Electrochemical behaviour of a heme model complex in aqueous solution

Abstract

The electrochemical behaviour of the meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,4,8,11-tetraene iron(II) complex has been studied in aqueous solution, in the presence of the N-methyl imidazole ligand. Below pH 5, the bis(imidazole) complex (λ max = 692 nm, E 1 2 = 0.78 V vs. SHE, I = 0.50 M NaCl) dissociates, yielding the monosubstituted complex ( E 1 2 = 0.64 V). The cyclic voltammograms were analysed successfully in terms of Shain and Nicholson's formalisms for a chemical reaction coupled with a reversible charge transfer process. The equilibrium and kinetic constants for the dissociation reactions in the iron(II) complexes were calculated as K = k d/ k f = 5.4 × 10 −5 M −1 k d = 1.2 s −1 and k f = 2.3 × 10 M −1 s −1. In the case of the iron(III) complexes, the dissociation constant was evaluated as K' = k d'/ k f' = 1.2 × 10 −2 M −1, where k d' = 0.13 s −1 and k f' = 11 M −1 s −1. The reduction of the iron(II) macrocyclic complexes proceeds at about −0.6 V, leading to a red species (λ max = 520 nm) which is believed to be a hydride product. By reversing the potential scan, the starting Fe(II) complex is regenerated quantitatively.