Iron(II, III)-Chlorin and Isobacteriochlorin Complexes. Models of the Heme Prosthetic Groups in Nitrite and Sulfite Reductases: Means of Formation and Spectroscopoic and Redox Properties

Article abstract of DOI:10.1021/ja00406a018

Extensive series of iron(II, III)-hydroporphyrin complexes of the types Fe(P)L, ^0,+, 2O, and Fe(P), with P = octaethylchlorin (OEC) and octaethylisobacteriochlorin (OEiBC) and L, L' = neutral or uninegative axial ligands, have been synthesized and isolated or generated in solution.Means of synthesis and reactivity properties of OEC and OEiBC complexes parallel those of octaethylporphyrin (OEP) complexes.This behavior, together with a detailed body of physicochemical properties (absorption, MCD, 1H NMR, EPR, and infrared spectra and voltammetry), serves to identity all new complexes.Certain of the OEiBC species are pertinent as possible analogues of the siroheme prosthetic group of nitrite and sulfite reductases.Physicochemical properties of OEP, OEC, and OEiBC complexes at parity of axial ligation are compared in an attempt to identify any intrinsic features of isobacteriochlorin species that might render them particularly suitable for mediation of multielectron reductions of substrates as executed by siroheme enzymes.Properties such as Fe(III)/Fe(II) potentials and ν_CO of Fe(P)L(CO) and Fe(P)(CO)1,2 were found to be nearly invariant to P, indicating little cis effect of these macrocycles, which are in different reduction levels.That property most dependent on macrocycle structure was found to be the potential for ring-based oxidation which increases in the order OEiBC < OEC < OEP.Comparative properties are discussed in some detail and are related to available information on sirohemes, including the question of axial ligation in the native enzyme.This research affords the first comprehensive examination of the preparation and chemical, spectroscopic, and redox properties of iron(II, III)-hydroporphyrin complexes.