Direct N–N Coupling between a Transient Iron(V)-Nitrido Complex and Nitrosobenzene Affording an Iron(III)-Phenyldiazene- N -Oxide

The reactivity of metal nitrides has attracted considerable attention owing to their proposed role as key intermediates in the critical N–N bond breaking and formation steps in N2 fixation and NH3 oxidation processes. Despite intensive efforts over the past decades, no clear-cut examples of direct N–N coupling initiated by metal-nitrides have yet been reported. Herein, we describe the reaction of a transient iron(V)-nitrido complex, [FeV(N)(cyclam-ac)][BArF4] (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetate), with pentafluoronitrosobenzene (PFPhNO), which quantitatively affords an iron(III)-phenyldiazene-N-oxide [FeIII(NN(O)PFPh)(cyclam-ac)][BArF4] (3). Complex 3 has been structurally and spectroscopically characterized using EPR, Mössbauer, and UV–vis–NIR techniques, coupled to wave function-based ab initio calculations, to contain a low-spin ferric center coordinated by a –N═N(O)PFPh monoanionic ligand featuring a N═N double bond. Further in situ measurements reveal that analogous reactions can occur upon treatment of 2 with PhNO and 4-NO2-PhNO. Detailed electronic structure analyses suggest that exceedingly efficient mutual frontier orbital interactions between 2 and PhNO render the title reaction barrierless and highly exothermic. This can be traced back to the unusual electronic feature of 2, viz., the 2-fold orbital near-degeneracy of the Fe–N π* orbitals and the dominant N-2p character thereof, both of which maximize stabilizing HOMO–LUMO overlaps. Furthermore, under vacuum, 3 was found to release N2, quantitatively furnishing a low-spin ferric phenolate complex, [FeIII(OPFPh)][BArF4] (4). The present work reveals previously unrecognized reactivity of high-valent iron-nitrido species and sheds further light on the N–N coupling reaction mediated by metal-nitrides.