Evaluating the identity and diiron core transformations of a (μ-Oxo)diiron(III)complex supported by electron-rich tris(pyridyl-2-methyl) amine ligands

The composition of a (μ-oxo)diiron(III)complex coordinated by tris[(3,5-dimethyl-4-methoxy)pyridyl-2-methyl]amine (R₃TPA)ligands was investigated. Characterization using a variety of spectroscopic methods and X-ray crystallography indicated that the reaction of iron(III)perchlorate, sodium hydroxide, and R₃TPA affords [Fe₂(μ-O)(μ-OH) (R₃TPA)₂](ClO₄)₃ (2)rather than the previously reported species [Fe₂(μ-O)(OH)(H₂O)(R ₃TPA)₂](ClO₄)₃ (1). Facile conversion of the (μ-oxo)(μ-hydroxo)diiron(III)core of 2 to the (μ-oxo)(hydroxo)(aqua)diiron(III)core of 1 occurs in the presence of water and at low temperature. When 2 is exposed to wet acetonitrile at room temperature, the CH₃CN adduct is hydrolyzed to CH₃COO-, which forms the compound [Fe₂(μ-O)(μ-CH₃COO)(R ₃TPA)₂](ClO₄)₃ (10). The identity of 10 was confirmed by comparison of its spectroscopic properties with those of an independently prepared sample. To evaluate whether or not 1 and 2 are capable of generating the diiron(IV)species [Fe₂(μ-O)(OH)(O)(R ₃TPA)₂]³⁺ (4), which has previously been generated as a synthetic model for high-valent diiron protein oxygenated intermediates, studies were performed to investigate their reactivity with hydrogen peroxide. Because 2 reacts rapidly with hydrogen peroxide in CH ₃CN but not in CH₃CN/H₂O, conditions that favor conversion to 1, complex 1 is not a likely precursor to 4. Compound 4 also forms in the reaction of 2 with H₂O₂ in solvents lacking a nitrile, suggesting that hydrolysis of CH₃CN is not involved in the H₂O₂ activation reaction. These findings shed light on the formation of several diiron complexes of electron-rich R₃TPA ligands and elaborate on conditions required to generate synthetic models of diiron(IV)protein intermediates with this ligand framework.