Several ruthenium-based olefin metathesis catalysts of the formula (PR3)2X2RuCHCHCPh2 have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing metathesis of the acyclic diene diethyl diallylmalonate. The following order of increasing activity was determined:  X = I < Br < Cl and PR3 = PPh3 ≪ P i Pr2Ph < PCy2Ph < P i Pr3 < PCy3. Additional studies were conducted with the catalyst (PCy3)2Cl2RuCH2 to probe the mechanism of olefin metathesis by this class of catalysts. The data support a scheme in which there are two competing pathways:  the dominant one in which a phosphine dissociates from the ruthenium center and a minor one in which both phosphines remain bound. Higher catalyst activites could be achieved by the addition of CuCl to the reaction.

The syntheses of ruthenium(II) and rhodium(III) model complexes of the putative active species in the iron(II)/MMAO polymerization system are reported. The cationic ruthenium methyl ethylene complex 3 is a stable, isolable compound, which does not produce polyethylene under a variety of conditions. Likewise, the analogous rhodium complex 12 shows no appreciable polymerization activity, despite its similarity to existing rhodium complexes which are active catalysts for olefin polymerization. X-ray crystal structures of the cationic ruthenium bis(ethylene) complex 7 and the five-coordinate rhodium bis(triflate) methyl complex 9 are also presented.(17) (a) Gabe, E.

The previously reported ruthenium carbenes (PCy3)2Cl2RuCHR (R = CHCPh2 (1a), Ph (1b)) react with the bridged-chloride dimers [(p-cymene)RuCl2]2, [(p-cymene)OsCl2]2, and [( t Bu2Cp)RhCl2]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.

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