Reaction of (μ-Oxo)diiron(III) Core with CO₂ in <i>N</i>-Methylimidazole: Formation of Mono(μ-carboxylato)(μ-oxo)diiron(III) Complexes with <i>N</i>-Methylimidazole as Ligands

Several iron(III) complexes with N-methylimidazole (N-MeIm) as the ligand have been synthesized by using N-MeIm as the solvent. Under anaerobic conditions, [Fe(N-MeIm)6](ClO4)3 (1) reacts with stoichiometric amounts of water in N-MeIm to afford the (μ-oxo)diiron(III) complex, [Fe2(μ-O)(N-MeIm)10](ClO4)4 (3). Exposure of a solution of 3 in N-MeIm to stoichiometric and excess CO2 gives rise to the (μ-oxo)(μ-carboxylato)diiron(III) species [Fe2(μ-O)(μ-HCO2)(N-MeIm)8](ClO4)3 (4) and the methyl carbonate complex [Fe2(μ-O)(μ-CH3OCO2)(N-MeIm)8](ClO4)3 (5), respectively. Formation of the formato-bridged complex 4 upon fixation of CO2 by 3 in N-MeIm is unprecedentated. Methyl transfer from N-MeIm to a bicarbonato-bridged (μ-oxo)diiron(III) intermediate appears to give rise to 5. Complex 3 is a good starting material for the synthesis of (μ-oxo)mono(μ-carboxylato)diiron(III) species [Fe2(μ-O)(μ-RCO2)(N-MeIm)8](ClO4)3 (where R = H (4), CH3 (6), or C6H5 (7)); addition of the respective carboxylate ligand in stoichiometric amount to a solution of 3 in N-MeIm affords these complexes in high yields. Attempts to add a third bridge to complexes 4, 6, and 7 to form the (μ-oxo)bis(μ-carboxylato)diiron(III) species result in the isolation of the previously known triiron(III) μ-η3-oxo clusters [{Fe(μ-RCO2)2(N-MeIm)}3O](ClO4) (8). The structures of 3, 4, 6, and 7 allow one, for the first time, to inspect the various features of the {Fe2(μ-O)(μ-RCO2)}3+ moiety with no strain from the ligand framework.