Tetranuclear iron(iii) complexes of an octadentate pyridine-carboxylate ligand and their catalytic activity in alkane oxidation by hydrogen peroxide

Abstract

Reaction of the octadentate ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (LH) with Fe(ClO₄)₃ leads to the formation of the tetranuclear complexes [Fe₄(µ-O)₂(LH)₂(ClCH₂-CO₂)₄](ClO₄)₄ (1), [{Fe₂(µ-O)L(R-CO₂)}₂](ClO₄)₄ (2 R = C₆H₅-, 3 R = CH₃-, 4, R = ClCH₂-). The crystal structures of complexes 1 and 2 reveal that they consist of two Fe^III₂(µ-O)(µ-RCO₂)₂ cores that are linked via the two LH/L ligands to give a “dimer of dimers” structure. Complex 1 assumes a helical shape, with protonated carboxylic acid moieties of the two ligands forming a hydrogen-bonded pair at the center of the cation. In complexes 2, 3 and 4, central carboxylates of the two ligands bridge the iron ions in each of the two Fe₂O units, with an interdimer iron–iron separation of approximately 10 Å and an intradimer separation of approximately 3.1 Å. The second carboxylate bridge within the Fe₂O units is defined by exogenous benzoate (2), acetate (3) or chloroacetate (4) ligands. The aqua complex [{Fe₂(µ-O)L(H₂O)₂}₂](ClO₄)₆ (5) is proposed to have a similar structure, but with the exogenous bridging carboxylates replaced by two terminal water ligands. These complexes exhibit electronic and Mössbauer spectral features that are similar to those of (µ-oxo)diiron(III) proteins as well as other related (µ-oxo)bis(µ-carboxylato)diiron(III) complexes. This similarity shows that these properties are not significantly affected by the nature of the bridging exogenous carboxylate, and that the octadentate framework ligand is essential in stabilizing the “dimer of dimers” structure. This structural feature remains in highly diluted solution (10⁻⁵ M) as evidenced by electrospray ionization mass-spectroscopy (ES MS). Cyclic voltammetric studies of complexes 2 and 5 showed two irreversible two-electron reductions, indicating that the two Fe₂O units of the tetranuclear complexes behave as distinct redox entities. Complexes 2, 3 and, especially, the aqua complex 5 are active alkane oxidation catalysts. Catalytic reactions carried out with alkane substrate molecules and hydrogen peroxide predominantly gave alcohols. High stereospecificity in the oxidation of cis-1,2-dimethylcyclohexane supports the metal-based molecular mechanism of O-insertion into C–H bonds postulated for non-heme iron enzymes such as methane monooxygenase.