Ethylene Tetramerization Catalysis: Effects of Aluminum-Induced Isomerization of PNP to PPN Ligands

Abstract

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.

Impact of Al Activators on Structure and Catalytic Performance of Cr Catalysts in Homogeneous Ethylene Oligomerization – A Multitechnique in situ/operando Study

“…This agrees with the fact that only a very small signal of catalytically inactive Cr I was measured, amounting to only 2 % of the total Cr content (Figure ). Based on our previous study it can be anticipated that the PNP ligand is properly coordinated to the Cr center, which has been found to suppress reduction to Cr I and promote 1‐octene selectivity . Surprisingly, even less Cr I has been detected, when AlMe 3 was used as activator, though in this case only about half of the ethylene conversion was reached and only short‐chain products were formed.…”

MAO‐free and extremely active catalytic system for ethylene tetramerization

“…1 J P-C = 6.0 Hz), 62.27 (t, 2 J P-C = 9.0 Hz), 133.14 (d, 2 J P-C = 21 Hz), 134.16, 138.89 ppm. 31 [B(C 6 F 5 ) 4 ] − 2 (0.337 g, 99%), which was used for the oligomerization studies without further purification. Table 3)…”

“…Catalyst development for ethylene oligomerization is still an active research topic in both academia and industry. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] 2 | EXPERIMENTAL…”

Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

“…[ 13–16 ] In this way, significant efforts have been dedicated to the synthesis of new families of ligands based on a wide variety of donor‐group combinations aiming to generate more efficient chromium catalyst systems that are capable of selectively forming α‐olefins with high productivities. [ 17–81 ] In particular, chromium catalysts stabilized by pyrazolyl‐based tridentate ligands have been successfully synthetized and their catalytic application in ethylene oligomerization explored in details. [ 51,81–91 ] Selected examples are presented in Chart 1.…”