Isolation of a Cationic Chromium(II) Species in a Catalytic System for Ethylene Tri- and Tetramerization

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.