Why Less Coordination Provides Higher Reactivity Chromium Phosphinoamidine Ethylene Trimerization Catalysts
2020
Cr phosphine catalysts
are uniquely suited for industrial selective
ethylene trimerization to 1-hexene. We recently introduced a Cr N-phosphinoamidine catalyst ((P,N)Cr) transition-state
model for selectivity, and here, we use density functional theory
calculations to address catalyst reactivity for ethylene trimerization.
This is particularly important because there are currently no empirical parameters or design principles
that provide prediction of high catalyst activity while maintaining
trimerization selectivity. Specifically, using transition states and
the energetic span model, we examined the ethylene trimerization catalytic
cycle with the bidentate (P,N)Cr catalyst 1a and compared this highly productive catalyst to the surprisingly
inactive tridentate (P,N,N)Cr catalyst. For (P,N)Cr
1a, this analysis revealed that for the high-spin CrI/III chromacycle mechanism, there are multiple CrI ethylene-coordinated
resting states and multiple turnover-controlling transition states,
which is consistent with previous experimental rate studies and can
account for a partial rate order in ethylene. Based on the calculated
energy landscape, the calculated 1-hexene productivity of 6.5 mol
s–1 and mass of 2.0 × 106 g h–1 is close to the experimental value. This analysis
also revealed that the tridentate (P,N,N)Cr catalyst
has a much larger energy span and is ∼107 slower,
which results from the stabilization of the energy landscape around
the chromacyclopentane intermediate. In addition to this reactivity/inactivity
comparison, we also calculated and compared the reactivity of several
other experimentally reported 1-hexene Cr tridentate catalysts. Based
on the catalytic energy spans, our calculations were able to qualitatively
and semi-quantitatively replicate relative catalyst reactivity.
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