Synthesis, Structures, and Ethylene Oligomerization Activity of Bis(phosphanylamine)pyridine Chromium/Aluminate Complexes
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A trivalent chromium complex of a PN(pyridine) ligand system, {[(2,6-Ph2P-NH)2C5H3N]CrCl3}(THF)2 (1), was prepared and tested as a catalyst for ethylene oligomerization and polymerization, with the purpose of probing the ability of a pyridine ring substituent as a stabilizing factor on catalytically active intermediates. Its nonselective catalytic behavior indicated that ready reduction of the metal center to the divalent state occurred during the activation process. To substantiate this point, we have reacted 1 with a few common aluminate activators and isolated both the divalent complexes {[2,6-Ph2PNHC5H3NAlClEt2NPPh2]Cr(μ-Cl)2AlEt2}(toluene) (3) and {[2,6-Ph2PNHC5H3NAlCl-i-Bu2NPPh2]Cr(μ-Cl)2Al-i-Bu2}2 (toluene) (4) and the trivalent complexes {[2,6-Ph2PNHC5H3NAlClMe2NPPh2]CrMe(μ-Cl)2AlMe2}(toluene)1.5 (2) and {[2,6-Ph2PNHC5H3 NHNPPh2]CrEt(μ-Cl)2AlEt2}AlEtCl3(hexane)0.5 (5). The reaction of the ligand with the divalent chromium precursor CrCl2(THF)2 in the presence of alkylaluminum afforded {[2,6-Ph2PNHC5H3NCl2EtAlNPPh2]Cr(μ-Cl)2AlEt2}2(toluene) (6) containing aluminate residues, where the metal has preserved the initial divalent state. All of these species showed moderate to high activities toward ethylene oligomerization.
