Nitrogen Lewis acids unveiled

Lewis acids and bases are one of chemistry’s fundamental concepts, depicting a molecule’s ability to accept an electron pair from a partner molecule or to donate an electron pair to a partner, respectively. For example, electron-rich nitrogen-centered molecules such as ammonia function as Lewis bases. Mark Gandelman of Technion—Israel Institute of Technology and his group have found a situation in which they can turn that donor-acceptor reactivity around, so that a nitrogen-centered molecule functions as a Lewis acid (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.6b12360). Gandelman’s group had previously studied a triazolium salt and found it to be an analog of imidazole-based N-heterocyclic carbenes, including the ability to serve as a ligand for transition metals. The central nitrogen of the saturated N–N–N triazolium unit in the molecule can accept electron density from a metal into a vacant p orbital, even though it weakly donates its lone pair of electrons residing in an sp²-type orbital to the metal. With that information in hand, the team decided to treat various triazolium compounds with different Lewis bases such as phosphides, phosphines, and carbanions and found that the nitrogen-centered molecules functioned as the Lewis acid partner. Gandelman and coworkers think these new nitrogen-based Lewis acids could join other well-established Lewis acids based on boron, phosphorus, aluminum, and tin and serve in reactive frustrated Lewis pairs or other types of catalysts.