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

[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 DBC 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 O₂ 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 O₂ 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.