Hydrogen- and Oxygen-Driven Interconversion between Imido-Bridged Dirhodium(III) and Amido-Bridged Dirhodium(II) Complexes

The reaction of [Cp*RhCl2](2) (CP* = eta(5)-C-5(CH3)(5)) with 2 equiv of p-toluenesulfonamide in the presence of KOH resulted in the formation of the sulfonylimido-bridged dirhodium(III) complex [(Cp*Rh)(2)(mu-NTs)(2)] (1a; Ts = SO2C6H4CH3-p). The imido complex 1a reacted with hydrogen donors such as H-2 and 2-propanol to give the sulfonylamido-bridged dirhodium(II) complex [(Cp*Rh)(2)(mu-NHTs)(2)] (2). Treatment of the (amido)rhodium(II) complex 2 with O-2 regenerated the (imido)rhodium(III) complex la. Complex la also underwent reversible protonation to afford the cationic amido- and imido-bridged dirhodium(III) complex [(Cp*Rh)(2)(mu-NHTs)(mu-NTs)](+) (4), which further reacted with H-2 or 2-propanol to give the (hydrido)bis(amido)dirhodium(III) complex [(Cp*Rh)(2)(mu-H)(mu-NHTs)(2)](+) (5). On the basis of DFT calculations and experimental results using 4 and 5, the reaction of la with H-2 Proved to proceed via heterolytic cleavage of H-2 assisted by the sulfonyl oxygen atom followed by proton migration from the metal center. Furthermore, the redox interconversion between 1a and 2 was applied to catalytic aerobic oxidation of H-2 and an alcohol by using la as a well-defined dinuclear catalyst. The iridium complex [(Cp*lr)(2)(mu-NTs)(2)] (1b) as well as a rhodium complex [Cp*RhCl2](2) without bridging imido ligands did not catalyze these aerobic oxidation reactions.