The selectivity-determining mechanistic steps of ethylene tetramerization and trimerization are evaluated in light of isotopic labeling experiments.

Diphosphinoamines (PNP) are commonly used to support Cr-catalyzed ethylene trimerization and tetramerization. Although isomerization of PNP to a PPN (iminobisphosphine) species has been established, such reactivity has not been studied in detail in the context of Cr-based selective ethylene oligomerization catalysis. Herein, we show that precursors that are stable as PNP frameworks can isomerize to PPN species in the presence of chlorinated aluminum activators relevant to ethylene oligomerization catalysis. Isomerization changes the pattern of reactivity of the ligands, making them more susceptible to nucleophilic attack by alkyl groups, resulting in a variety of degradation products. The isomerization-mediated degradation of PNP ligands leads to the formation of unwanted polymerization catalysts in ethylene tetramerization systems, thus providing insight into the formation of Cr species that affect the overall selectivity and activity values. For example, independently prepared [R 2 PNR]  leads to potent Cr polymerization catalysts. The susceptibility for isomerization is dependent on the nature of the N-substituent of the PNP precursor. Electron donating N-substituent i-Pr, which disfavors the PPN isomer compared to p-tolyl, and minimization of water contamination correlate with higher oligomerization activity and lower polymer byproducts. More broadly, the present study demonstrates the significant impact that Al-activators can have on the structure and behavior of the supporting ligand leading to detrimental reactivity.

A new, stoichiometric activation mode is presented for Cr-PNP (PNP = diphosphinoamine) complexes for ethylene tetramerization catalysis. To access suitable precatalysts, two robust Cr(III) multiaryl compounds were synthesized as THF adducts. These complexes are supported by a facially coordinated bis(aryl) ligand with an additional ether donor. From these precursors, Cr-PNP tris(hydrocarbyl) complexes were synthesized. Using 1 equiv of Brønsted acid as an activator, an active species for the catalytic tetramerization of ethylene was produced, without the need for excess alkylaluminum reagents.