Alternating α-Olefin Distributions via Single and Double Insertions in Chromium-Catalyzed Ethylene Oligomerization

Abstract

The catalytic oligomerization of ethylene with chromium-based complexes containing bis­(benzimidazolemethyl)­amine (BIMA) ligands results in alternating distributions of linear α-olefins (LAOs). Extremely high activities are obtained (>100 000 g mmol–1 h–1 bar–1) with N-alkyl-substituted BIMA ligands, whereas bulky groups on the central nitrogen or alternative central donors result in much lower activities. Variations in the ligand backbone, as well as methylation of the benzimidazole units, lead to reduction in activity. The alternating LAO distributions have been mathematically analyzed using second-order recurrence relations. The shape of the distributions is affected by ethylene pressure (1–4 bar) and by the cocatalyst to some degree. On the basis of the results and analysis presented herein, we propose that the alternating behavior originates from the ability of these chromium BIMA catalysts to undergo single as well as double ethylene insertion reactions. A minor second distribution (<5 wt %) of 2-ethyl-1-alkenes is obtained under certain conditions, resulting from incorporation of 1-butene. DFT studies (M06L) and experimental observations regarding the reaction between AlMe3 and the N-methyl BIMA ligand 2 have shown that deprotonation of the benzimidazole N–H units can occur, which suggests a change in coordination of the BIMA ligand under oligomerization conditions.

The Mathematics of Ethylene Oligomerisation and Polymerisation

“…A final note should be added, that for ethylene oligomerisations at lower pressures with these catalysts, it is possible that the most abundant oligomeric product, 1-butene in this case, is incorporated during the oligomerisation reaction. We have observed this with some of the most active catalysts, resulting in a minor distribution of 2-ethyl-1-alkenes with the opposite alternating pattern, alongside the major distribution of 1-alkenes with the normal alternating behaviour [17].…”

“…1 and 2. In addition, a number of alternating LAO distributions with chromiumbased systems have been reported in the literature [11][12][13][14][15][16][17]. These four types of ethylene product distribution are depicted in Fig.…”

“…Distribution of 1-alkene oligomers as mol(n)% vs n (units of ethylene) produced via N,N-bis((1H-benzimidazol-2-yl)methyl)-N-methylamine chromium(III) chloride/MAO[16,17]. Conditions: 32 nmol Cr, 4 bar continuous ethylene pressure, 50 °C, MAO (7 mmol), toluene (200 mL), 1 h. Activity = 102,300 g mmol −1 h −1 bar −Catalytic scheme for the production of alternating 1-alkene oligomer distributions.…”

Recent advances in homogeneous chromium catalyst design for ethylene tri-, tetra-, oligo- and polymerization

“…A family of chromium complexes containing bis(benzimidazolemethyl)-amine ligands proved extremely active towards ethylene oligomerization ( Figure 4) [80]. For example, up to 100000 g (mmol Cr) -1 h -1 bar -1 was achieved using 16b in combination with MAO as co-catalyst.…”

Extremely Active Ethylene Tetramerization Catalyst Avoiding the Use of Methylaluminoxane: [iPrN{P(C6H4‐p‐SiR3)2}2CrCl2]+[B(C6F5)4]−

“…Catalyst development for ethylene oligomerization is an ongoing research topic in both academia and industry. [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] 4 ] À (2.0 eq), CrCl 3 (THF) 3 , and PNP ligand that exhibited high activity (up to 4000 kg/g-Cr/h) when combined with common and inexpensive alkylaluminum iBu 3 Al (Scheme 1). [46] We presumed that the chloride-bridged adduct (i. e., [(PNP)Cr(Cl)(THF)(μ 2 -Cl) 2 Al(acac)] 2 + [B(C 6 F 5 ) 4 ] À 2 ) might be formed between the cationic Cr species [(PNP)CrCl 2 ] + [B(C 6 F 5 ) 4 ] À and the cationic Al species [ClAl(acac)] + [B(C 6 F 5 ) 4 ] À .…”