Electrochemical oxidation of Rh(I) to Rh(III) in rhodium(I) β-diketonato carbonyl phosphine complexes

Cyclic voltammetric (CV) studies revealed a direct relationship between the irreversible oxidation potentials of Rh(I) to Rh(III) in complexes of the type [Rh(I)(β-diketonato)(CO)(PPh3)] and the pKa-values of the free β-diketones dibenzoylmethane (Hdbm), benzoylacetone (Hba), benzoyltrifluoroacetone (Hbtfa) and trifluoroacetone (Htfaa). More electronegative β-diketone ligands remove electron density from the rhodium metal, making the complex a stronger Lewis acid and less reactive towards oxidative addition. The effect of different solvents on the reduction potential of these complexes proved solvent co-ordination during the electrochemical oxidation of square planar Rh(I) to the octahedral Rh(III). A direct relationship was obtained between the irreversible reduction potential and the solvent donicity of the solvent acetonitrile, methanol, dimethylformamide and pyridine.