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