Ruthenium(IV)-Oxo Complexes: The Novel Utilization of Tertiary Pnictogen Ligands

The novel complexes [(bpy)₂(PnR₃)Ru^IV(O)](ClO₄)₂(where bpy = 2, 2'-bipyridine and PnR₃ = tertiary phosphine or arsine) have been generated from the analogous ruthenium(II)-aquo species, through electrochemical means or by the addition of two equivalents of cerium(IV). Characterization of the ruthenium(II)-aquo and ruthenium(IV)-oxo complexes was accomplished through UV-vis spectroscopy, IR spectroscopy, ¹⁸O labeling experiments, NMR spectroscopy, cyclic voltammetry, coulometry, and conductivity measurements. These complexes are stable both in the solid state and in various solutions, where the phosphine and arsine ligands do not undergo oxidation by the ruthenium(IV)-oxo center. Cyclic voltammetric measurements of the ruthenium(II)-aquo complexes are consistent with the following redox couples: Ru^IV═O/Ru^III—OH/Ru^II—OH₂, which can be described as two sets of concomitant one-electron, one-proton transfers. The ruthenium(IV)-oxo complexes act as selective oxidation reagents toward organic substrates, where the nature of the pnictogen ligand greatly affects the rate of substrate oxidation. In addition, the use of a pnictogen ligand cis to the oxo moiety in these complexes simplifies the mechanism of substrate oxidation relative to other ruthenium-based oxidants. Finally, pnictogen ligands exert unusual effects on the redox chemistry of ruthenium (IV)-oxo complexes, including hydrophobic selectivity and aerobically driven substrate oxidation catalysis.