The systematic influence of tripodal ligands on the catechol cleaving activity of iron(III) containing model compounds for catechol 1,2-dioxygenases

Abstract

A series of mononuclear iron(III) complexes as functional and structural model compounds for intradiol cleaving catechol dioxygenases were synthesized. For all model compounds the iron(III) cores are in a distorted octahedral environment derived from tripodal tetradentate N₄-donor ligands and a catechol. Model complexes for enzyme–substrate adducts were characterized by spectroscopic and electrochemical methods, and in four cases by single-crystal X-ray crystallography. The systematic variation of one ligand arm in the structurally characterized complexes yields a different steric shielding of the iron(III) center, significantly influencing the bonding of the catechol substrate and the subsequent reaction with dioxygen. The spectroscopic features and catechol cleaving activities of in situ generated complexes with the above ligands were probed. All complexes are highly reactive towards intradiol cleavage of various catechols in the presence of air. The catechol 1,2-dioxygenase reaction depends on the redox potential of both the iron(III) complex and the catechol derivative as well as the steric demand of the tripodal ligand. Some complexes show high catalytic activities with yields up to 84% with respect to aerial cleavage of catechols.