Oxygen Reduction Reactions of Monometallic Rhodium Hydride Complexes

Selective reduction of oxygen is mediated by a series of monometallic rhodium­(III) hydride complexes. Oxidative addition of HCl to trans-RhICl­(L)­(PEt3)2 (1a, L = CO; 1b, L = 2,6-dimethylphenylisocyanide (CNXy); 1c, L = 1-adamantylisocyanide (CNAd)) produces the corresponding RhIII hydride complex cis-trans-RhIIICl2H­(L)­(PEt3)2 (2a–c). The measured equilibrium constants for the HCl-addition reactions show a pronounced dependence on the identity of the “L” ligand. The hydride complexes effect the reduction of O2 to water in the presence of HCl, generating trans-RhIIICl3(L)­(PEt3)2 (3a–c) as the metal-containing product. In the case of 2a, smooth conversion to 3a proceeds without spectroscopic evidence for an intermediate species. For 2b/c, an aqua intermediate, cis-trans-[RhIII(OH2)­Cl2(L)­(PEt3)2]Cl (5b/c), forms along the pathway to producing 3b/c as the final products. The aqua complexes were independently prepared by treating peroxo complexes trans-RhIIICl­(L)­(η2-O2)­(PEt3)2 (4b/c) with HCl to rapidly produce a mixture of 5b/c and 3b/c. The reactivity of the peroxo species demonstrates that they are plausible intermediates in the O2-reduction chemistry of hydride complexes 2a–c. These results together show that monometallic rhodium hydride complexes are capable of promoting selective reduction of oxygen to water and that this reaction may be controlled with systematic alteration of the ancillary ligand set.