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-BuIm)3}]+ (4, PhB(tBuIm)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 ferryl 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 g⊥2 + (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.
Highlights
High-valent iron complexes featuring oxo (O2−) or nitrido (N3−) coordination are invoked as key intermediates in O2 and N2 activation processes.[1]
The samples obtained are closer to the usual conditions of chemical reactions in comparison with the previous work, where complex 1 was generated by photolysis of a solid thin film of 1pro deposited on a cold tip in the incident Raman beam at 30 K.11
Despite the different preparation protocol employed, the resonance Raman (rR) spectra measured for our photolyzed samples revealed signals at 883, 1371, and 1569 cm−1 (Figure S5), which reasonably match the Fe N
Summary
High-valent iron complexes featuring oxo (O2−) or nitrido (N3−) coordination are invoked as key intermediates in O2 and N2 activation processes.[1].
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