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.

A synthetic precedent for the [Fe IV 2 (μ-O) 2 ] diamond core proposed for methane monooxygenase intermediate Q

“…Thus the higher oxidative reactivity of the Fe IV AO complex may be attributed to the stronger basicity of the corresponding Fe III OO Ϫ species. 2 ] diamond core (24)(25)(26). The generation and characterization of 2b as a complex with a significant lifetime at Ϫ40°C attests to the chemical viability of an Fe IV 2 O 2 diamond core structure and lends credence to the postulation of such a core structure for MMO-Q.…”

“…Resonance Raman spectroscopy has proven to be a valuable tool for the characterization of species with [Fe 2 ( -O) 2 ] diamond core structures. A characteristic of such complexes is a band in the 660-700 cm Ϫ1 region, which downshifts Ϸ30 cm Ϫ1 upon 18 O labeling and has been assigned to a deformation of the [Fe 2 ( -O) 2 ] core (24)(25)(26). The Raman spectrum of 2b obtained with 514.5-nm excitation shows one intense band at 674 cm Ϫ1 (Fig.…”

Spectroscopy and electronic structures of mono- and binuclear high-valent non-heme iron–oxo systems

“…The crystal structure displays a bis-l-oxo core. MCD, Mö ssbauer and EPR spectroscopy indicate the presence of a low-spin Fe III S = 1/2 and low-spin Fe IV S = 1 ferromagnetically coupled to yield a total S = 3/2, class III valence-delocalized ground state for the complex [21][22][23][24]. The complex [Fe III Fe IV (l-O)( [9]aneN 3 ) 2 (l-CH 3 CO 2 ) 2 ] 3+ displays a 35:65 mixture of S = 1/2 and S = 3/2 species, but Mö ssbauer and EPR results were ambiguous as to the spin-state nature of the Fe IV [25].…”

Structural and Electrochemical Studies of Novel Bis(μ‐methoxo)diiron complexes: Observation of FeIIIFeIV and FeIVFeIV States Resonating with Phenoxyl Radicals

“…[1,2] To elucidate the structural and functional aspects of these intermediates, several molecular models have been proposed by using synthetic complexes [3][4][5][6][7][8][9] and by the DFT computational approach. [7,10,11] Although only one example of an Fe III Fe IV complex has been crystallographically characterized, [3] there is spectroscopic evidence for the formation of a high-valence diiron state through chemical or electrochemical oxidation. [4][5][6][7][8] Because of their low stability, the study of these high-valence species normally requires low-temperature analyses.…”