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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

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journal contribution
posted on 2003-05-21, 00:00 authored by Andrew J. Skulan, Melissa A. Hanson, Hua-fen Hsu, Lawrence Que,, Edward I. Solomon
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 C2h geometric distortion of the Fe2O2 core results in different stretching force constants for adjacent Fe−O bonds (kstr(Fe−Olong) = 1.66 and kstr(Fe−Oshort) = 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.

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