ELECTRON TRANSFER REACTIONS OF BRIDGED BINUCLEAR IRON(III)- COMPLEXES: A REVIEW

Binuclear iron (III) complexes comprising of the -oxo-bridged form, [Fe2O]4+ or [Fe(OH)2]4+, and the dicarboxylic bridged chains incorporating salen or saloph ligands have been synthesized and characterized. Their close resemblance to some haeme and non-haeme iron-based enzymes has elicited a lot of interest in studying them. Biological studies have shown some of them to be good mimics of superoxide dismutase. The saloph and salen analogues also showed physiological actions very close to the porphyrins. The study of their electron transfer reactions, with a view of understanding their potential as electron transfer enzymes, has generated interesting results and some unusual kinetic behaviours. Reactions of [(FeHEDTA)2O]2+ with ascorbic acid and some thiols were second order and followed outer-sphere path mediated by ion-pairs formation. However, reaction of [Fe2(bipy)O]4+ with some reductants were zero order with respect to the oxidant and also followed outer-sphere path. Reduction of the adipato-bridged dimer, [Fe(saloph)2µ-adi], by SO32- and SO42-, S2O52-, benzyl mercaptan, mercaptobenzothiazole and thioglycolic acid followed inner-sphere paths. However, all the reactions were acid-catalysed and were mediated by intervening protonated oxidant precursor complexes.