Ruthenium indenylidene complexes bearing bis(N-Alkyl/N’-Mesityl)-sided heterocyclic carbene ligands

We report on the synthesis and characterization of new ruthenium indenylidene complexes bearing two unsymmetrical N-heterocyclic carbene (NHC) ligands denoted as RuCl2(3-phenyl-1-indenylidene)bis(1-mesityl-3-R-4,5-dihydroimidazole-2-ylidene) in which R is methyl 7a and cyclohexyl 7b. Complexes 7a and 7b were analyzed using single-crystal X-ray diffraction analysis, elemental analysis, IR, NMR spectroscopy, and HRMS. The catalytic activities of complexes 7a and 7b were evaluated in olefin metathesis reactions: ring-opening metathesis polymerization (ROMP) of cis,cis-1,5-cyclooctadiene (COD) and ringclosing metathesis (RCM) of diethyl diallyl malonate (DEDAM) as well as in the isomerization of allylic alcohols. Complexes 7a and 7b failed to initiate the reactions at room temperature in all tested reactions, which might be due to the high thermal stability and low degree of lability of the Ru-CNHC bonds. At 80 °C, the complex 7a showed the best performance due to an increased initiation and a decreased steric obstruction towards the incoming substrates.