MAO‐free and extremely active catalytic system for ethylene tetramerization

• Published in: Applied organometallic chemistry Vol. 33; no. 4
• Main Authors: Kim, Tae Hee, Lee, Hyun Mo, Park, Hee Soo, Kim, Sung Dong, Kwon, Su Jin, Tahara, Atsushi, Nagashima, Hideo, Lee, Bun Yeoul
• Format: Journal Article
• Language: English
• Published: Chichester Wiley Subscription Services, Inc 01-04-2019
• Subjects: 1‐octene production; Aromatic compounds; Catalysis; Chemistry; chromium catalyst; Equivalence; Ethylene; ethylene tetramerization; High temperature; Ligands; NMR; Nuclear magnetic resonance; PNP ligand derivative
• ISSN: 0268-2605; 1099-0739
• DOI: 10.1002/aoc.4829

The original Sasol catalytic system for ethylene tetramerization is composed of a Cr source, a PNP ligand, and MAO (methylaluminoxane). The use of expensive MAO in excess has been a critical concern in commercial operation. Many efforts have been made to replace MAO with non‐coordinating anions (e.g., [B(C6F5)4]−); however, most of such attempts were unsuccessful. Herein, an extremely active catalytic system that avoids the use of MAO is presented. The successive addition of two equivalent [H(OEt2)2]+[B(C6F5)4]− and one equivalent CrCl3(THF)3 to (acac)AlEt2 and subsequent treatment with a PNP ligand [CH3(CH2)16]2C(H)N(PPh2)2 (1) yielded a complex presumably formulated as [1‐CrAl (acac)Cl3(THF)]2+[B(C6F5)4]−2, which exhibited high activity when combined with iBu3Al (1120 kg/g‐Cr/h; ~4 times that of the original Sasol system composed of Cr (acac)3, iPrN(PPh2)2, and MAO). Via the introduction of bulky trialkylsilyl substituents such as –SiMe3, –Si(nBu)3, or –SiMe2(CH2)7CH3 at the para‐position of phenyl groups in 1 (i.e., by using [CH3(CH2)16]2C(H)N[P(C6H4‐p‐SiR3)2]2 instead of 1), the activities were dramatically improved, i.e., tripled (2960–3340 kg/g‐Cr/h; more than 10 times that of the original Sasol system). The generation of significantly less PE (<0.2 wt%) even at a high temperature is another advantage achieved by the introduction of bulky trialkylsilyl substituents. NMR studies and DFT calculations suggest that increase of the steric bulkiness on the alkyl‐N and P‐aryl moieties restrict the free rotation around (alkyl)N–P (aryl) bonds, which may cause the generation of more robust active species in higher proportion, leading to extremely high activity along with the generation of a smaller amount of PE.
MAO‐free catalytic system that was constructed with [CrAlCl3(acac)]2+[B(C6F5)4]−2 and PNP ligand substituted with R3Si‐group at para‐positions exhibits extremely high activity for ethylene tetramerization with generation of a negligible amount of PE.