Activation of nitrite ion by biomimetic iron(III) complexes

Abstract

The rate of reaction of NO 2 − ion with various FeIII porphyrins in the presence of PPh3 is shown to depend on the redox potential of the FeIII center. There is a linear relationship between the ease of reduction of the FeIII to FeII and the kinetics for the formation of the FeII porphyrin nitrosyl adduct, with concomitant oxidation of PPh3 to PPh3O. Cyclic voltammograms show reversible one-electron reductions that can be ascribed to the FeIII/FeII couple ranging from E1/2 = −343 to −145 mV (versus Ag/AgCl). The order of increasing half-wave reduction potentials for the FeIII/FeII porphyrin redox centers studied is octaethylporphyrin > etioporphyrin I > deuteroporphyrin IX dimethyl ester > protoporphyrin IX dimethyl ester > α,β,γ,δ-tetraphenylporphyrin. This sequence of redox potentials complements the pseudo first-order kinetics ( ${k_{\rm obs}=2.2\times10^{-3}}$ to ${13\times10^{-3}}$ m s −1) for the oxidation of PPh3 and subsequent FeII porphyrin nitrosyl adduct formation. The rates of reaction of biomimetic FeIII porphyrins with NO 2 − ion demonstrate how metal center redox properties are influenced by the surrounding ligand. In this paper we have elucidated a possible mechanistic control for the rate of this reaction.