Cyclic Voltammetry of Rh(I) Complexes and the Oligomers. A Correlation between the Anodic Peak Potentials and the Rate Constants for the Electron Transfer Reactions with Inorganic Oxidants

Article abstract of DOI:10.1246/bcsj.55.2892

The cyclic voltammogram of a CH3CN solution of ⁺ exhibits three anodic current peaks which correspond to the oxidations of the monomer, the dimer, and the trimer, with no cathodic wave on the reverse scan.The anodic peak potentials E^p_ox in such irreversible cyclic voltammograms were determined for various Rh(I) monomers and the oligomers such as ⁺, ²⁺, ²⁺, ²⁺, ³⁺, and ⁴⁺ (R=p-MeOC6H4 and Ph, etc; dppm=bis(diphenylphosphino)methane, dicp=1,3-diisocyanopropane).The anodic peak potentials E^p_ox vary mainly with the degree of oligomerization of the Rh(I) complexes, decreasing in the order monomer > dimer > trimer > tetramer, in parallel with the energies of the highest occupied molecular orbitals E_HOMO.It has been found that the E^p_ox values are linearly correlated with logarithm of the rate constants for the electron transfer reactions with inorganic oxidants such as ³⁺ and ³⁺ (bpy=2,2'-bipyridine) in the context of the Marcus theory as expected when the standard oxidation potentials E⁰_ox would be used.It is thus suggested that the anodic peak potentials of the Rh(I) complexes in the irreversible system can be used as the standard oxidation potentials as far as the relative values are concerned.