Synthesis of Robust Phosphazane Hybrid NHCs for Small Molecule Activation: Unique sp2 and sp C–H insertion.

The ability of chemists to master catalysis and small molecule activation can be largely attributed to the rational design of a wide range of tunable ligand frameworks. Striking results in this area have been achieved using N-heterocyclic carbenes (NHCs) due to their tunable steric and electronic properties. Among these systems, NHCs with inorganic backbones have demonstrated significant potential in these areas due to their distinctive electronic properties compared to their organic counterparts. Within this group, phosphazane hybrid NHCs (PHNHCs) species have been proposed as highly versatile and promising species for reactivity and catalytic studies. However, their application in the realm of small molecule activation has been largely underexplored due to the high intrinsic electrophilic nature of their phosphorus centers. In this study, a decade after their first literature report, we successfully synthesized a series of robust PHNHCs. These new systems exhibit notable electronic characteristics, showing high net donor strength (TEP = 2048 cm–1) and appreciable π-acceptor properties (δse = 269 ppm in CDCl3; δse = 299 ppm in (CD3)2CO). Furthermore, the free carbenes of PHNHCs demonstrate the ability to insert into C–H bonds of both olefins and alkynes. Notably, the olefin insertion has not been previously observed in other NHCs. This unique reactivity profile positions PHNHCs as a valuable and complementary addition to the existing NHC toolkit.