Mononuclear non-heme iron(III) complexes as functional models for catechol dioxygenases

This article provides an overview of our work on mononuclear iron(III) complexes of phenolate and non-phenolate ligands as structural and functional models for the intradiol-cleaving non-heme catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) enzymes. All the complexes are cis-facially coordinated to iron(III) with a distorted octahedral geometry. The iron(III) complexes of linear tridentate 3N ligands and tetradentate tripodal phenolate ligands possess octahedral geometries with cis-coordination positions available for bidentate coordination of catechols. In two of these complexes with sterically demanding -NMe2 pendant, the Fe-O-C bond angle is around 135.7 degrees, which is close to those (Fe-O-C, 133 degrees, 148 degrees) in 3,4-PCD, enzyme. Also, interestingly, one of the bis-phenolate complexes displays trigonal bipyramidal coordination geometry as in the enzymes. The efficiency of the complexes to catalyze the intradiol-cleavage of 3,5-di-tert-butylcatechol (H2DBC) could be illustrated not only on the basis of Lewis acidity of the iron(III) center alone, but also by assuming that product release is the rate-determining phase of the catalytic reaction.