Quantitative evaluation of lewis acidity of metal ions derived from the g values of ESR spectra of superoxide: Metal ion complexes in relation to the promoting effects in electron transfer reactions

The g values of ESR spectra of superoxide - metal ion complexes (O₂^.- Mⁿ⁺, n = 1, 2, 3) are determined in acetonitrile at 143 K. The binding energies (ΔE) of metal ions with O₂^.- have been evaluated from deviation of the g_zz values from the free spin value. The ΔE values are well correlated with the catalytic reactivities of metal ions in electron transfer from cobalt(II) tetraphenylporphyrin to O₂ and p-benzoquinone, which does not occur in the absence of metal ions under otherwise the same experimental conditions. The ΔE values can thereby be used as the first quantitative measure for Lewis acidity of metal ions in relation with the catalytic reactivities in electron transfer reactions.