Oxo vs Imido Alkylidene
d<sup>0</sup>‑Metal Species: How and Why Do They Differ in
Structure, Activity, and Efficiency in Alkene Metathesis?
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Abstract
Density functional calculations have been carried out
to analyze the origin of the differences in reactivity, selectivity,
and stability toward deactivation in metathesis of d<sup>0</sup> oxo
alkylidene complexes vs their isoelectronic imido counterparts. DFT
calculations show that the elementary steps and geometries of the
extrema are similar for the oxo and imido complexes, but that the
energy profiles are different, the greatest difference occurring for
the deactivation pathway. For the alkene metathesis pathway, replacing
the imido by an oxo ligand slightly lowers the energy barrier for
alkene coordination but raises that for the [2+2]-cycloaddition and
cycloreversion; it also destabilizes the trigonal bipyramidal (<b>TBP</b>) metallacyclobutane intermediate with respect to the separated
reactants. The isomeric square-based pyramid (<b>SP</b>) metallacyclobutane
is in general more stable, and its stability relative to the separated
reactants is similar for oxo and imido systems. Consequently, the
oxo complex is associated with a slightly larger energy difference
between the lowest energy intermediate (<b>SP</b> or separated
reactants) and the highest energy transition state (cycloreversion)
than the imido complex, which accounts for a slightly lower activity.
Changing the imido into an oxo ligand disfavors strongly the deactivation
pathway by raising considerably the energy barrier of the β-H
transfer at the <b>SP</b> metallacycle that begins the entry
into the channel for deactivation and byproduct formation as well
as that of the subsequent ethene insertion. This makes the oxo catalysts
more selective and stable toward deactivation than the corresponding
imido catalysts, when dimerization can be avoided