Cadmium(II) Cysteine Complexes in the Solid State: A Multispectroscopic Study

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) center dot H2O (1) and {Cd(HCys)(2) center dot H2O}(2) center dot H3O+ClO4- (2), by means of vibrational (Raman and IR absorption), solid-state NMR (Cd-113 and C-13), and Cd K- and L-3-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 113 Cd solid-state cross-polarization magic angle spinning NMR spectra showed a broad signal in the similar to 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 angstrom 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 angstrom was obtained. The Cd L-3-edge XANES spectra of 1 and 2 resembled that of the CdS3O model compound and showed that the coordination around Cd(II) ions in I 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.