Redox chemistry of trinuclear complexes possessing a hexathiolatomolybdate(IV) core: in situ syntheses, characterization and geometry optimization

Abstract

The trinuclear heterobimetallic clusters [(Ph₃P)Cu(µ-SR)₃Mo(µ-SR)₃Cu(PPh₃)](R = C₆H₄Me-4 2, C₆H₄F-4 3, C₆H₄Cl-4 4 or C₆H₄Br-4 5) have been found to undergo reversible one-electron reductions. The reduction potential is sensitive to the para substituent on the thiolate and has been correlated with the electron-donating properties of the substituent. In addition, 2 undergoes a reversible one-electron oxidation. According to EPR measurements, the oxidation of 2 is molybdenum-based. Geometry optimizations based on density functional theory of the oxidized and reduced model cluster [(H₃P)Cu(µ-SH)₃Mo(µ-SH)₃Cu(PH₃)], assuming retention of the D₃ point-group symmetry, have shown that both redox processes will lead to an elongation of the Mo–Cu vector. Reduction of the central molybdenum fragment will decrease the strength of the Cu to Mo donor bond. Oxidation of the central molybdenum fragment will result in a decrease of electrostatic interaction with the [(H₃P)CuCu(PH₃)]²⁺ fragment. The reaction of 2 with NOBF₄ results in oxidation of the cluster. The IR spectrum of the diamagnetic reaction product shows a band at 1657 cm^–1 suggesting the presence of co-ordinated NO^–. Further evidence is provided by ¹⁴N NMR spectroscopy. The reactions of 2 and 5 with arenediazonium salts result in degradation of the clusters, with the liberation of organic sulfides and disulfides, suggesting the involvement of free thiyl radicals.