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

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 [Fe V (N)(TPP)] (1, TPP 2- = tetraphenylporphyrinato), and [Fe V (N)(cyclam-ac)] + (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance (EPR) and 57 Fe 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, [Fe V (N)(MePy 2 tacn)](PF 6 ) 2 (3, MePy 2 tacn = methyl- N', N″-bis(2-picolyl)-1,4,7-triazacyclononane), [Fe V (N){PhB( t-BuIm) 3 }] + (4, PhB( t BuIm) 3 - = phenyltris(3- tert-butylimidazol-2-ylidene)borate), and [Fe V (O)(TAML)] - (5, TAML 4- = 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.