Reversible Redox Transformations of Bridging Sulfide Ligands within Bioctahedral Rhenium Cluster Anions.

The present study provides new experimental data on the reactivity of sulfur-based ligands connected to Re atoms. It was recently discovered that oxidation of µ-S ligands within the bioctahedral [Re12CS14(µ-S)3(CN)6]6- cluster anion using H2O2 led to the formation of µ-SO2 and µ-SO3 products. This work is focused on the investigation of the opposite process - reduction of the [Re12CS14(µ-SO2)3(CN)6]6- anion to [Re12CS14(µ-S)3(CN)6]6- by sulfide ions in an aqueous medium under mild conditions. The reduction results in a slow change of the solution colour, making it possible to investigate in detail the intermediate products. Analysis of the reaction mixture by ESI mass spectrometry revealed that a series of cluster anions [Re12CS17O n(CN)6]6- ( n = 0-5) was generated during the process. This indicates stepwise chemical reduction of bridging SO2 ligands proceeding through the formation of µ-SO ligands. In this way, twelve-nuclear rhenium clusters demonstrate a unique, fully reversible redox cycle of their µ-bridging ligands, keeping the cluster core unchanged. Two mixed-ligand cluster anions, [Re12CS14(µ-SO2)2(µ-S)(CN)6]6- and [Re12CS14(µ-SO2)(µ-S)2(CN)6]6-, were found to be the major reaction intermediates. The former was crystallised with [Cu(NH3)5]2+ and [Cd(NH3) n]2+ ammine cations and investigated by single-crystal X-ray diffraction. To understand the electronic structure and charge distribution in [Re12CS17O n(CN)6]6- cluster anions, DFT calculations were also carried out. [ABSTRACT FROM AUTHOR]