Outer Sphere Oxidations of Alcohols and Formic Acid by Charge Transfer Excited States of Iron(III) Species

Abstract

When methanol, 2-propanol, and formic acid are used as scavengers in the ligand to metal charge transfer (l.m.c.t.) photolysis of Fe(OH2)63+, Fe(OH2)5CI2+, Fe2(OH2)8(OH)24+, or FeEDTA, there is a linear relationship between quantum yield for Fe(II) production and scavenger concentration, [S], at higher [S] values. Extrapolation of the linear portions to [S] = 0 gives an intercept corresponding to the limiting yields observed for scavenging with tert-butyl alcohol. Butanol scavenging at the limit has been shown to give the primary free radical yields from photolysis of aquo iron(III) species. Nuclear magnetic resonance relaxation time studies show that alcohols do not coordinate to Fe(III) and calculations from known stability constants indicate that formic acid does not coordinate under the experimental conditions. The increase of Fe(II) yields with [S] is attributed to an outer sphere oxidation of noncoordinated organic species by the charge transfer excited states of Fe(III) species. There is no discrimination among the organic reductants. The results may be understood without postulating a long lifetime for the Fe(III) l.m.c.t. states if the reaction is assumed to occur only with organic molecules in encounter with the Fe(III) complex at the time of excitation. Organic products were formaldehyde from methanol oxidation and acetone from 2-propanol oxidation. The Fe(II): formaldehyde stoichiometry was 2:1.