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 <i>trans</i>-Rh^ICl­(L)­(PEt₃)₂ (<b>1a</b>, L = CO; <b>1b</b>, L = 2,6-dimethylphenylisocyanide (CNXy); <b>1c</b>, L = 1-adamantylisocyanide (CNAd)) produces the corresponding Rh^III hydride complex <i>cis</i>-<i>trans</i>-Rh^IIICl₂H­(L)­(PEt₃)₂ (<b>2a</b>–<b>c</b>). 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 <i>trans</i>-Rh^IIICl₃(L)­(PEt₃)₂ (<b>3a</b>–<b>c</b>) as the metal-containing product. In the case of <b>2a</b>, smooth conversion to <b>3a</b> proceeds without spectroscopic evidence for an intermediate species. For <b>2b/c</b>, an aqua intermediate, <i>cis</i>-<i>trans</i>-[Rh^III(OH₂)­Cl₂(L)­(PEt₃)₂]Cl (<b>5b/c</b>), forms along the pathway to producing <b>3b/c</b> as the final products. The aqua complexes were independently prepared by treating peroxo complexes <i>trans</i>-Rh^IIICl­(L)­(η²-O₂)­(PEt₃)₂ (<b>4b/c</b>) with HCl to rapidly produce a mixture of <b>5b/c</b> and <b>3b/c</b>. The reactivity of the peroxo species demonstrates that they are plausible intermediates in the O₂-reduction chemistry of hydride complexes <b>2a</b>–<b>c</b>. 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