Rate-Limiting Steps in the Intramolecular C–H Activation of Ruthenium N‑Heterocyclic Carbene Complexes

Ruthenium
(II) complexes with N-heterocyclic carbenes (NHC) are
commonly used as efficient catalysts in hydrogenation of olefins with
simultaneous intramolecular C–H activation. Using the DFT approach,
we have investigated the entire hydrogenation reaction pathway for
four new potential catalysts and ethylene, a model substrate. Our
calculations imply that the dissociation of phosphine is the rate-limiting
step of hydrogenation, contrary to recent computational results. We
also found that catalysts bearing NHCs with aliphatic and aromatic
side groups are energetically favorable over other aliphatic cyclohexyl-substituted
NHC. To examine how electronic properties of various catalysts influence
the energetic barrier in the crucial steps of the reaction, we applied
the Noncovalent Interaction analysis, which allowed us to reveal crucial
interactions which stabilize/destabilize important intermediates and
transition states in the hydrogenation reaction.