σ/π Plasticity of NHCs on the Ruthenium–Phosphine and RutheniumYlidene Bonds in Olefin Metathesis Catalysts

The
nature of the Ru bonding environment in a set of Grubbs catalysts
has been studied by means of density functional theory (DFT). On the
one hand, for a set of 20 [Ru­(CH2)­(NHC)­(PCy3)]­[Cl]2 second-generation adducts, the results
show that calculated 31P NMR shieldings exhibit a good
correlation with the calculated R(Ru–P) bond
lengths, which are in turn strongly correlated with the calculated
PCy3 ligand dissociation energies. Bond energy decomposition
analysis (EDA) also indicates that there is a strong correlation between
the σ and π orbital interaction energies for the Ru–PCy3 bond, while no correlation was found for the case of the
bond between the Ru moiety and the N-heterocyclic carbene (NHC) ligands,
Ru–NHC. Furthermore, π orbital interaction energies of
the Ru–PCy3 bond were found to be strongly correlated
with the calculated PCy3 ligand dissociation energies,
as well as with the R(Ru–P) bond lengths,
confirming the significance of the π back-donation component
from Ru to PCy3 in determining the lability of the PCy3 ligand in the studied adducts. On the other, for a set of
17 [Ru­(CHR)­(NHC)x(PCy3)2–x]­[Cl]2 first (x = 0)- and second-generation (x = 1) complexes,
DFT results show that changes occurring in the 13C NMR
shielding of the Ruylidene bond are mainly due to σ­(RuC)
→ π*­(RuC) molecular orbital (MO) transitions.
Good correlations are observed between σ­(13C) and
the energy gaps of the MOs involved in such transitions, between the
binding energies of the ylidene moiety and the rest of the Ru fragment,
as well as with the R(RuC) bond lengths.
Finally, our novel preliminary results suggest that, once the metallacycle
intermediate is formed by reaction with ethylene, 13C′(β)
NMR shielding retains the NMR information from σ­(13C) in the 16e species, in contrast to what happens with the 13C­(α) NMR shielding.