Disentangling Ligand Effects on Metathesis Catalyst Activity: Experimental and Computational Studies of Ruthenium–Aminophosphine Complexes

Second-generation ruthenium olefin
metathesis catalysts bearing
aminophosphine ligands were investigated with systematic variation
of the ligand structure. The rates of phosphine dissociation (k1; initiation rate) and relative phosphine reassociation
(k–1) were determined for two series
of catalysts bearing cyclohexyl­(morpholino)­phosphine and cyclohexyl­(piperidino)­phosphine
ligands. In both cases, incorporating P–N bonds into the architecture
of the dissociating phosphine accelerates catalyst initiation relative
to the parent [Ru]–PCy3 complex; however, this effect
is muted for the tris­(amino)­phosphine-ligated complexes, which exhibit
higher ligand binding constants in comparison to those with phosphines
containing one or two cyclohexyl substituents. These results, along
with X-ray crystallographic data and DFT calculations, were used to
understand the influence of ligand structure on catalyst activity.
Especially noteworthy is the application of phosphines containing
incongruent substituents (PR1R′2); detailed
analyses of factors affecting ligand dissociation, including steric
effects, inductive effects, and ligand conformation, are presented.
Computational studies of the reaction coordinate for ligand dissociation
reveal that ligand conformational changes contribute to the rapid
dissociation for the fastest-initiating catalyst of these series,
which bears a cyclohexyl-bis­(morpholino)­phosphine ligand. Furthermore,
the effect of amine incorporation was examined in the context of ring-opening
metathesis polymerization, and reaction rates were found to correlate
well with catalyst initiation rates. The combined experimental and
computational studies presented in this report reveal important considerations
for designing efficient ruthenium olefin metathesis catalysts.