Mechanism of oxygenative cleavage of catechols by nonheme iron complexes in relevance to catechol dioxygenases studied by quantum chemical calculations

Abstract

Mechanism of oxygenative cleavage of catechols by nonheme iron complexes was studied by quantum chemical calculations. Calculations based on the density-functional theory indicate that all carbon atoms of the DMC and Cat ligands of [Fe III(NH 3) 4(DMC)] + (DMC: 3,5-dimethylcatecholate), [Fe III(NH 3) 4(Cat)] + and [Fe II(NH 3) 4(Cat)] (Cat: catecholate) are positively charged, that is not favorable for the electrophilic attack. Significant amounts of the spin density, that are greater on oxygen than carbons, appear on the catecholate ligand. The spin density on aromatic carbon atoms is greater in the ferric complex than in the ferrous complex, supporting the Fe(II)-semiquinonate character of the ferric catecholate complexes. Results are obtained to support the probability of the initial binding of molecular oxygen to the iron center rather than to the aromatic carbons. In the step of the oxygen insertion into the C–C bond, formation of an epoxide-like structure is proposed. It is shown that the postulated intermediate can be converted to an oxygen-inserted product in the change of the electronic state from the anionic ligand to the neutral product.