A novel iron(III) complex with a tridentate ligand as a functional model for catechol dioxygenases: Properties and reactivity of [Fe(BBA)DBC]ClO4

Abstract

[Fe-III(BBA)DBC]ClO4 as a new functional model for catechol dioxygenases has been synthesized, where BBA is a bis(benzimidazolyl-2-methyl)amine and DEC is a 3,5-di-tert-butylcatecholate dianion. The BBA complex has a structural feature that iron center has a five-coordinate geometry similar to that of catechol dioxygenase-substrate complex. The BBA complex exhibits strong absorption bands at 560 and 820 nm in CH3CN which are assigned to catecholate to Fe(III) charge transfer transitions. It also exhibits EPR signals at g = 9.3 and 4.3 which are typical values for the high-spin Fe-III (S = 5/2) complex with rhombic symmetry. Interestingly, the BBA complex reacts with O-2 within an hour to afford intradiol cleavage (35%) and extradiol cleavage (60%) products. Surprisingly, a green color intermediate is observed during the oxygenation process of the BBA complex in CH3CN. This green intermediate shows a broad isotropic EPR signal at g = 2.0. Based on the variable temperature EPR study, this isotropic signal might be originated from the [Fe(III)-peroxo-catecholate] species having low-spin Fe-III center, not from the simple organic radical. Consequently, it allows O-2 to bind to iron center forming the Fe(III)-superoxide species that converts to the Fe((III))-peroxide intermediate. These present data can lead us to suggest that the oxygen activation mechanism take place for the oxidative cleaving catechols of the five-coordinate model systems for catechol dioxygenases.