Ruthenium Complexes Bearing Two N-Heterocyclic Carbene Ligands in Low Catalyst Loading Olefin Metathesis Reactions

The synthesis of ruthenium indenylidene complexes containing mixed N-heterocyclic carbene ligands featuring one sterically small NHC [IMeMe (1,3,4,5-tetramethylimidazol-2-ylidene), (IPrMe)-Pr-i (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), and ICy (1,3-dicyclohexylimidazol-2-ylidene)] and one larger congener SIMes (1,3-bis-(2,4,6 trimethylphenyl)imidazolin-2-ylidene) is described. Characterization by X-ray diffraction allowed for calculation of percent buried volume of the small NHCs in these complexes and for speculation on the relationship NHC/catalytic activity. In addition, these complexes displayed an enhanced stability but also a high ability to promote ring-closing metathesis reactions at catalyst loadings as low as 0.05 mol %.