Oxygen Reduction Mechanism of Monometallic Rhodium Hydride Complexes

The
reduction of O2 to H2O mediated by a series
of electronically varied rhodium hydride complexes of the form cis,trans-RhIIICl2H­(CNAd)­(P­(4-X-C6H4)3)2 (2) (CNAd = 1-adamantylisocyanide; X = F (2a), Cl (2b), Me (2c), OMe (2d)) was examined through synthetic and kinetic studies. Rhodium­(III)
hydride 2 reacts with O2 to afford H2O with concomitant generation of trans-RhIIICl3(CNAd)­(P­(4-X-C6H4)3)2 (3). Kinetic studies of the reaction of
the hydride complex 2 with O2 in the presence
of HCl revealed a two-term rate law consistent with an HX reductive
elimination (HXRE) mechanism, where O2 binds to a rhodium­(I)
metal center and generates an η2-peroxo complex intermediate, trans-RhIIICl­(CNAd)­(η2-O2)­(P­(4-X-C6H4)3)2 (4), and a hydrogen-atom abstraction (HAA) mechanism,
which entails the direct reaction of O2 with the hydride.
Experimental data reveal that the rate of reduction of O2 to H2O is enhanced by electron-withdrawing phosphine
ligands. Complex 4 was independently prepared by the
addition of O2 to trans-RhICl­(CNAd)­(P­(4-X-C6H4)3)2 (1). The reactivity of 4 toward HCl reveals
that such peroxo complexes are plausible intermediates in the reduction
of O2 to H2O. These results show that the given
series of electronically varied rhodium­(III) hydride complexes facilitate
the reduction of O2 to H2O according to a two-term
rate law comprising HXRE and HAA pathways and that the relative rates
of these two pathways, which can occur simultaneously and competitively,
can be systematically modulated by variation of the electronic properties
of the ancillary ligand set.