Redox Non-Innocence of a N‑Heterocyclic Nitrenium Cation Bound to a Nickel–Cyclam Core

The redox properties of Ni complexes bound to a new ligand, [DMC-nit]+, where a N-heterocyclic nitrenium group is anchored on a 1,4,8,11-tetraazacyclotetradecane backbone, have been examined using spectroscopic and DFT methods. Ligand-based [(DMC-nit)­Ni]2+/+ reduction and metal-based [(DMC-nit)­Ni]2+/3+ oxidation processes have been established for the [(DMC-nit)­Ni]+/2+/3+ redox series, which represents the first examples of nitrenium nitrogen (Nnit)-bound first-row transition-metal complexes. An unprecedented bent binding mode of Nnit in [(DMC-nit)­Ni]2+ is observed, which possibly results from the absence of any Nnit→Ni σ-donation. For the corresponding [(DMC-nit)­Ni­(F)]2+ complex, σ-donation is dominant, and hence a coplanar arrangement at Nnit is predicted by DFT. The binding of the triazolium ion to Ni enables new chemistry (formate oxidation) that is not observed in a derivative that lacks this functional group. Thus the N-heterocyclic nitrenium ligand is a potentially useful and versatile reagent in transition-metal-based catalysis.