Role of anionic ligands on the activity of Ruthenium Olefin Metathesis Catalyst : A DFT investigation

Olefin metathesis has evolved into a widespread route to unsaturated compounds over the last few decades. The introduction of N-heterocyclic carbene ligands with improved catalytic efficiency, activity and selectivity is undoubtedly the prime reason. Depending upon the choice of anionic ligands (X) the N-heterocyclic ruthenium carbene complex ((NHC)(X)₂Ru=CHPhSPh) catalyzes Ring Opening Polymerization Metathesis (ROMP) of several complexes with a different rate. While choosing Cl as anionic ligand, complexes like norbornene, cyclooctaene and cycloocadiene undergoes ROMP, whereas replacing Cl by I prevents the reactions to occur. Moreover, in case of dicyclopentadiene it leads to different ROMP products by opening up of six membered and five membered ring double bond with Cl and I ligated complex respectively. We carried out mechanistic study of the catalysts activation followed by the overall metathesis reaction of norbornene and dicyclopentadiene to understand the difference in reactivity. Our DFT study showed that along with steric effects variation of electronic effects between I and Cl plays a major role on such different catalytic activity and selectivity.