A mechanistic study of nitrite reduction on iron(ii) complexes of methylated N-confused porphyrins

Abstract

Proton delivery to the prosthetic group is a crucial step to sustain the activity of nitrite reductase. An iron N-confused porphyrin (NCP) complex, which is capable of relaying protons from the outer pyrrolic nitrogen (N_out–H) of the inverted pyrrole ring to the axial coordinated ligand, has been demonstrated to facilitate facile nitrite reduction. Time-dependent FTIR studies on the reaction between [Fe^II(HCTPPMe)Br] (1) and a nitrite anion revealed a two-step process involving conversion of the starting complex 1 to an {Fe(NO)}⁷ intermediate, [Fe(CTPPMe)(NO)] (5), before the detection of [Fe(CTPPCH₂)(NO)] (3), an {Fe(NO)}⁶ end product. Moreover, spectroscopic data confirm that N_out–H on the NCP core is indispensable to the proceeding of the nitrite reduction reaction. Mass spectra have detected the coordination of a nitrite to the iron center while DFT theoretical calculations suggest that subsequent intramolecular proton transfer to a nitro group to form [Fe(CTPPMe)(HNO₂)] (6a) evokes a homolytic N–OH bond fission on axial nitrous acid due to an enhanced π-back-bonding to produce an {Fe(NO)}⁷ intermediate and to release a hydroxyl radical. The subsequent oxidation of an {Fe(NO)}⁷ intermediate by the hydroxyl radical gave the final product, {Fe(NO)}⁶ [Fe(CTPPCH₂)(NO)] (3). This study illustrates a proton assisted small molecule activation on the iron N-confused porphyrin coordination sphere and provides complemental insights into the mechanism of enzymatic nitrite reduction reactions.