Nitrenium Ligand Fills A Carbene Gap

Attaching a pair of phosphine arms onto a triazole skeleton has led to the first example of a transition-metal nitrenium complex. The nitrenium ion fills a missing link in the series of ubiquitous imidazole-based N-heterocyclic carbene (NHC) ligands (Nat. Chem., DOI: 10.1038/nchem.1068). NHCs, which were discovered 20 years ago, have opened up new pathways in catalysis and materials chemistry. The original NHCs spawned many derivatives, including imidazoles in which main-group elements such as silicon, germanium, boron, and phosphorus substitute for the carbene carbon atom. But until now, a version in which a nitrogen stands in for the carbene carbon and forms stable metal complexes had been elusive. Mark Gandelman of Technion—Israel Institute of Technology and coworkers determined that the pincerlike phosphine arms could embrace an electron-rich metal and then draw it close enough to coordinate to the nitrogen atom in the same way the metal would coordinate to the carbene carbon in a standard NHC. They prepared the rhodium complex shown, as well as a ruthenium version. The researchers continue to investigate the properties of the nitrenium complexes, and they plan to explore potential catalysis applications.