Ethylene Oligomerization Promoted by a Silylated-SNS Chromium System

The ethylene trimerization SNS ligand has been modified by replacing the methylene carbons flanking the nitrogen atom with dimethyl silyl groups. Three ligands, CySCH 2 Si(CH 3 ) 2 N(H)Si(CH 3 ) 2 CH 2 SCy ( a ), ( t -Bu)SCH 2 Si(CH 3 ) 2 N(H)Si(CH 3 ) 2 CH 2 S( t -Bu) ( b ), and PhSCH 2 Si(CH 3 ) 2 N(H)Si(CH 3 ) 2 CH 2 SPh ( c ), have been prepared. Ligand a in either protonated or deprotonated forms was reacted with CrCl 3 (THF) 3 to afford the corresponding monomeric [CySCH 2 Si(CH 3 ) 2 N(H)Si(CH 3 ) 2 CH 2 SCy]CrCl 3 ( 1a ) or dimeric {[CySCH 2 Si(CH 3 ) 2 NSi(CH 3 ) 2 CH 2 SCy]CrCl(μ-Cl)} 2 ( 2a ). One-pot reaction of a in the presence of Et 2 AlCl with either Cr(III) or Cr(II) chlorides afforded in either case the divalent {[CySCH 2 Si(CH 3 ) 2 N(H)Si(CH 3 ) 2 CH 2 SCy]Cr{(μ-Cl)Al(CH 2 CH 3 ) 2 Cl} 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 {[CySCH 2 Si(CH 3 ) 2 NSi(CH 3 ) 2 CH 2 SCy]Cr(μ-Cl)} 2 ( 4a ) and {[( t -Bu)SCH 2 Si(CH 3 ) 2 NSi(CH 3 ) 2 CH 2 S( 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 AlCl 3 or AlMe 3 and using divalent instead of trivalent chromium salts. In the cases of ligands a and c and AlCl 3, two isostructural complexes, {[CySCH 2 Si(CH 3 ) 2 N(H)Si(CH 3 ) 2 CH 2 SCy]Cr{(μ-Cl)AlCl 3 } 2 ( 5a ) and {[PhSCH 2 Si(CH 3 ) 2 N(H)Si(CH 3 ) 2 CH 2 SPh]C{(μ-Cl)AlCl 3 } 2 ( 5c ), have been obtained. The reaction with AlMe 3 afforded {[CySCH 2 Si(CH 3 ) 2 N(Al(CH 3 ) 2 -μ-Cl)Si(CH 3 ) 2 CH 2 SCy]Cr{(μ-Cl)Al(CH 3 ) 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 Me 2 AlCl and if a divalent chromium precursor was used. The reaction afforded the catalytically inactive divalent {[CySCH 2 Si(CH 3 ) 2 N(CH 3 )Si(CH 3 ) 2 CH 2 SCy]Cr(μ-Cl)} 2 {(Al(CH 3 ) 2 Cl) 2 )(μ-Cl)} 2 ( 7d ). Most of these species showed good catalytic activity upon activation but produced only statistical distributions of oligomers.