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.