Raman Signature of the Fe₂O₂ “Diamond” Core

Abstract

We report the resonance Raman (RR) spectra of iron complexes containing the Fe2(μ-O)2 core. Frozen CH3CN solutions of the FeIIIFeIV intermediate [Fe2(μ-O)2L2](ClO4)3 (where L = TPA, 5-Me3-TPA, 5-Me2-TPA, 5-MeTPA, 5-Et3-TPA, or 3-Me3-TPA) show numerous resonance-enhanced vibrations, and among these, an oxygen-isotope-sensitive vibration around 667 cm-1 that shifts ca. 30 cm-1 when the samples are allowed to exchange with 18OH2, and whose Raman shift does not vary with methyl substitution of the TPA ligand. Spectra of iron-isotope-substituted samples of [Fe2(μ-O)2(L)2](ClO4)3 (54Fe and 57Fe for L = TPA, and 54Fe and 58Fe for L = 5-Me3-TPA) show that this vibration is also iron-isotope-sensitive. These isotopic data taken together strongly suggest that this vibration involves motion of the Fe2(μ-O)2 core that is isolated from motions of the ligand. A frozen CH3CN solution of the diiron(III) complex [Fe2(μ-O)2(6-Me3-TPA)2](ClO4)2 shows one intense resonance-enhanced vibration at 692 cm-1 that shifts −30 cm-1 with 18O labeling. Normal coordinate analysis of the Fe2(μ-O)2 core in [Fe2(μ-O)2(5-Me3-TPA)2](ClO4)3 supports the assignment of the Fermi doublet centered around 666.2 cm-1 as an A1 vibration of this core. Furthermore, we propose that this unique feature found in the region between 650 and 700 cm-1 is indicative of a diamond core structure and is the Raman signature of an iron cluster containing this core.

Nuclear resonance vibrational spectroscopic and computational study of high-valent diiron complexes relevant to enzyme intermediates

“…Finally, in the energy region above 450 cm −1 , the Fe(IV) 2 and Fe(III)Fe(IV) complexes of 2 show two weaker features at ∼475 and ∼515 cm −1 , and in the case of the Fe(III)Fe(IV) complex, an additional higher-energy peak was observable in the NRVS data at 680 cm −1 , consistent with rR data on the related 56 Fe species (Fig. S3 and Table S2) (35).…”

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.…”

A High‐Valent Heterobimetallic [CuIII(μ‐O)2NiIII]2+ Core with Nucleophilic Oxo Groups

“…Furthermore, single oxygen-isotope labeling experiments [8] show an additional intermediate band at 615 cm −1 corresponding to the vibration of the partially labelled species [ν( 3 - 16 O - 18 O )]. In analogy to the well-studied [M 2 (μ-O) 2 ] 2+ (M= Fe, Ni, Cu) systems, for which similar rRaman signatures and oxygen isotope effects were reported, [9] we assign the rRaman feature at 625 cm −1 to a tetra-atomic vibration (Fig. S2) mode of the [Ni(μ-O) 2 Cu] 2+ core.…”