Analogues of the iron-binding site in catechol 1,2-dioxygenase: iron(III) complexes of benzimidazole and pyridine-containing tridentate ligands

Abstract

A series of iron(III) complexes of the type [FeLCl₂]·H₂O [HL =N-(pyridin-2-ylmethyl)salicylideneamine, (2-hydroxy-5-nitrobenzyl)(pyridin-2-ylmethyl)amine, (2-hydroxy-5-nitrobenzyl)(2-pyridin-2-ylethyl)amine, N-(2-imidazol-4-ylethyl)salicylideamine, N-(benzimidazole-2-ylmethyl)salicylaldimine, (benzimidazol-2-ylmethyl)(2-hydroxybenzyl)amine, or (benzimidazol-2-ylmethyl)(2-hydroxy-5-nitrobenzyl)amine] have been prepared. Also [FeLCl₃]·H₂O [L = bis(benzimidazol-2-ylmethyl)amine] has been isolated. These complexes have been characterised using IR, UV/VIS and EPR spectral and electrochemical techniques. Their interaction with a variety of mono- and bi-dentate heterocyclic bases as well as phenols has been investigated using electronic and EPR spectroscopy. Their interaction with 3,5-di-tert-butylcatechol (H₂dbcat) and its anions reveals changes in the phenolate-to-iron(III) chargetransfer (c.t.) band similar to those for catechol dioxygenase-substrate complexes. The redox behaviour of the iron(III) complexes and their 1 : 1 adducts with dbcat anions generated in situ has been investigated in methanol. There is a linear correlation between the Fe^III-Fe^II redox potential and the c.t. band energy of the complexes. All the complexes catalyse the oxidative cleavage of H₂dbcat by molecular oxygen to yield cis,cis-muconic anhydride. The catalytic activity has been correlated with the Fe^III-Fe^II as well as dbsq-H₂dbcat (dbsq = 3,5-di-tert-butyl-1,2-benzosemiquinone) redox potentials of the dbcat adducts.