Mechanisms of ligand-to-metal intramolecular electron transfer in cobalt(III)-amine complexes containing a coordinated radical

Abstract

The one-electron reduction of Co(III) complexes containing nitrophenyl ligands possessing differing lead-in and bridging groups by radiolytically generated CO/sub 2//sup -/ and C(CH/sub 3/)/sub 2/OH radicals in neutral and acidic aqueous solution results in the formation of coordinated nitrophenyl ligand radicals. The uv-visible absorption spectra, the acid-base properties, and the decay kinetics of the transient intermediates were examined by pulse radiolysis. In neutral solution, the coordinated ligand radicals decay via intramolecular electron transfer from the coordinated nitrophenyl radical donor to the Co(III) acceptor. The values of the intramolecular electron-transfer rate constants depend on the isomeric position of the nitro group on the phenyl moiety, the structure of the bridging molecule between the redox sites, and the nature of the lead-in group to the metal center. Bridging structures between the initial radical site and the metal center of varying length, flexibility, and ..pi..-conjugation are incorporated into the 18 complexes studied. Correlation of the values of ..delta..H double dagger and ..delta..S double dagger of electron transfer with the structural relationship of the donor and acceptor sites leads to the proposition that four different mechanisms of intramolecular electron transfer operate in these complexes: through chain, direct and indirect ligand bypass, and nonadiabatic transfer.more » Protonation of the coordinated nitro radical greatly diminishes the rate of intramolecular electron transfer in the nitrophenol carboxylato complexes; in most cases, protonation affects only the driving force for electron transfer while leaving the mechanism unchanged. 37 references, 3 figures, 4 tables.« less