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Electron Paramagnetic Resonance Signature of Tetragonal Low Spin Iron(V)-Nitrido and -Oxo Complexes Derived from the Electronic Structure Analysis of Heme and Non-Heme Archetypes

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Chang,  Hao-Ching
Research Group Ye, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Neese,  Frank
Research Department Neese, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Ye,  Shengfa
Research Group Ye, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Citation

Chang, H.-C., Mondal, B., Fang, H., Neese, F., Bill, E., & Ye, S. (2019). Electron Paramagnetic Resonance Signature of Tetragonal Low Spin Iron(V)-Nitrido and -Oxo Complexes Derived from the Electronic Structure Analysis of Heme and Non-Heme Archetypes. Journal of the American Chemical Society, 141(6), 2421-2434. doi:10.1021/jacs.8b11429.


Cite as: http://hdl.handle.net/21.11116/0000-0003-867C-B
Abstract
Iron(V)-nitrido and -oxo complexes have been proposed as key intermediates in a diverse array of chemical transformations. Herein we present a detailed electronic-structure analysis of [FeV(N)(TPP)] (1, TPP2- = tetraphenylporphyrinato), and [FeV(N)(cyclam-ac)]+ (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance (EPR) and 57Fe Mössbauer spectroscopy coupled with wave function based complete active-space self-consistent field (CASSCF) calculations. The findings were compared with all other well-characterized genuine iron(V)-nitrido and -oxo complexes, [FeV(N)(MePy2tacn)](PF6)2 (3, MePy2tacn = methyl-N',N ''-bis(2-picolyl)-1,4,7-triazacyclononane), [FeV(N){PhB(t-Bulm)3}]+ (4, PhB(tBulm)3 - = phenyltris(3-tert-butylimidazol-2-ylidene)borate), and [FeV(O)(TAML)]- (5, TAML4- = tetraamido macrocyclic ligand). Our results revealed that complex 1 is an authenticated iron(V)-nitrido species and contrasts with its oxo congener, compound I, which contains a ferry] unit interacting with a porphyrin radical. More importantly, tetragonal iron(V)-nitrido and-oxo complexes 1-3 and 5 all possess an orbitally nearly doubly degenerate S = 1/2 ground state. Consequently, analogous near-axial EPR spectra with g|| < g ≤ 2 were measured for them, and their g|| and g values were found to obey a simple relation of g2 + (2 - g||)2 = 4. However, the bonding situation for trigonal iron(V)-nitrido complex 4 is completely different as evidenced by its distinct EPR spectrum with g|| < 2 < g. Further in-depth analyses suggested that tetragonal low spin iron(V)-nitrido and-oxo complexes feature electronic structures akin to those found for complexes 1-3 and 5. Therefore, the characteristic EPR signals determined for 1-3 and 5 can be used as a spectroscopic marker to identify such highly reactive intermediates in catalytic processes.