Small Reorganization Energy for Ligand-Centered Electron-Transfer Reduction of Compound I to Compound II in a Heme Model Study.

The electron-transfer (ET) processes from electron-donor substrates to oxoiron(IV) porphyrin π-cation-radical species (Cpd I) are key steps in their oxygenation reactions. Here, we have evaluated the rate constants of the outer-sphere ET reduction of Cpd I model complexes of meso-tetramesitylporphyrin (1) and 2,7,12,17-tetramesityl-3,8,13,18-tetramethylporphyrin (2) in light of the Marcus theory of ET to determine the ET reorganization energies (λ). The λ values of the ligand-centered ET reduction of Cpd I model complexes are much smaller than those of the metal-centered ET reduction of various oxoiron(IV) complexes. Moreover, the λ value of 1 is larger than that of 2, resulting from the difference in the nature of the a1u/a2u porphyrin π-cation-radical orbitals.