Computational Monitoring of Oxidation States in Olefin Metathesis

Ruthenium-based catalysts play a pivotal role in the formation of carbon-carbon double bonds in olefin metathesis. The reaction mechanism always involves the formation of a four-membered metallacycle where the Ru centers should exhibit a formal +4 oxidation state. The steric and electronic properties of the so-called spectator N-heterocyclic carbene (NHC) ligand of Ru-benzylidene complexes are, presumably, responsible for the ability/inability of the metal center to achieve the necessary formal oxidation state for the catalytic process run on, and hence should be an essential ingredient in the design of efficient catalysts. In the present study we make use of density functional theory (DFT) calculations combined with the so-called effective oxidation state (EOS) analysis to examine the evolution of the metal oxidation state along the reaction mechanism. In particular, we tackle the Ru²⁺ versus Ru⁴⁺ debate in Grubbs catalysts, deepening into details about the different nature of the intermediates of the reaction pathway. We also analyze the picture of (cheaper and cleaner) iron-and osmium-based catalysts. Moreover, the nature and occupation number of the frontier effective fragment orbitals of the ligands sheds light into the subtle electronic differences between first-and second-generation Grubbs catalysts that nevertheless do not affect the assignment of oxidation states.