Synthesis of an extremely sterically shielding N-heterocyclic carbene ligand

The N-heterocyclic carbene (NHC) ligand IPr** features substituents of unprecedented steric demand (IPr** = 1,3-bis[2,6-bis[(4-tert-butylphenyl)methyl]-4-methylphenyl]imidazol-2-ylidene). The NHC structure is an advanced derivative of the IPr system, and of the IPr* by Berthon-Gelloz and Marko. In the IPr** ancillary ligand, two para-methyl are introduced and eight methyl groups of IPr are formally replaced by a 4-tert-butylphenyl substituent, respectively, thereby sterically shielding both a coordinated metal and its second ligand. Favorable features of ligand and late transition metal complexes include high solubility, aesthetic NMR spectra, and a tendency towards crystallization.