Synthesis and Investigation of Homo- and Heterobimetallic Ruthenium Olefin Metathesis Catalysts Exhibiting Increased Activities

The previously reported ruthenium carbenes (PCy_3)_2Cl_2RuCHR (R = CHCPh_2 (1a), Ph (1b)) react with the bridged-chloride dimers [(p-cymene)RuCl_2]_2, [(p-cymene)OsCl_2]_2, and [(tBu_2Cp)RhCl_2]_2 to quantitatively form the bimetallic, bridged-chloride ruthenium carbenes 2a,b, 4a,b, and 6a,b and 1 equiv of each corresponding piano-stool complex. In the ring-opening metathesis polymerization of 1,5-cyclooctadiene, catalyst activity was found to increase in the order M = Ru < Os < Rh for the ancillary metal centers, with all of the bimetallic catalysts having higher activities than 1a,b. The kinetics of ROMP of the derivatized norbornene 9 were studied using catalyst 2a, and the data support an associative mechanism of olefin metathesis, contrary to the mechanism of olefin metathesis proposed for the parent catalysts 1a,b.