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(mu-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(mu-Cl)}2{AlCl4}2.(C7H8)1.5 (2), monomeric [(CySCH2CH2N(H)CH2CH2SCy)CrCl2 (THF)][AlCl4] (3), and {[CySCH2CH2N(H)CH2CH2SCy]Cr(eta2-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)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 [(mu-CySCH2CH2NCH2CH2SCy)CrCl]2 (7), which did not produce oligomers.

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Treatment of the ethylene tetramerization catalyst precursor [PNP]CrCl3 with Me3Al afforded the new divalent catalyst precursor {[PNP]2Cr(m-Cl)AlMe3}[ClAlMe3]0.34[Me4Al]0.66·0.125(hexane)·0.25(toluene) (1). The formation of 1 implies reduction to the divalent state, cationization, and ligand scrambling. The fact that this species displays, upon activation with MAO, a catalytic activity similar to that of the trivalent precursor indicates that reduction of the metal to the divalent state is a step toward the formation of the actual catalytically active species. The failure of CrCl2(THF)2 to react with [PNP] indicates that cationization and acquisition of the second ligand and of a bridging ClAlMe3 residue are central to the stabilization of this new divalent catalyst precursor.