Researchers in Canada, under the leadership of Eric Rivard from Alberta University, have created a novel N-heterocyclic carbene (NHC) ligand boasting a record-setting buried volume. This ligand, incorporating N-bound trityl groups and a benzylated backbone, exhibits outstanding steric and electronic characteristics. These qualities enable it to stabilize low-coordinate complexes of gallium and lithium.
Bulky ligands such as NHCs are recognized for promoting rare, low coordination in inorganic elements, resulting in unique traits that are beneficial for catalysis. NHCs are notably adaptable owing to their capacity to establish robust metal-carbon bonds while having adjustable steric and electronic features.
In 2017, Rivard’s group presented an exceptionally bulky NHC ITr with nitrogen-bound trityl groups, reaching the highest percent buried volume (%V_bur) for an NHC up to that date. This innovation paved the way for a thermally stable quasi-monocoordinate thallium(I) complex.
Recently, Rivard and Ludwig Zapf synthesized an even more massive benzylated NHC ligand, BnITr, which has a %V_bur surpassing 60%, exceeding the previous 58.8% record set by a biphenyl-wingtip NHC from Michal Szostak’s group at Rutgers University in 2025.
X-ray diffraction analyses demonstrated that the benzylated backbone of the new ligand causes the trityl groups to rotate towards the carbene, enhancing steric shielding around the carbene center relative to its non-benzylated predecessor.
Employing this larger carbene, Zapf and Rivard stabilized a quasi-monocoordinated gallium(I) cation complex and successfully isolated a quasi-monocoordinated lithium cation complex, representing the first monomeric molecular complex of lithium devoid of additional ligands or solvents at its core. Despite efforts to create a quasi-monocoordinated palladium(0) complex, which was anticipated as an intermediate in palladium-catalyzed cross-couplings, they instead found an unanticipated migration of a trityl group to the palladium center.