In situ formed Cr(III) based silicon-bridged PNS systems for selective ethylene tri-/tetramerization

In-situ formed and activated chromium(III) catalysts based on silicon-bridged PNS (Si-PNS) ligands of the form Ph₂PN(R¹)Si(CH₃)₂S(R²) have been investigated for selective ethylene tri-/tetramerization. Ligand substituents at the backbone N and S atoms have been realized to have a marked impact on the product selectivity in these systems. Ligands having the substituents such as cyclohexyl (in ligand 3), ortho-tolyl (in ligand 5) or ethyl (in ligand 6) at backbone S, and a low steric bulk moiety (isopropyl) at backbone N in these ligands favor tetramerization of ethylene and respectively deliver 75%, 76% and 66% C₈ selectivity. Ligands having higher steric bulk substituents like 2,6-diisoprpylphenyl (in ligand 1) or cyclopentyl (in ligand 2) at backbone N, and a cyclohexyl moiety at backbone S in these ligands promote trimerization of ethylene and produce C₆ fraction. While the ortho-tolyl moiety at backbone S and a 2,6-diisopropylphenyl substituent at backbone N completely diminish the C₆ and C₈ selectivity in ligand 4 based system and produce higher oligomers. DFT calculations revealed that the sterically relaxed ligand 3 provide a facile and energetically favorable route for C₈ formation, while sterically encumbered substituents tend to block the formation of C₆ and C₈ fractions in ligand 4 based system.