Synthesis and characterization of a family of binuclear non-heme iron monooxygenase model compounds: Evidence for a “phenolate/amide carbonyl (PAC) shift” upon oxidation

A series of binuclear iron compounds has been synthesized using diamide, bis-phenolate ligands in which the carbon-linker between the amide nitrogen atoms has been varied. Two diferrous compounds in the series, View the MathML sourceFe2II(H2Hbach)2(N-MeIM)2andFe2II(H2Hbame)2(N-MeIM)2 along with their two-electron oxidized, di-μ-methoxy-bridged counterparts, View the MathML sourceFeIII(H2Hbach)2(OMe)2andFeIII(H2Hbame)2(OMe)2 have been crystallographically characterized, as have the di-μ-methoxy compounds View the MathML sourceFe2III(H2Hbap)2(OMe)2,Fe2III(H2Hbabn)2(OMe)2andFe2III(H2Hbapen)2(OMe)2 (H2Hbab = 1,2-bis(2-hydroxybenzamido) benzene, H2Hbach = trans -1,2-bis(2-hydroxybenzamido) cyclohexane, H2Hbame = 1,2-bis(2-hydroxybenzamido) ethane, H2Hbap = 1,3-bis(2-hydroxybenzamido) propane, H2Hbabn = 1,4-bis(2-hydroxybenzamido) butane, H2Hbapen = 1,5-bis(2-hydroxybenzamido) pentane, N-MeIM = N -methylimidazole and OMe = methoxide). View the MathML sourceFe2II(H2Hbach)2(N-MeIM)2andFe2II(H2Hbame)2(N-MeIM)2 are structurally very similar to previously reported diferrous compounds of this family of ligands that have been shown to be active as oxygen atom transfer catalysts. Flexibility in the carbon-linker allows some variability in the orientation of the phenolate arms of the ligands in the diferric di-μ-methoxy compounds, but the Fe2O2 core remains largely unchanged across the series. Two-electron oxidation of the ferrous compounds in methanol shows a substantial ligand rearrangement that is consistent with other spectroscopic, electrochemical and kinetic investigations. The loss of both phenolate bridges upon oxidation is reminiscent of the “carboxylate shift” observed in binuclear non-heme enzymes and could provide insight into the driving force behind this family of compounds’ function as a catalyst.