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2-Phenoxypyridyl Dinucleating Ligands for Assembly of Diiron(II) Complexes: Efficient Reactivity with O2 To Form (μ-Oxo)diiron(III) Units
journal contributionposted on 2009-11-16, 00:00 authored by Loi H. Do, Stephen J. Lippard
A series of 2-phenoxypyridyl and 2-phenoxyimino ligands, H2LR,R′ [2,2′-(5,5′-(1,2-phenylenebis(ethyne-2,1-diyl))bis(pyridine-5,2-diyl))diphenol, where R = H, Me, or t-Bu, and R′ = H or Ph] and H2BIPSMe,Ph [(3,3′-(1E,1′E)-(3,3′-sulfonylbis(3,1-phenylene)bis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)bis(5-methylbiphenyl-2-ol)], were synthesized as platforms for nonheme diiron(II) protein model complexes. UV−vis spectrophotometric studies and preparative-scale reactions of LR,R′ or BIPSMe,Ph, where LR,R′ and BIPSMe,Ph are the deprotonated forms of H2LR,R′ and H2BIPSMe,Ph, respectively, with iron(II) revealed that the presence of sterically protective o-phenol substituents is necessary to obtain discrete dinuclear species. The reaction of LMe,Ph with iron(II) in tetrahydrofuran (THF) afforded the doubly bridged compound [Fe2(LMe,Ph)2(THF)3] (1), which was characterized in the solid state by X-ray crystallography. A large internal cavity in this complex facilitates its rapid reaction with dioxygen, even at −50 °C, to produce the thermodynamically stable [Fe2(μ-O)(LMe,Ph)2] (2) species. Reaction of 18O2 instead of 16O2 with 1 led to a shift in the Fe−O−Fe vibrational frequency from 833 to 798 cm−1, confirming the presence of the (μ-oxo)diiron(III) core and molecular oxygen as the source of the bridging oxo group. The LMe,Ph ligand is robust toward oxidative decomposition and does not display any reversible redox activity.