New Ruthenium(II) Indenylidene Complexes Bearing Unsymmetrical N-Heterocyclic Carbenes

New robust and air-stable ruthenium­(II) indenylidene second-generation precatalysts with unsymmetrical N-heterocyclic carbene (NHC) ligands were synthesized. These complexes were studied using ring-closing metathesis (RCM) of diethyl diallylmalonate (5; DEDAM) as a model substrate. Two new complexes containing OMe and NMe₂ groups in the NHC ligand, namely [1-(2,4,6-trimethylphenyl)-3-(2-methoxybenzyl)-2-imidazolidinylidene]­dichloro­(3-phenyl-1H-inden-1-ylidene)­(tricyclohexylphosphine)­ruthenium­(II) (4a) and [1-(2,4,6-trimethylphenyl)-3-(2-N,N-dimethylaminobenzyl)-2-imidazolidinylidene]­dichloro­(3-phenyl-1H-inden-1-ylidene)­(tricyclohexylphosphine)­ruthenium­(II) (4b), show increased activity in comparison to the parent [1,3-bis­(2,4,6-trimethylphenyl)-2-imidazolidinylidene]­dichloro­(3-phenyl-1H-inden-1-ylidene)­(tricyclohexylphosphine)­ruthenium­(II) (Umicore M2). Interestingly, the complex [1-(2,4,6-trimethylphenyl)-3-(2-thiomethylbenzyl)-2-imidazolidinylidene]­dichloro­(3-phenyl-1H-inden-1-ylidene)­(tricyclohexylphosphine)­ruthenium­(II) (4c), bearing a SMe substituent, showed very low activity in the model RCM reaction with diethyl diallylmalonate (5). Application of the active complexes for metathesis reactions with a broad spectrum of olefins in commercial-grade solvents in air was examined.