2-Phenoxypyridyl Dinucleating Ligands for Assembly of Diiron(II) Complexes: Efficient Reactivity with O₂ To Form (μ-Oxo)diiron(III) Units

A series of 2-phenoxypyridyl and 2-phenoxyimino ligands, H₂L^R,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 H₂BIPS^Me,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 L^R,R′ or BIPS^Me,Ph, where L^R,R′ and BIPS^Me,Ph are the deprotonated forms of H₂L^R,R′ and H₂BIPS^Me,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 L^Me,Ph with iron(II) in tetrahydrofuran (THF) afforded the doubly bridged compound [Fe₂(L^Me,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)(L^Me,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 L^Me,Ph ligand is robust toward oxidative decomposition and does not display any reversible redox activity.