Iron(III) complexes of phenolate ligands as models for catechol dioxygenases

Abstract

Catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) enzymes catalyse the oxidative cleavage of catechols tocis, cis-muconic acids with the incoporation of molecular oxygen. In our laboratory two series of iron(III) complexes of linear tridentate and tripodal tetradentate phenolate ligands have been characterised using IR, UV-Vis and EPR spectral and electrochemical techniques. The X-ray crystal structure of a few of the complexes have been determined. The interactions of the complexes with a variety of monodentate and bidentate heterocyclic bases as well as phenols have been investigated. The interactions with catecholate anions reveal changes in the phenolate-to-iron(III) charge transfer band, which are remarkably similar to catechol dioxygenase-substrate complexes. The redox behaviour of the complexes and their 1:1 adducts with 3,5-di-t-butylcatechol (H₂DBC) has been investigated. All the complexes catalyse the oxidative cleavage of H₂DBC by molecular oxygen to yieldcis,cis-muconic anhydride. The structure, redox and catalytic activities of the iron(III) complexes have been discussedvis-a-vis those of the enzymes.