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

Abstract

The reaction of [Cp*RhCl(2)](2) (Cp* = eta(5)-C(5)(CH(3))(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 = SO(2)C(6)H(4)CH(3)-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 1a. Complex 1a 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 1a 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 1a as a well-defined dinuclear catalyst. The iridium complex [(Cp*Ir)(2)(mu-NTs)(2)] (1b) as well as a rhodium complex [Cp*RhCl(2)](2) without bridging imido ligands did not catalyze these aerobic oxidation reactions.