Synthesis, Structure, and Catalytic Activity of New Ruthenium(II) Indenylidene Complexes Bearing Unsymmetrical N-Heterocyclic Carbenes

New robust and air-stable ruthenium(II) indenylidene type second-generation precatalysts with unsymmetrical N-heterocyclic carbene (NHC) ligands were synthesized. The reaction profiles of these complexes were studied in commercial-grade solvents in air with model substrates leading to the di-, tri-, and tetrasubstituted olefins. In addition, application of selected precatalysts for olefin metathesis reactions on a broad spectrum of substrates with different functional groups was examined. Studies of solvent effects for the selected precatalysts as well as thermal activation of the corresponding complexes showed significant differences in their activities. Observed relationships in connection with single-crystal X-ray analysis revealed the influence of the unsymmetrical NHC ligands on the initiation rate of precatalysts. Dissociation of the phosphine ligand, the rate-determining step for our most active precatalysts, is accelerated by the steric repulsion of the dangling benzyl arm and the tricyclohexylphosphine ligand. A precatalyst with a hemilabile benzyl arm evinced decreased activity, probably due to coordination of the heteroatom to the ruthenium core taking place after dissociation of the phosphine ligand.