Models for α-Keto Acid-Dependent Non-heme Iron Enzymes: Structures and Reactivity of [FeII(L)(O2CCOPh)](ClO4) Complexes

Yu Min Chiou, Larry Que

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Abstract

The first iron(II)—α-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 α-keto acid to model the putative iron—cofactor interaction in the active site of α-keto acid-dependent non-heme iron enzymes. [FeII(6TLA)(BF)](ClO4) (1) crystallizes in the triclinic system, space group P1 (no. 2) with cell constants a = 8.931(6) Å, b = 13.366(7) A, c = 15.160(7) A, α = 75.92(4)°, β = 81.06(5)°, γ = 70.78(5)°, V = 1652(4) A3, and Z = 2; R = 0.071 and Rw= 0.082. [FeII(TPA)(BF)(MeOH)](ClO4)·2MeOH (2·MeOH) crystallizes in the orthorhombic system, space group Pca21(no. 29) with cell constants a = 19.875(6) Å, b = 8.916(4) Å, c = 18.02(1) Å, V = 3193(4) Å3, and Z = 4; R = 0.054 and Rw = 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 [FeII(6TLA)(OBz)]+ (5) and [FeIII 2O(TPA)2(OBz)2]2+ (6), respectively. Both 1 and 2 react with substrates 2,4-di-tert-butylphenol and triphenylphosphine under an O2 atmosphere to afford the corresponding biphenol and OPPh3, respectively. 18O2-labeling experiments show incorporation of one 18O atom into the respective benzoate products and one into the OPPh3. Kinetic studies on a series of [FeII(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 ϱValue 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 a-keto acid-dependent non-heme iron enzymes.

Original languageEnglish (US)
Pages (from-to)3999-4013
Number of pages15
JournalJournal of the American Chemical Society
Volume117
Issue number14
DOIs
StatePublished - Apr 1995

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