Why Less Coordination Provides Higher Reactivity Chromium Phosphinoamidine Ethylene Trimerization Catalysts

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.