Direct N–N Coupling between a Transient Iron(V)-Nitrido Complex and Nitrosobenzene Affording an Iron(III)-Phenyldiazene- <i>N</i> -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, [Fe^V(N)(cyclam-ac)][BAr^F₄] (<b>2</b>, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetate), with pentafluoronitrosobenzene (^PFPhNO), which quantitatively affords an iron(III)-phenyldiazene-<i>N</i>-oxide [Fe^III(NN(O)^PFPh)(cyclam-ac)][BAr^F₄] (<b>3</b>). Complex <b>3</b> 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 <b>2</b> with PhNO and 4-NO₂-PhNO. Detailed electronic structure analyses suggest that exceedingly efficient mutual frontier orbital interactions between <b>2</b> and PhNO render the title reaction barrierless and highly exothermic. This can be traced back to the unusual electronic feature of <b>2</b>, 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, <b>3</b> was found to release N₂, quantitatively furnishing a low-spin ferric phenolate complex, [Fe^III(O^PFPh)][BAr^F₄] (<b>4</b>). 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.