Mono- and Bis-NHC Ruthenium Complexes─Theoretical Insights into Stability and Ligand Exchange Selectivity

Abstract Ruthenium–N-heterocyclic carbene complexes used as catalysts for olefin metathesis may exist as either mono-NHC or bis-NHC species. We used a number of computational methods, ranging from semiempirical and DFT approaches to highly accurate symmetry-adapted perturbation theory (SAPT) approaches to determine which of the electronic and structural features are accountable for the formation and the stability of the final catalyst. We found that modern DFT functionals accurately describe the formation of the majority of ruthenium complexes and correctly identify the most stable rotamer, while other approaches combining GFN2-xTB and DFT or SAPT methods give accurate geometries but inaccurate energies. We also showed that the calculated bond dissociation energies of ligands and intramolecular interactions between substituents may serve as computationally less expensive metrics for the estimation of the stability of ruthenium complexes. Finally, we also analyzed the influence of electronic and steric effects on the stability of studied complexes.