Mechanistic Studies of O2 Reduction Effected by Group 9 Bimetallic Hydride Complexes

Synthetic and kinetic studies are used to uncover mechanistic details of the reduction of O2 to water mediated by dirhodium complexes. The mixed-valence Rh20,II(tfepma)2(CNtBu)2Cl2 (1, tfepma = MeN[P(OCH2CF3)2]2, CNtBu = tert-butyl isocyanide) complex is protonated by HCl to produce Rh2II,II(tfepma)2(CNtBu)2Cl3H (2), which promotes the reduction of O2 to water with concomitant formation of Rh2II,II(tfepma)2(CNtBu)2Cl4 (3). Reactions of the analogous diiridium complexes permit the identification of plausible reaction intermediates. Ir20,II(tfepma)2(CNtBu)2Cl2 (4) can be protonated to form the isolable complex Ir2II,II(tfepma)2(CNtBu)2Cl3H (5), which reacts with O2 to form Ir2II,II(tfepma)2(CNtBu)2Cl3(OOH) (6). In addition, 4 reacts with O2 to form Ir2II,II(tfepma)2(CNtBu)2Cl2(η2-O2) (7), which can be protonated by HCl to furnish 6. Complexes 6 and 7 were both isolated in pure form and structurally and spectroscopically characterized. Kinetics examination of hydride complex 5 with O2 and HCl furnishes a rate law that is consistent with an HCl-elimination mechanism, where O2 binds an Ir0 center to furnish an intermediate η2-peroxide intermediate. Dirhodium congener 2 obeys a rate law that not only is also consistent with an analogous HCl-elimination mechanism but also includes terms indicative of direct O2 insertion and a unimolecular isomerization prior to oxygenation. The combined synthetic and mechanistic studies bespeak to the importance of peroxide and hydroperoxide intermediates in the reduction of O2 to water by dirhodium hydride complexes.