Reactions of Rh(CHCOO) with thiols and thiolates: a structural study.

The structural differences between the aerobic reaction products of Rh(AcO) (1; AcO = CHCOO) with thiols and thiolates in non-aqueous media are probed by X-ray absorption spectroscopy. For this study, ethanethiol, dihydrolipoic acid (DHLA; a dithiol) and their sodium thiolate salts were used. Coordination of simple thiols to the axial positions of Rh(AcO) with Rh-SH bonds of 2.5-2.6 Å keeps the Rh-Rh bond intact (2.41 ± 0.02 Å) but leads to a colour change from emerald green to burgundy. Time-dependent density functional theory (TD-DFT) calculations were performed to explain the observed shifts in the electronic (UV-vis) absorption spectra. The corresponding sodium thiolates, however, break up the Rh(AcO) framework in the presence of O to form an oligomeric chain of triply S-bridged Rh(III) ions, each with six Rh-S (2.36 ± 0.02 Å) bonds. The Rh...Rh distance, 3.18 ± 0.02 Å, in the chain is similar to that previously found for the aerobic reaction product from aqueous solutions of Rh(AcO) and glutathione (HA), {Na[Rh(HA)]·7HO}, in which each Rh(III) ion is surrounded by about four Rh-S (2.33 ± 0.02 Å) and about two Rh-O (2.08 ± 0.02 Å). The reaction products obtained in this study can be used to predict how dirhodium(II) tetracarboxylates would react with cysteine-rich proteins and peptides, such as metallothioneins.