Synthesis and Properties of Bimetallic Hoveyda–Grubbs Metathesis Catalysts

The catalytic activity of ruthenium Hoveyda–Grubbs
complexes
in olefin metathesis is a function of complex steric and electronic
effects acting on initiation and propagation steps. In order to study
the π-electron factors influencing the initiation process, we
attempted syntheses of bimetallic complexes with common organic ligands
bearing two chelate rings. While most of the studied ligand exchange
reactions of the isomeric bis-chelating benzene derivatives gave mixtures
of unstable complexes, a homodinuclear derivative of 1,4-dimethoxy-2,5-divinylbenzene
was sparingly soluble and precipitated from the reaction mixture in
a pure form. The complex was studied with spectroscopic and X-ray
methods, which confirmed the symmetrical bimetallic structure. However,
in model metathesis reactions the catalyst displayed activity very
comparable to the related monometallic complexes. This suggests that
in the bimetallic system two consecutive initiation processes of the
metathesis catalyst (first, bimetallic complex + olefin → monometallic
complex + propagating species; second, monometallic complex + olefin
→ styrene + propagating species) proceed at similar rates and,
thus, no cooperativity between the two steps is displayed. Properties
of the family of bimetallic complexes were probed with NMR studies,
and π-electronic effects operating in the systems were discussed.