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)­(R3TPA)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)­(R3TPA)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)­(R3TPA)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 CH3CN 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.