Role of the metal oxidation state in the SNS-Cr catalyst for ethylene trimerization: Isolation of Di- and trivalent cationic intermediates

Abstract

The reaction of the highly selective [CySCH2CH 2N(H)CH2CH2SCy]CrCl3 catalyst precursor with alkyl aluminum activators was examined with the aim of isolating reactive intermediates. Reaction with Me3Al afforded a cationic trivalent chromium alkyl species {[CySCH2CH2N(H)CH 2CH2SCy]CrMe(μ-Cl)}2{(AlMe3) 2(m-Cl}2·(C7H8)2 (1a). Although it was not possible to obtain crystalline samples of sufficient quality from the reaction with MAO (the most preferred activator), the near-to-identical EPR spectra indicated a very close structural similarity with 1a. Ethylene oligomerization tests clearly revealed that 1 and other cationic trivalent dimeric complexes {[CySCH2CH2N(H)CH 2CH2SCy]CrCl(M-Cl)}2{AlCl4} 2·(C7H8)1.5 (2), monomeric [(CySCH2CH2N(H)CH2CH2SCy)CrCl 2 (THF)][AlCl4] (3), and {[CySCH2CH 2N(H)CH2CH2SCy]Cr(η2-AlCl 4)}{Al2Cl7} (4) adducts display the same catalyst selectivity as the [CySCH2CH2N(H)CH 2CH2SCy]CrCl3 complex and, therefore, are probably all precursors to the same catalytically active species. 2, 3, and 4 were obtained upon treatment of [CySCH2CH2N(H)CH 2CH2SCy] CrCl3 with different stoichiometric ratios of AlCl3. When i-BAO activator was used, reduction of the metal center occurred readily, affording {([CySCH2CH 2N(H)CH2CH2S Cy]Cr)(μ-Cl)] 2}{(i-Bu)2AlCl2}2 (5). 5 is also a selective catalyst, thus indicating that trivalent species are most probably precursors to a divalent catalytically active complex. Reaction of CrCl 2(THF)2 with the ligand afforded the labile divalent adduct [CySCH2CH2N(H)CH2CH2SCy] CrCl2(THF) (6), also catalytically active and selective. Instead, deprotonation of the ligand with n-BuLi followed by reaction with CrCl 2(THF)2 gave the dinuclear complex [(μ-CySCH 2CH2NCH2CH2SCy)CrCl]2 (7), which did not produce oligomers. © 2006 American Chemical Society.