Electron spin resonance studies. Part 66. Characterization of copper(II) complexes in the oxidation of D-penicillamine, L-cysteine, and related sulphur-containing compounds
E.s.r. spectroscopy has been employed to characterize a series of copper(II)–dithiolate complexes formed in aqueous solution at pH ca. 10. Some examples, e.g. that from D-penicillamine [Cu(D-pen)2], are long-lived whereas rapid-flow and fast-freezing methods are required for the study of more transient species (e.g. the analogous complex from L-cysteine). Spectra have been described for related copper complexes of S2N2, S2O2, and S4 type and both isotropic and anisotropic parameters are interpreted in terms of significant spin-delocalization onto S (especially) and N ligands. Some transient ternary complexes [e.g. from L-cysteine and mercaptoethanoic acid)] have also been identified. Stopped-flow experiments on the kinetics of the (second-order) decay of the transient complexes reveal the importance of steric and electronic effects in governing their lifetimes.