Chemical and Electrochemical Properties of [Cp*Rh] Complexes Supported by a Hybrid Phosphine-Imine Ligand

A series of [Cp*Rh] complexes (Cp* = eta(5)-pentamethylcyclopentadienyl) bearing the kappa(2)-[P,N]-8-(diphenylphosphino)quinoline (PQN) ligand have been prepared and characterized. Chemical or electrochemical reduction of the rhodium(III) form generates an isolable rhodium(I) complex; this rhodium(I) complex reacts with a range of organic acids to yield a rhodium(III) hydride bearing [Cp*] in the eta(5) mode and [PQN] in the expected kappa(2) mode. Solid-state structures of these three compounds from X-ray diffraction studies reveal only small changes in the intraligand bond distances across the series, suggesting the redox events associated with interconversion of these compounds are primarily metal centered. Cyclic voltammetry data show that the rhodium(III) chloride complex undergoes a two-electron reduction at -1.19 V vs ferrocenium/ferrocene, whereas the analogous solvento rhodium(III) acetonitrile complex undergoes two sequential one-electron reductions. The rhodium(III) hydride undergoes an irreversible, ligand-centered reduction near -1.75 V vs ferrocenium/ferrocene. Carrying out this reduction alone or in the presence of added of triethylammonium as a source of protons results in only modest yields of H-2, as shown by bulk electrolyses and chemical reduction experiments. These results are discussed in the context of recent work with [Cp*Rh] complexes bearing more symmetric 2,2'-bipyridyl and diphosphine ligands.