Electron-transfer properties of high-valent metal-oxo complexes

Electron-transfer properties of heme and non-heme high-valent metal-oxo complexes are overviewed in relation to their reactivity toward oxidation of substrates. The rate constants of electron transfer from a series of electron donors to various heme and non-heme high-valent metal-oxo complexes such as compound I and compound II of horseradish peroxidase (HRP), (TMP)Mn^IV(O) ((TMP=tetramesityl-porphyrinate dianion), (TBP₈Cz)Mn^V(O) (TBP₈Cz=octa-tert-butylphenylcorrolazinate trianion) and [(L)Fe^IV(O)]²⁺, where L=TMC, 1,4,8,11-tetra-methyl-1,4,8,11-tetraazacyclotetradecane; Bn-TPEN, N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine; N4Py, N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine; bisp, 3,7-diazabicyclo[3.3.1]nonane) were evaluated in light of the Marcus theory of electron transfer in order to determine the reorganization energy (λ) for electron transfer. The λ value increases in the order: 1.3eV (compound I of HRP), 1.53eV [(TBP₈Cz)Mn^V(O)], 1.6eV (compound II of HRP), 1.7eV [(TMP)Mn^IV(O)], 2.05eV [(bisp)Fe^IV(O)]²⁺, 2.37eV [(TMC)Fe^IV(O)]²⁺), 2.55eV ([(Bn-TPEN)Fe^IV(O)]²⁺), 2.74eV ([(N4Py)Fe^IV(O)]²⁺). The λ value of compound I of HRP is the smallest among those of high-valent metal-oxo complexes, because the site of the reduction is on the porphyrin ligand, whereas the site of the reduction is on the metal for the other high-valent metal-oxo complexes. The λ values of high-valent metal-oxo porphyrins are generally smaller than those of non-heme iron(IV)-oxo complexes. The effects of Lewis acidic metal ions and Brønsted acids on the one-electron reduction of high-valent metal-oxo complexes are also overviewed in relation to their enhancement of the oxidizing ability towards reductants.