Synthetic analogue of the {Fe2(μ-OH)2(μ-O2CR)}3+ core of soluble methane monooxygenase hydroxylase via synthesis and dioxygen reactivity of carboxylate-bridged diiron(II) complexes

We describe the synthesis and dioxygen reactivity of diiron(H) tetracarboxylate complexes [Fe-2(mu-O2CArTol)(2)(O2CArTol )(2)(N,N-Me(2)en)(2)] (2) and [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)(NN-Bn(2)en)(2)] (6), where (ArCO2-)-C-Tol = 2,6-di(p-tolyl)benzoate. These complexes were prepared as models for the diiron(II) center in the hydroxylase component of soluble methane monooxygenase (MMOH). Compound 6 reacts with dioxygen to afford PhCHO in similar to60(5)% yield, following oxidative N-dealkylation of the pendant benzyl group on the diamine ligand. The diiron(III) complex [Fe-2(mu-OH)(2)(mu-O2CArTol)(O-2- CArTol)(3)(N-Bnen)(N,N-Bn(2)en)] (8) was isolated from the reaction mixture. The 4.2 K Mossbauer spectrum of 8 displays a single quadrupole doublet with parameters delta = 0.48(2) mm s(-1) and DeltaE(Q) = 0.61(2) mm s(-1). The {Fe-2(mu-OH)(2)(mu-O2CR)}(3+) core structure in 8 matches that of the fully oxidized form of MMOH. The conversion of 6 to 8 closely parallels the chemistry of MMOH in which an O-2-derived oxygen atom is inserted into the C-H bond of methane. Several reaction pathways are considered to account for this novel chemical transformation, and these are compared with mechanistic frameworks previously developed for related cytochrome P450 and copper(l) dioxygen chemistry.