Raman Signature of the Fe₂O₂ “Diamond” Core

We report the resonance Raman (RR) spectra of iron complexes containing the Fe 2 (μ-O) 2 core. Frozen CH 3 CN solutions of the Fe III Fe IV intermediate [Fe 2 (μ-O) 2 L 2 ](ClO 4 ) 3 (where L = TPA, 5-Me 3 -TPA, 5-Me 2 -TPA, 5-MeTPA, 5-Et 3 -TPA, or 3-Me 3 -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 18 OH 2, and whose Raman shift does not vary with methyl substitution of the TPA ligand. Spectra of iron-isotope-substituted samples of [Fe 2 (μ-O) 2 (L) 2 ](ClO 4 ) 3 ( 54 Fe and 57 Fe for L = TPA, and 54 Fe and 58 Fe for L = 5-Me 3 -TPA) show that this vibration is also iron-isotope-sensitive. These isotopic data taken together strongly suggest that this vibration involves motion of the Fe 2 (μ-O) 2 core that is isolated from motions of the ligand. A frozen CH 3 CN solution of the diiron(III) complex [Fe 2 (μ-O) 2 (6-Me 3 -TPA) 2 ](ClO 4 ) 2 shows one intense resonance-enhanced vibration at 692 cm - 1 that shifts −30 cm - 1 with 18 O labeling. Normal coordinate analysis of the Fe 2 (μ-O) 2 core in [Fe 2 (μ-O) 2 (5-Me 3 -TPA) 2 ](ClO 4 ) 3 supports the assignment of the Fermi doublet centered around 666.2 cm - 1 as an A 1 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.