Extremely Active Ethylene Tetramerization Catalyst Avoiding the Use of Methylaluminoxane: [iPrN{P(C 6 H 4 ‐ p ‐SiR 3 ) 2 } 2 CrCl 2 ] + [B(C 6 F 5 ) 4 ] −

Sasol's original ethylene tetramerization catalyst requires the use of expensive MMAO, a low working temperature (∼60 °C), and generates polyethylene (PE) as a side product. In this study, we developed an upgraded catalytic system that successfully avoids the need for MMAO. [(PNP)CrCl 2 ] + [B(C 6 F 5 ) 4 ] − ‐type species was obtained from the reaction of CrCl 3 (THF) 3 , [PhN(H)Me 2 ] + [B(C 6 F 5 ) 4 ] − , and iPrN[P(C 6 H 4 ‐ p ‐Si(nBu) 3 ) 2 ] 2 ( 2 ) as well as from simply reacting 2 with [CrCl 2 (NCCH 3 ) 4 ] + [B(C 6 F 5 ) 4 ] − . The bulky (nBu) 3 Si‐substituents play the crucial role of preventing the formation of the inactive [(PNP) 2 CrCl 2 ] + [B(C 6 F 5 ) 4 ] − . The prepared [ 2 ‐CrCl 2 ] + [B(C 6 F 5 ) 4 ] − combined with iBu 3 Al was extremely active (>4000 kg/g‐Cr/h), performed well at a high temperature of up to 90 °C, and generated a negligible amount of PE (0.03 wt%). Screening the performance with a series of iPrN[P(C 6 H 4 ‐ p ‐SiR 3 ) 2 ] 2 further supported that bulky R 3 Si‐substituents are crucial not only to achieve extremely high activities but also to minimize the generation of PE. Structure of a [(PNP)CrCl 2 ] + [B(C 6 F 5 ) 4 ] − species was elucidated by X‐ray crystallography.