Spectroscopic Study of [Fe₂O₂(5-Et₃-TPA)₂]³⁺:  Nature of the Fe₂O₂ Diamond Core and Its Possible Relevance to High-Valent Binuclear Non-Heme Enzyme Intermediates

The spectroscopic properties and electronic structure of an Fe₂(III,IV) bis-μ-oxo complex, [Fe₂O₂(5-Et₃-TPA)₂](ClO₄)₃ where 5-Et₃-TPA = tris(5-ethyl-2-pyridylmethyl)amine, are explored to determine the molecular origins of the unique electronic and geometric features of the Fe₂O₂ diamond core. Low-temperature magnetic circular dichroism (MCD) allows the two features in the broad absorption envelope (4000−30000 cm^-1) to be resolved into 13 transitions. Their C/D ratios and transition polarizations from variable temperature−variable field MCD saturation behavior indicate that these divide into three types of electronic transitions; t₂ → t₂* involving excitations between metal-based orbitals with π Fe−O overlap (4000−10000 cm^-1), t₂/t₂* → e involving excitations to metal-based orbitals with σ Fe−O overlap (12500−17000 cm^-1) and LMCT (17000−30000 cm^-1) and allows transition assignments and calibration of density functional calculations. Resonance Raman profiles show the C_2h geometric distortion of the Fe₂O₂ core results in different stretching force constants for adjacent Fe−O bonds (k_str(Fe−O_long) = 1.66 and k_str(Fe−O_short) = 2.72 mdyn/Å) and a small (∼20%) difference in bond strength between adjacent Fe−O bonds. The three singly occupied π*-metal-based orbitals form strong superexchange pathways which lead to the valence delocalization and the S = ³/₂ ground state. These orbitals are key to the observed reactivity of this complex as they overlap with the substrate C−H bonding orbital in the best trajectory for hydrogen atom abstraction. The electronic structure implications of these results for the high-valent enzyme intermediates X and Q are discussed.