Ruthenium Metathesis Catalysts Bearing Anionic N-Heterocyclic Carbenes: A Computational Study on Failed Approaches to Their Synthesis

Despite recent efforts, the synthesis of ruthenium metathesis catalysts bearing anionic N-heterocyclic carbenes has not been successful. Using a computational density functional theory approach, we show that previously synthesized anionic N-heterocyclic carbenes are, with one exception, too weak Lewis bases to replace a chloride ion in the structure of common ruthenium metathesis catalysts. On the other hand, the transmetalation from silver or copper complexes is not feasible because of the very high affinity of anionic N-heterocyclic carbenes to these transition metals in comparison to ruthenium. We also show that the heterobimetallic Ag/Ru and Cu/Ru complexes obtained during such transmetalation are unlikely to catalyze olefin metathesis but may be good candidates for other catalysts as a result of facile dissociation of the phosphine moiety to produce an active Ru complex.

Automated summary

Despite efforts, synthesizing ruthenium metathesis catalysts with anionic N-heterocyclic carbenes has not been successful. Computational studies show that the previously synthesized anionic N-heterocyclic carbenes are generally too weak to replace chloride ions in common ruthenium metathesis catalysts. Transmetalation from silver or copper complexes is not feasible due to the strong affinity of anionic N-heterocyclic carbenes for these transition metals compared to ruthenium. Additionally, the heterobimetallic Ag/Ru and Cu/Ru complexes formed during transmetalation are unlikely to catalyze olefin metathesis but may have potential as other catalysts.