Chromium Catalysts Based on Tridentate Pyrazolyl Ligands for Ethylene Oligomerization

Abstract

A set of chromium(III) complexes CrCl 3 (L) based on tridentate ligands (1, L ) bis [2-(3,5dimethylpyrazolyl)have been prepared and characterized by elemental analysis, ESI-HRMS, and an X-ray diffraction study for 3. Upon activation with methylaluminoxane (MAO), these precatalysts show high activity in ethylene oligomerization (TOF ) (3.4-131.0) × 10 3 (mol ethylene) (mol Cr) -1 h -1 at 80 °C), producing R-olefins in the range C 4 -C 14+ with high selectively. The catalytic performances are substantially affected by the ligand environment, especially the bridge donor atom, and reaction conditions. Under optimized conditions ([Cr] ) 10 µmol, 80 °C, 40 bar ethylene, MAOto-Cr ) 300), precatalyst 5 leads to TOF ) 131.0 × 10 3 h -1 and selectivity for R-olefins in the range 89-97%.

Substituted tridentate pyrazolyl ligands for chromium and nickel-catalyzed ethylene oligomerization reactions: effect of auxiliary ligand on activity and selectivity

“…These results are consistent with those observed in 6-membered ring nickel and chromium catalysts bearing an analogous tridentate sulfur-bridged ligand. 4,6 The chromium complexes 1a and 2a produce oligomers ranging from C 4 to C 14+ with a high selectivity for a-olefins attaining 67.71 and 73.47%, respectively. Both catalyst systems show similar behaviour for production of a-olefins as shown in Figure 1.…”

“…This result is consistent with those ones found for MAO-activated nickel and chromium complexes bearing analogous tridentate ligands. 4,6 It is interesting to note that the presence of phenyl groups at 3-position of the pyrazolyl rings in CrCl 3 {bis[2-(3-diphenyl-1-pyrazolyl)ethyl)]amine} (1a) determines a dramatic effect on activity and selectivity as compared to the similar complex CrCl 3 {bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl)]amine} (1b) 6 (compare entries 1 and 3, Table 1). For instance, catalyst 1a (22,900 mol(ethylene) mol(Cr)…”

“…On the other hand, the introduction of relatively bulky phenyl substituents at 5-position of the pyrazolyl rings in 2a as compared to methyl groups in CrCl 3 {bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl)]sulfide} (2b) 6 leads to a noticeable decrease in catalytic activity in (NSN)Cr III systems (compare entries 2 and 4, Table 1) which can be associated to the electronic effects of these phenyl groups to the chromium metal center. The selectivity for C 4 -C 12 fractions remains almost unchanged (compare entries 2 and 4).…”

Recent Research Progress in Preparation of Ethylene Oligomers with Chromium-Based Catalytic Systems

“…1-Hexene and 1-octene, used as co-monomers in the production of linear lowdensity polyethylene (LLDPE), have gained special attention [1][2][3][4][5]. The conventional method of producing 1-hexene and 1-octene is by oligomerization of ethylene.…”

“…The oligomerization of ethylene typically leads to a geometric distribution of linear α-olefins, and fractional distillation is required to isolate these individual α-olefins. Therefore, there is increasing interest to develop catalytic systems that give better selectivity toward desirable alkenes [3,6,7].…”