2-Phenoxypyridyl Dinucleating Ligands for Assembly of Diiron(II) Complexes: Efficient Reactivity with O[subscript 2] to Form (-Oxo)diiron(III) Units

A series of 2-phenoxypyridyl and 2-phenoxyimino ligands, H2L (2,2’-(5,5’-(1,2phenylenebis(ethyne-2,1-diyl))bis(pyridine-5,2-diyl))diphenol, where R = H, Me, or t-Bu, and R’ = H, or Ph) and H2BIPS ((3,3’-(1E,1’E)-(3,3’-sulfonylbis(3,1-phenylene)bis(azan-1-yl-1ylidene))bis(methan-1-yl-1-ylidene)bis(5-methylbiphenyl-2-ol)) were synthesized as platforms for non-heme diiron(II) protein model complexes. UV-vis spectrophotometric studies and preparativescale reactions of LR,R’ or BIPSMe,Ph, where LR,R’ and BIPSMe,Ph are the deprotonated forms of H2L and H2BIPS, respectively, with Fe(II) revealed that the presence of sterically protective ortho phenol substituents is necessary to obtain discrete dinuclear species. Reaction of LMe,Ph with Fe(II) in THF afforded the doubly-bridged compound [Fe2(L)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)2] (2) species. Reaction of O2 instead of O2 with 1 led to a shift in the Fe–O–Fe vibrational frequency from 833 cm-1 to 798 cm-1, confirming the presence of the μ-oxodiiron(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.