Together We Are Stronger: An Unexpected Synergistic Effect During Anionic Ligand Exchange in Ruthenium Olefin Metathesis Catalysts

A family of mixed cis‐ chloro‐iodo sulfur‐chelated ruthenium benzylidenes was investigated. These complexes were generated by exploiting the spontaneous exchange of anionic ligands in solution, achieved by mixing the corresponding cis‐ dichloro and cis‐ diiodo analogs. The equilibrium between the different mixed anionic species was influenced by both steric and electronic factors, as supported by DFT computations. The hetero‐anionic catalysts significantly outperformed their homo‐anionic analogs in benchmark ring‐closing metathesis reactions. This was observed under both thermal and photochemical activation and was especially pronounced with sterically hindered substrates. Additionally, the mixed species substantially suppressed isomerization side reactions in the self‐metathesis of methyl oleate at high temperatures. This study presents a rare case in which combining two catalysts for the same reaction generates a more efficient catalytic species in situ.