MODELS FOR ALPHA-KETO ACID-DEPENDENT NONHEME IRON ENZYMES - STRUCTURES AND REACTIVITY OF [FE-II(L)(O(2)CCOPH)](CLO4) COMPLEXES

The first iron(II)-alpha-ketocarboxylate complexes were synthesized with the use of tetradentate tripodal ligands tris[(6-methyl-2-pyridyl)methyl] amine (6TLA) and tris(2-pyridylmethyl)amine (TPA) and benzoylformate (BF) as the alpha-keto acid to model the putative iron-cofactor interaction in the active site of alpha-keto acid-dependent non-heme iron enzymes. [Fe-II(6TLA)(BF)](ClO4) (1) crystallizes in the triclinic system, space group P (1) over bar (no. 2) with cell constants a 8.931(6) Angstrom, b = 13.366(7) Angstrom, c = 15.160(7) Angstrom, alpha = 75.92(4)degrees, beta = 81.06(5)degrees, gamma = 70.78(5)degrees, V = 1652(4) Angstrom(3), and Z = 2; R = 0.071 and R(W) = 0.082. [Fe-II(TPA)(BF)(MeOH)](ClO4). 2MeOH (2 . MeOH) crystallizes in the orthorhombic system, space group Pca2(1) (no. 29) with cell, constants a = 19.875(6) Angstrom, b = 8.916(4) Angstrom, c = 18.02(1) Angstrom, V = 3193(4) Angstrom(3), and Z = 4; R = 0.054 and R(W) = 0.056. The BF ligand chelates to the iron in 1 via one carboxylate oxygen and the carbonyl oxygen, but binds to the iron in 2 only through a carboxylate oxygen, with a methanol solvate occupying the other site. Both complexes react with dioxygen and quantitatively convert to decarboxylated complexes [Fe-II(6TLA)(OBz)](+) (5) and [(Fe2O)-O-III(TPA)(2)(OBz)(2)](2+) (6), respectively. Both 1 and 2 react with substrates 2,4-di-tert-butylphenol and triphenylphosphine under an O-2 atmosphere to afford the corresponding biphenol and OPPh(3), respectively. O-18(2)-labeling experiments show incorporation of one O-18 atom into the respective benzoate products and one into the OPPh(3). Kinetic studies on a series of [Fe-II(6TLA)(X-BF)](ClO4) complexes show pseudo-first-order disappearance of their characteristic color in the presence of excess dioxygen. The rate of the oxidative decarboxylation is sensitive to the nature of the phenyl substituent, exhibiting a Hammett rho value of +1.07 which indicates a nucleophilic mechanism. A reaction mechanism is proposed consisting of dioxygen binding to the iron(II) center, forming an iron(III)-superoxide species, attack of the nascent superoxide on the BF keto carbon, oxidative decarboxylation affording the oxidizing species, and substrate oxidation. Complexes 1 and 2 represent the first structural and functional models for alpha-keto acid-dependent non-heme iron enzymes.