Nitrenium ions as ligands for transition metals

Abstract

Unlike N-heterocyclic carbenes (NHCs), which are now used ubiquitously in metal-based chemistry, the nitrogen-derived analogue (in which a carbon is replaced with the isoelectronic nitrogen cation, a nitrenium ion) has remained elusive as a ligand for metals. This is especially intriguing, because several other main-group analogues of NHCs have been prepared, and have been shown to coordinate with transition-metal complexes. Here, we describe the preparation of several N-heterocyclic nitrenium ions that are isoelectronic and isostructural to NHCs, and study their ligand properties. The formation of relatively strong nitrenium–metal bonds is unambiguously confirmed, in solution by selective 15N-labelling experiments, and in the solid state by X-ray crystallography. Experimental and computational studies of the electronic properties of this novel type of ligand suggest that they are poor σ-donors and good π-acceptors. Nitrenium salts have been known for more than 100 years. Despite being isoelectronic and isostructural to the ubiquitous N-heterocyclic carbene ligands and other main-group analogues, their coordination to metals has remained elusive. Here, the first examples of nitrenium ions as ligands for transition metals are described, along with investigations of their electronic properties.