New Ruthenium(II) Indenylidene Complexes Bearing Unsymmetrical N<i>-</i>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 (<b>5</b>; DEDAM) as a model substrate. Two new complexes containing OMe and NMe<sub>2</sub> groups in the NHC ligand, namely [1-(2,4,6-trimethylphenyl)-3-(2-methoxybenzyl)-2-imidazolidinylidene]­dichloro­(3-phenyl-1<i>H</i>-inden-1-ylidene)­(tricyclohexylphosphine)­ruthenium­(II) (<b>4a</b>) and [1-(2,4,6-trimethylphenyl)-3-(2-<i>N,N</i>-dimethylaminobenzyl)-2-imidazolidinylidene]­dichloro­(3-phenyl-1<i>H</i>-inden-1-ylidene)­(tricyclohexylphosphine)­ruthenium­(II) (<b>4b</b>), show increased activity in comparison to the parent [1,3-bis­(2,4,6-trimethylphenyl)-2-imidazolidinylidene]­dichloro­(3-phenyl-1<i>H</i>-inden-1-ylidene)­(tricyclohexylphosphine)­ruthenium­(II) (Umicore <b>M2</b>). Interestingly, the complex [1-(2,4,6-trimethylphenyl)-3-(2-thiomethylbenzyl)-2-imidazolidinylidene]­dichloro­(3-phenyl-1<i>H</i>-inden-1-ylidene)­(tricyclohexylphosphine)­ruthenium­(II) (<b>4c</b>), bearing a SMe substituent, showed very low activity in the model RCM reaction with diethyl diallylmalonate (<b>5</b>). Application of the active complexes for metathesis reactions with a broad spectrum of olefins in commercial-grade solvents in air was examined.