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

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posted on 08.04.2009, 00:00 by Koji Ishiwata, Shigeki Kuwata, Takao Ikariya
The reaction of [Cp*RhCl2]2 (Cp* = η5-C5(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(μ-NTs)2] (1a; Ts = SO2C6H4CH3-p). The imido complex 1a reacted with hydrogen donors such as H2 and 2-propanol to give the sulfonylamido-bridged dirhodium(II) complex [(Cp*Rh)2(μ-NHTs)2] (2). Treatment of the (amido)rhodium(II) complex 2 with O2 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(μ-NHTs)(μ-NTs)]+ (4), which further reacted with H2 or 2-propanol to give the (hydrido)bis(amido)dirhodium(III) complex [(Cp*Rh)2(μ-H)(μ-NHTs)2]+ (5). On the basis of DFT calculations and experimental results using 4 and 5, the reaction of 1a with H2 proved to proceed via heterolytic cleavage of H2 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 H2 and an alcohol by using 1a as a well-defined dinuclear catalyst. The iridium complex [(Cp*Ir)2(μ-NTs)2] (1b) as well as a rhodium complex [Cp*RhCl2]2 without bridging imido ligands did not catalyze these aerobic oxidation reactions.