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(A-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(A-Cl)}(2){AlCl4}(2),(C7H8)(1.5) (2), monomeric [(CySCH2CH2N(H)CH2CH2SCy)CrCl2(THF)][AlCl4] (3), and {[CySCH2CH2N(H)CH2CH2SCy]Cr(A(2)-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)(mu-Cl)](2)} {(i-Bu)(2)AlCl2}(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 [(mu-CySCH2CH2NCH2CH2SCy)CrCl](2) (7), which did not produce oligomers.