Evaluating the Identity and Diiron Core Transformations of a (μ-Oxo)diiron(III) Complex Supported by Electron-Rich Tris(pyridyl-2-methyl)amine Ligands

Abstract

The composition of a (μ-oxo)diiron(III) complex coordinated by tris[(3,5-dimethyl-4-methoxy)pyridyl-2-methyl]amine (R(3)TPA) 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 R(3)TPA affords [Fe(2)(μ-O)(μ-OH)(R(3)TPA)(2)](ClO(4))(3) (2) rather than the previously reported species [Fe(2)(μ-O)(OH)(H(2)O)(R(3)TPA)(2)](ClO(4))(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 CH(3)CN adduct is hydrolyzed to CH(3)COO(-), which forms the compound [Fe(2)(μ-O)(μ-CH(3)COO)(R(3)TPA)(2)](ClO(4))(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 [Fe(2)(μ-O)(OH)(O)(R(3)TPA)(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(3)CN but not in CH(3)CN/H(2)O, 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 H(2)O(2) in solvents lacking a nitrile, suggesting that hydrolysis of CH(3)CN is not involved in the H(2)O(2) activation reaction. These findings shed light on the formation of several diiron complexes of electron-rich R(3)TPA ligands and elaborate on conditions required to generate synthetic models of diiron(IV) protein intermediates with this ligand framework.