Role of the Metal Oxidation State in the SNS−Cr Catalyst for Ethylene Trimerization: Isolation of Di- and Trivalent Cationic Intermediates

The reaction of the highly selective [CySCH2CH2N(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)CH2CH2SCy]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)CH2CH2SCy] CrCl(μ-Cl)}2{AlCl4}2·(C7H8)1.5 (2),
monomeric [(CySCH2CH2N(H)CH2CH2SCy)CrCl2 (THF)][AlCl4] (3), and {[CySCH2CH2N(H)CH2CH2SCy]Cr(η2-AlCl4)}{Al2Cl7} (4) adducts display the same catalyst selectivity as the [CySCH2CH2N(H)CH2CH2SCy]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)CH2CH2SCy] CrCl3 with different stoichiometric
ratios of AlCl3.. When i-BAO activator was used, reduction of the metal center occurred readily, affording
{([CySCH2CH2N(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
CrCl2(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 CrCl2(THF)2 gave the dinuclear complex [(μ-CySCH2CH2NCH2CH2SCy)CrCl]2 (7), which did
not produce oligomers.