Critical issues raised in the Sasol's original ethylene tetramerization catalyst composed of Cr(acac)3, iPrN(PPh2)2 and MMAO (modified-methylaluminoxane) are the use of expensive MMAO, low working temperature around 60 °C, and generation of PE side product. Generation of polyethylene, though its amount is small, also poses a critical problem that is often referred to as 'fouling' of the operation of the continuous commercial process. In this work, we disclosed an upgraded catalytic system that avoids the use of MAO solving the issues. In precedent trials to prepare the catalyst precursors, [(PNP)CrCl2]+[B(C6F5)4]–-type species, the main product isolated was inactive bis(PNP)-CrIII complex ([(PNP)2CrCl2]+[B(C6F5)4]–). The targeted mono(PNP)-CrIII complex was obtained from CrCl3(THF)3, [PhN(H)Me2]+[B(C6F5)4]–, and iPrN[P(C6H4-p-Si(nBu)3]2 (2). The bulky (nBu)3Si-substituents in 2 play a crucial role, preventing the formation of the inactive bis(PNP)-CrIII species and hindering the rotation around P-N bonds to block demetallation process. The prepared species [2-CrCl2]+[B(C6F5)4]– combined with iBu3Al was extremely active (> 6000 Kg/g-Cr/h), worked well at a high temperature up to 90 °C, and generated a negligible amount of PE (0.03 wt%). The extremely active [2-CrCl2]+[B(C6F5)4]– was also prepared by simply reacting 2 and [CrCl2(NCCH3)4]+[B(C6F5)4]–. Screening the performance with a series of iPrN[P(C6H4-p-SiR3]2 (R3Si- = (nBu)3Si-, iPr3Si-, Et3Si-, Me3Si-, Et2(iPr)Si-, Me2(iPr)Si-, and Me2(1-octyl)Si-) further supported that bulky R3Si-substiuents are crucial not only to gain the extremely high activities but also to minimize generation of the problematic side product PE even at a high temperature.
