Functional Models for Catechol 1,2-Dioxygenase. The Role of the Iron(III) Center

A series of [Fe(L)DBC] complexes, where L is a tetradentate tripodal ligand and DBC is 3,5-di-terr-butylcatecholate, has been prepared to serve as functional mimics for the catechol dioxygenases. The tripodal ligands, (RCH₂)2N(CH₂R') (NTA, R = R' = COO^-; PDA, R = COO^-, R'= 2-pyridyl; BPG, R = 2-pyridyl, R' = COO^-; HDP, R = 2-pyridyl, R' = 2-hydroxy-3,5-dimethylphenyl), serve to tune the Lewis acidity of the ferric center. This tuning is manifested in differences in the energies of the catecholate-to-Fe^III charge-transfer bands, the potentials of the semiquinone/catecholate couple, and the shifts of the DBC protons found for the complexes. These complexes react with O₂ with high specificity to yield a product resulting from C1-C2 oxidative cleavage. Kinetic studies show that the rate-determining step involves the attack of dioxygen on the complex, and the rates of reaction of the complexes increase in the order of HDP, NTA, PDA, and BPG. This order in general follows the increase in Lewis acidity of the ferric center but more specifically correlates with increasing semiquinone character on the DBC ligand as indicated by the NMR shifts. As a further check, [Fe(BPG)DBC].2CH₃OH (P₂1/n) was crystallographically characterized and compared with the previously reported [Fe(NTA)DBC]^2- The structures are similar; both are six-coordinate high-spin ferric complexes with unsymmetrically chelated DBC ligands (ΔrFe-O(Dbc)= 0.1 Å for both). Taken together, these observations suggest that the substrate activation proposed for the dioxygenase mechanism results from the delocalization of unpaired spin density from the ferric center onto the coordinated catecholate arising from ligand-to-metal charge transfer.