Magnetic circular dichroism and computational study of mononuclear and dinuclear iron(iv) complexesElectronic supplementary information (ESI) available: VT MCD spectra, VT and VTVH MCD intensity analysis of complex 1, energies, 〈Sx〉, 〈Sz〉 values and Boltzmann populations of S = 1 magnetic sublevels as a function of the applied magnetic field, derivation of the excited states arising from the 1b2 → 2b1 transition, determination of the C-term sign of band 1 and the E(2e → 2a1) transitions for comp

• Main Authors: Ye, Shengfa, Xue, Genqiang, Krivokapic, Itana, Petrenko, Taras, Bill, Eckhard, Que Jr, Lawrence, Neese, Frank
• Format: Journal Article
• Language: English
• Published: 20.04.2015
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c4sc03268c

High-valent iron( iv )-oxo species are key intermediates in the catalytic cycles of a range of O 2 -activating iron enzymes. This work presents a detailed study of the electronic structures of mononuclear ([Fe IV (O)(L)(NCMe)] 2+ , 1 , L = tris(3,5-dimethyl-4-methoxylpyridyl-2-methyl)amine) and dinuclear ([(L)Fe IV (O)(μ-O)Fe IV (OH)(L)] 3+ , 2 ) iron( iv ) complexes using absorption (ABS), magnetic circular dichroism (MCD) spectroscopy and wave-function-based quantum chemical calculations. For complex 1 , the experimental MCD spectra at 2-10 K are dominated by a broad positive band between 12 000 and 18 000 cm −1 . As the temperature increases up to ∼20 K, this feature is gradually replaced by a derivative-shaped signal. The computed MCD spectra are in excellent agreement with experiment, which reproduce not only the excitation energies and the MCD signs of key transitions but also their temperature-dependent intensity variations. To further corroborate the assignments suggested by the calculations, the individual MCD sign for each transition is independently determined from the corresponding electron donating and accepting orbitals. Thus, unambiguous assignments can be made for the observed transitions in 1 . The ABS/MCD data of complex 2 exhibit ten features that are assigned as ligand-field transitions or oxo- or hydroxo-to-metal charge transfer bands, based on MCD/ABS intensity ratios, calculated excitation energies, polarizations, and MCD signs. In comparison with complex 1 , the electronic structure of the Fe IV &z.dbd;O site is not significantly perturbed by the binding to another iron( iv ) center. This may explain the experimental finding that complexes 1 and 2 have similar reactivities toward C-H bond activation and O-atom transfer. The electronic structures of mononuclear and dinuclear iron( iv ) complexes are studied using magnetic circular dichroism and wavefunction-based ab initio methods, and then correlated with their similar reactivities toward H- and O-atom transfer.