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 NMe 2 groups in the NHC ligand, namely [1-(2,4,6-trimethylphenyl)-3-(2-methoxybenzyl)-2-imidazolidinylidene]dichloro(3-phenyl-1 H -inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) ( 4a ) and [1-(2,4,6-trimethylphenyl)-3-(2- N,N -dimethylaminobenzyl)-2-imidazolidinylidene]dichloro(3-phenyl-1 H -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-1 H -inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) (Umicore M2 ). Interestingly, the complex [1-(2,4,6-trimethylphenyl)-3-(2-thiomethylbenzyl)-2-imidazolidinylidene]dichloro(3-phenyl-1 H -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.