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]Cr(m-CI)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.