title: 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.
abstract: The spectroscopic properties and electronic structure of an Fe(2)(III,IV) bis-mu-oxo complex, [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3) where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, are explored to determine the molecular origins of the unique electronic and geometric features of the Fe(2)O(2) 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; t(2) --> t(2) involving excitations between metal-based orbitals with pi Fe-O overlap (4000-10000 cm(-)(1)), t(2)/t(2) --> e involving excitations to metal-based orbitals with sigma 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(2)(h)() geometric distortion of the Fe(2)O(2) 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/A) and a small ( approximately 20%) difference in bond strength between adjacent Fe-O bonds. The three singly occupied pi-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.
authors.affiliation: Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Skulan
authors.forename: Melissa A
authors.initials: MA
authors.lastname: Hanson
authors.forename: Hua-Fen
authors.initials: HF
authors.lastname: Hsu
authors.forename: Lawrence
authors.initials: L
authors.lastname: Que
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
mesh_terms: D002942:Circular Dichroism; D004798:Enzymes / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry*; D006418:Heme / Q000737:chemistry; D008968:Molecular Conformation; D011725:Pyridines / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman; D013696:Temperature; D013816:Thermodynamics
chemical_list: D004798:Enzymes; C475845:Fe(2)O(2)-tris(5-ethyl-2-pyridylmethyl)amine; D005290:Ferric Compounds; D011725:Pyridines; C431843:tris(2-pyridylmethyl)amine; C000499:ferric oxide; D006418:Heme