Ethylene Oligomerization Promoted by a Silylated-SNS Chromium System

Abstract

The ethylene trimerization SNS ligand has been modified by replacing the methylene carbons flanking the nitrogen atom with dimethyl silyl groups. Three ligands, CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy (a), (t-Bu)SCH2Si(CH3)2N(H)Si(CH3)2CH2S(t-Bu) (b), and PhSCH2Si(CH3)2N(H)Si(CH3)2CH2SPh (c), have been prepared. Ligand a in either protonated or deprotonated forms was reacted with CrCl3(THF)3 to afford the corresponding monomeric [CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]CrCl3 (1a) or dimeric {[CySCH2Si(CH3)2NSi(CH3)2CH2SCy]CrCl(μ-Cl)}2 (2a). One-pot reaction of a in the presence of Et2AlCl with either Cr(III) or Cr(II) chlorides afforded in either case the divalent {[CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]Cr{(μ-Cl)Al(CH2CH3)2Cl}2 (3a). To deprotonate the N–H function of the Si-SNS ligand, n-BuLi was used for the purpose of preparing the divalent chromium analogue. The reaction afforded in the case of both a and b the two nearly isostructural divalent complexes {[CySCH2Si(CH3)2NSi(CH3)2CH2SCy]Cr(μ-Cl)}2 (4a) and {[(t-Bu)SCH2Si(CH3)2NSi(CH3)2CH2S(t-Bu)]Cr(μ-Cl)}2 (4b) in crystalline form. To further clarify the interaction of 4 with aluminate species, we have carried out in situ complexation in the presence of either AlCl3 or AlMe3 and using divalent instead of trivalent chromium salts. In the cases of ligands a and c and AlCl3, two isostructural complexes, {[CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]Cr{(μ-Cl)AlCl3}2 (5a) and {[PhSCH2Si(CH3)2N(H)Si(CH3)2CH2SPh]C{(μ-Cl)AlCl3}2 (5c), have been obtained. The reaction with AlMe3 afforded {[CySCH2Si(CH3)2N(Al(CH3)2-μ-Cl)Si(CH3)2CH2SCy]Cr{(μ-Cl)Al(CH3)3} (6a). Its structure was informative, showing a possible catalyst deactivation pathway. To better evaluate the role of the N–H function, we have also methylated ligand a at the N atom. The complexation to chromium was successful only in the presence of Me2AlCl and if a divalent chromium precursor was used. The reaction afforded the catalytically inactive divalent {[CySCH2Si(CH3)2N(CH3)Si(CH3)2CH2SCy]Cr(μ-Cl)}2{(Al(CH3)2Cl)2)(μ-Cl)}2 (7d). Most of these species showed good catalytic activity upon activation but produced only statistical distributions of oligomers