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 Rh^II—Rh^II 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 Rh^III...Rh^III 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[Rh₂^III(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.