Activation of nitrite ion by biomimetic iron(III) complexes

Abstract

The rate of reaction of NO ⁻₂ ion with various Fe^III porphyrins in the presence of PPh₃ is shown to depend on the redox potential of the Fe^III center. There is a linear relationship between the ease of reduction of the Fe^III to Fe^II and the kinetics for the formation of the Fe^II porphyrin nitrosyl adduct, with concomitant oxidation of PPh₃ to PPh₃O. Cyclic voltammograms show reversible one-electron reductions that can be ascribed to the Fe^III/Fe^II couple ranging from E_1/2 = −343 to −145 mV (versus Ag/AgCl). The order of increasing half-wave reduction potentials for the Fe^III/Fe^II 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 Fe^II porphyrin nitrosyl adduct formation. The rates of reaction of biomimetic Fe^III 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.