Chromium-Based Complexes Bearing N-Substituted Diphosphinoamine Ligands for Ethylene Oligomerization

Abstract

A range of novel N-substituted diphosphinoamine (PNP) ligands Ph 2 PN(R)PPh 2 [R = F 2 CHCH 2 (1); R = Me 2 CHCH 2 (2); R = Me 2 CHCH 2 CH 2 (3)] have been synthesized via one-step salt elimination reaction. The ligand-coordinated chromium carbonyls [Ph 2 PN(R)PPh 2 ]Cr(CO) 4 (4−6) were further synthesized, and X-ray crystallography analysis of complex 6 revealed the κ 2 -P,P bidentate binding mode of Cr center and the molecular structure of PNP ligand 3. Then the catalytic ethylene oligomerization behaviors of PNP ligands 1−3 bridging chromium chloride complexes {[Ph 2 PN(R)PPh 2 ]CrCl 2 (μ-Cl)} 2 (7−9) were further discussed in depth. Experimental results showed that complex 7 with the strong electron-withdrawing F 2 CHCH 2 group can promote the nonselective ethylene oligomerization, while both complex 8 and complex 9 with the electron-donating Me 2 CHCH 2 and Me 2 CHCH 2 CH 2 groups can significantly enhance the selective ethylene tri/tetramerization. The good catalytic activity of 198.3 kg/(g Cr•h), the selectivity toward 1-hexene and 1-octene of 76.4%, and the low PE content of 0.2% were simultaneously achieved with the Al/Cr molar ratio of 600 using the complex 8/MMAO system at 45 °C and 45 bar. These excellent results were mainly attributed to the fact that the β-branching of bridging ligand 2 increased the steric bulk of the N-moiety for complex 8.

Chromium-Based Complexes Bearing Aminophosphine and Phosphine–Imine–Pyrryl Ligands for Selective Ethylene Tri/Tetramerization

“…At 45 bar, the 392.5 kg/(g·Cr·h) catalytic activity, 64.2% C 8 selectivity, and 0.5% PE content were obtained. Based on these results, it can be concluded that high ethylene pressure can facilitate ethylene insertion into metallacycloheptane, which leads to the formation of a metallacyclononane reaction intermediate …”

“…Based on these results, it can be concluded that high ethylene pressure can facilitate ethylene insertion into metallacycloheptane, which leads to the formation of a metallacyclononane reaction intermediate. 44 Next, the effects of reaction temperatures (40,45,50,55, and 60 °C) with a complex 7/MAO system at 45 bar were further investigated. As shown in Figure 2b, the catalytic activity of the complex 7/MAO system exhibited volcano shape, and the highest catalytic activity of 435.2 kg/(g•Cr•h) and the lowest PE content of 0.2% were obtained at 45 °C.…”

“…48 However, at high Al/Cr ratio, i.e., 600−800, the slightly decreased activity was observed as well, in agreement with the literature, which can be attributed to the over-reduced Cr active species caused by extra MAO. 44,49 It should be noted that both the Cr source and solvent type have the local influences on catalytic ethylene tri/tetramerization. 50 Unlike complex 7 bearing the ligand 2 system, Cr(acac) 3 or CrCl 3 (THF) 3 was stoichiometrically mixed with ligand 2 for preparing the in situ catalysts and then were subjected to evaluating the catalytic ethylene tri/tetramerization performance, as shown in Figure 4a.…”

Selective ethylene tetramerization: an overview

“…Relative to toluene, methylcyclohexane dissolves more ethylene (on mass and molar bases), providing a more ethylene-rich environment for the catalyst. In addition, Liu et al 148 similarly studied the electronic effects of the ligands 29-31. The strongly electron-with-drawing F 2 CHCH 2 group in ligand 29 had an effect on promoting the catalytic activity.…”

“…The strongly electron-with-drawing F 2 CHCH 2 group in ligand 29 had an effect on promoting the catalytic activity. 148 However, the selectivity toward C 8 was reduced to half when an electron-withdrawing group was present, which may be attributed to the weak interaction of F with H in the elimination of chromacycloheptane. In addition, a certain amount of C 4 was produced at the same time.…”