Functional Models for Catechol 1,2-Dioxygenase. Structure, Reactivity, and Mechanism

A series of [Fe(L)DBC]^2-complexes, where L is a tetradentate tripodal ligand and DBC is 3,5-di-tert-butylcatecholate, has been synthesized and characterized by visible spectroscopy and cyclic voltammetry. The tripodal ligands are derivatives of NTA, nitrilotriacetic acid, where one of the carboxymethyl arms has been replaced by a 2-hydroxybenzyl group. Crystals of (dabcoH)₂[Fe(NTA)DBC](P2₁/n; dabco, l,4-diazabicyclo[2.2.2]octane) reveal a six-coordinate high-spin ferric complex with an unsymmetrically chelated DBC_r(Fe-O(DBC)⁼1–889, 1.979 A). The visible spectra of the complexes exhibit catecholate-to-Fe(III) charge-transfer transitions in the 600-nm region, while their cyclic voltammograms show a reversible wave near 0 V vs. SCE due to the semiquinone/catecholate couple. The energy of the LMCT band and the E°' value for a particular complex shift in response to the Lewis acidity of the metal center, as expected. These complexes react with dioxygen over a period of days to yield two identified products, 3,5-di-tert-butylmuconic anhydride (trapped as the furanone) and 3,5-di-tert-butyl-2-hydroxy-p-benzoquinone. The product yields depend on the Lewis acidity of the metal center, with the NTA complex yielding the highest amount of anhydride (ca. 80%) and essentially no quinone. As the Lewis acidity of the metal center diminishes, the amount of anhydride decreases and the yield of quinone increases. These observations are interpreted in light of the substrate activation mechanism proposed for the catechol dioxygenases; important features include the reaction of a monodentate catecholate-iron(III) complex with 0₂to form an iron peroxide intermediate and its subsequent metal-facilitated decomposition to the anhydride.