heat-liberation peak, respectively.On the base of weight changes, the first endothermic weight loss process (8.90 %) corresponds to the loss of two water molecules (found 8.90 % calc.8.52 %); the second weight loss event may be related to the loss of one phenol (PhOH) molecule with the breakage of C-C bonds (found: 22.43 %, calc.22.00 %), an exothermic phenomenon.The weight loss of 3.58 % from 240ºC to 370ºC is attributed to the loss of one oxygen atom (found: 3.58 %, calc.3.79 %).There is a broad endothermic peak at 512.5ºC and ca.33.93 % weight loss in the TG curve between 380~540ºC, which is attributed to the loss of the phenol groups, while the residual weight of 31.16 % suggests that the residue may be CdO (found 31.16 % calcd.30.38 %).In the temperature range of 540~730ºC, there is about an 18.21 % weight loss.From this weight loss we can suggest that the corresponding transition process may be attributed to the sublimation of part of the CdO.[15] A summary of the key crystallographic information is given in Table 1.The selected crystal of [Cd(H 2 O) 2 (ο-HOC 6 H 4 COO) 2 ] 2 was mounted on an Rigaku Raxis-IV diffractometer using Mo-Kα radiation (λ=0.71073Å, T = 293 K) with a graphite monochromator.Intensities were corrected for Lorentz and polarization effects and empirical absorption, and the data reduction using the SADABS program [16].The structure was solved by the direct methods and all the non-hydrogen atoms were refined on F 2 anistropically by the full-matrix least squares method [17].The hydrogen atom positions were fixed geometrically at calculated distances and allowed to ride on the parent carbon atoms.The molecular graphics were plotted using SHELXTL [17].Atomic scattering factors and anomalous dispersion corrections were taken from International Tables for X-ray Crystallography [18].

Figure 2 :
Figure 2: Packing diagram of the unit cell along the b axis.

Figure 3 :
Figure 3: TG/DSC curves of the title compound