Decomposition of Ruthenium Olefin Metathesis Catalysts

The decomposition of a series of ruthenium metathesis catalysts has been examined using
methylidene species as model complexes. All of the phosphine-containing methylidene complexes
decomposed to generate methylphosphonium salts, and their decomposition routes followed first-order
kinetics. The formation of these salts in high conversion, coupled with the observed kinetic behavior for
this reaction, suggests that the major decomposition pathway involves nucleophilic attack of a dissociated
phosphine on the methylidene carbon. This mechanism also is consistent with decomposition observed in
the presence of ethylene as a model olefin substrate. The decomposition of phosphine-free catalyst
(H2IMes)(Cl)2RuCH(2-C6H4-O-i-Pr) (H2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) with ethylene
was found to generate unidentified ruthenium hydride species. The novel ruthenium complex (H2IMes)(pyridine)3(Cl)2Ru, which was generated during the synthetic attempts to prepare the highly unstable pyridine-based methylidene complex (H2IMes)(pyridine)2(Cl)2RuCH2, is also reported.