Dioxygen-Initiated Oxidation of Heteroatomic Substrates Incorporated into Ancillary Pyridine Ligands of Carboxylate-Rich Diiron(II) Complexes

摘要

Progress toward the development of functional models of the carboxylate-bridged diiron active site in soluble methane monooxygenase is described in which potential substrates are introduced as substituents on bound pyridine ligands. Thiol, sulfide, sulfoxide, and phosphine moieties incorporated into a pyridine ligand were allowed to react with the preassembled diiron(II) complex [Fe2(μ-O2CAr^R)2(O2CAr^R)2(THF)2], where ^-O2CAr^R is a sterically hindered 2,6-di(p-tolyl)- or 2,6-di(p-fluorophenyl)benzoate (R = Tol or 4-FPh). The resulting diiron(II) complexes were characterized crystallographically. Triply- and doubly-bridged compounds [Fe2(μ-O2CAr^Tol)3(O2CAr^Tol)(2-MeSpy)] (>4) and [Fe2(μ-O2CAr^Tol)2(O2CAr^Tol)2(2-MeS(O)py)₂] (>5) resulted when 2-methylthiopyridine (2-MeSpy) and 2-pyridylmethylsulfoxide (2-MeS(O)py), respectively, were employed. Another triply-bridged diiron(II) complex, [Fe₂(μ-O₂CAr^4-FPh)₃(O₂CAr^4-FPh)(2-Ph₂Ppy)] (>3), was obtained containing 2-diphenylphosphinopyridine (2-Ph₂Ppy). Use of 2-mercaptopyridine (2-HSpy) afforded the mononuclear complex [Fe(O₂CAr^Tol)₂(2-HSpy)₂] (>6a). Together with that of previously reported [Fe₂(μ-O₂CAr^Tol)₃(O₂CAr^Tol)(2-PhSpy)] (>2) and [Fe₂(μ-O₂CAr^Tol)₃(O₂CAr^Tol)(2-Ph₂Ppy)] (>1), the dioxygen reactivity of these iron(II) complexes was investigated. A dioxygen-dependent intermediate (>6b) formed upon exposure of >6a to O₂, the electronic structure of which was probed by various spectroscopic methods. Exposure of >4 and >5 to dioxygen revealed both sulfide and sulfoxide oxidation. Oxidation of >3 in CH₂Cl₂ affords [Fe₂(μ-OH)₂(μ-O₂CAr^4-FPh)(O₂CAr^4-FPh)₃(OH₂)(2-Ph₂P(O)py)] (>8), which contains the biologically relevant {Fe₂(μ-OH)₂(μ-O₂CR)}³⁺ core. This reaction is sensitive to the choice of carboxylate ligands, however, since the p-tolyl analog >1 yielded a hexanuclear species, >7, upon oxidation.