IPr**-Highly Hindered, Ring Extended N-Heterocyclic Carbenes.

There is a strong demand for the development of sterically-hindered N-heterocyclic carbenes due to their potential to stabilize reactive organometallic species. IPr* (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazo-2-ylidene) is a highly-hindered and sterically-flexible ligands, which has found broad utilization in coordination chemistry. Herein, we report the synthesis, structural and electronic characterization of IPr** - a class of novel, sterically-bulky and easily-accessible N-heterocyclic carbene ligands that bearing biphenyl wingtips. Coordination chemistry to Ag(I), Cu(I) and Pd(II) is presented. The biphenyl wingtip permits to extend the %V of IPr** to 58.8%, linear geometry, which is the highest reported to date for imidazol-2-ylidene ligands. The synthesis of an electron-rich congener, IPr**, which is analogous to the popular IPr* ligand is also presented. Coordination to Pd(II) demonstrates steric flexibility, where the %V changes to 42.8% for a square planar geometry. The study demonstrates that IPr** with extended biphenyl wingtip is sterically bulkier than common imidazol-2-ylidenes, including IPr, IPr*, IPr and IPr*. Considering the essential role of sterically-hindered N-heterocyclic carbene ligands in various areas of coordination chemistry and metal-catalysis, this new class of NHCs is poised for rapid and widespread application.