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

The previously reported ruthenium carbenes (PCy 3 ) 2 Cl 2 Ru CHR (R = CHCPh 2 ( 1a ), Ph ( 1b )) react with the bridged-chloride dimers [( p -cymene)RuCl 2 ] 2, [( p -cymene)OsCl 2 ] 2, and [( t Bu 2 Cp)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 .