Thursday, May 07, 2009

Inorg. Chem. Vol. 48, Iss. 9

Cadmium(II) Cysteine Complexes in the Solid State: A Multispectroscopic Study
Inorg. Chem., 2009, 48 (9), pp 4219–4230 DOI: 10.1021/ic900145n
Farideh Jalilehvand, Vicky Mah, Bonnie O. Leung, Janos Mink, Guy M. Bernard
and Laszlo Hajba
Cadmium(II) cysteinate compounds have recently been recognized to provide an environmentally friendly route for the production of CdS nanoparticles, used in semiconductors. In this article, we have studied the coordination for two cadmium(II) cysteinates, Cd(HCys)2·H2O (1) and {Cd(HCys)2·H2O}2·H3O+ClO4− (2), by means of vibrational (Raman and IR absorption), solid-state NMR (113Cd and 13C), and Cd K- and L3-edge X-ray absorption spectroscopy. Indistinguishable Cd K-edge extended X-ray absorption fine structure (EXAFS) and Cd L3-edge X-ray absorption near edge structure (XANES) spectra were obtained for the two compounds, showing similar local structure around the cadmium(II) ions. The vibrational spectra show that the cysteine amine group is protonated (NH3+) and not involved in bonding. The 113Cd solid-state cross-polarization magic angle spinning NMR spectra showed a broad signal in the 500−700 ppm range, with the peak maximum at about 650 ppm, indicating three to four coordinated thiolate groups. Careful analyses of low-frequency Raman and far-IR spectra revealed bridging and terminal Cd−S vibrational bands. The average Cd−S distance of 2.52 ± 0.02 Å that constantly emerged from least-squares curve-fitting of the EXAFS spectra is consistent with CdS4 and CdS3O coordination. Both structural models yielded reasonable values for the refined parameters, with a slightly better fit for the CdS3O configuration, for which the Cd−O distance of 2.27 ± 0.04 Å was obtained. The Cd L3-edge XANES spectra of 1 and 2 resembled that of the CdS3O model compound and showed that the coordination around Cd(II) ions in 1 and 2 cannot be exclusively CdS4. The small separation of 176 cm−1 between the infrared symmetric and antisymmetric COO− stretching modes indicates monodentate or strongly asymmetrical bidentate coordination of a cysteine carboxylate group in the CdS3O units. The combined results are consistent with a “cyclic/cage” type of structure for both the amorphous solids 1 and 2, composed of CdS4 and CdS3O units with single thiolate (Cd−S−Cd) bridges, although a minor amount of cadmium(II) sites with CdS3O2−3 and CdS4O coordination geometries cannot be ruled out.

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