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 15 N-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 μ-acceptors.
Bibliographical noteFunding Information:
The authors acknowledge financial support from the US–Israel Binational Science Foundation (grant no. 2008391), the Israel Science Foundation (grant no. 1292/07) and the FIRST Program of the Israel Science Foundation (grant no. 1514/07). The authors are also grateful to G. Molev for fruitful discussions and D. Milstein for ongoing support.