Reactions of Rh2(CH3COO)4 with thiols and thiolates: a structural study.

The structural differences between the aerobic reaction products of Rh2(AcO)4 (1; AcO- = CH3COO-) 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 Rh2(AcO)4 with Rh-SH bonds of 2.5-2.6 Å keeps the RhII-RhII 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 Rh2(AcO)4 framework in the presence of O2 to form an oligomeric chain of triply S-bridged Rh(III) ions, each with six Rh-S (2.36 ± 0.02 Å) bonds. The RhIII...RhIII distance, 3.18 ± 0.02 Å, in the chain is similar to that previously found for the aerobic reaction product from aqueous solutions of Rh2(AcO)4 and glutathione (H3A), {Na2[Rh2III(HA)4]·7H2O}n, 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.