IPr**(4-Bp)─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 ligand, which has found broad utilization in coordination chemistry. Herein, we report the synthesis, structural, and electronic characterization of IPr**^(4-Bp)─a class of novel, sterically bulky, and easily accessible N-heterocyclic carbene ligands that bear biphenyl wingtips. Coordination chemistry to Ag(I), Cu(I), and Pd(II) is presented. The biphenyl wingtip permits extension of the %V_bur of IPr**^(4-Bp) 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**^MeO(4-Bp), which is analogous to the popular IPr*^MeO ligand, is also presented. Coordination to Pd(II) demonstrates steric flexibility, where the %V_bur changes to 42.8% for square planar geometry. The study demonstrates that IPr^**(4-Bp) with an extended biphenyl wingtip is sterically bulkier than common imidazol-2-ylidenes, including IPr, IPr*, IPr^#, and IPr*^(2-Np). 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.