Switchable catalytic activity, from nonselective ethylene oligomerization to trimerization and even polymerization, is shown by 1, depending on the alkyl aluminum compound with which it is activated. AlMe3 or iBu2AlCl resulted in highly active single‐component polymerization catalysts (see scheme; R=Me, PE=polyethylene), the activity of which can be switched or inhibited by the presence of an additional alkyl aluminum compound.

Reaction of the divalent complex [(t-Bu)NPN(t-Bu)] 2 Cr (1) with different stoichiometric ratios of Al(i-Bu) 3 afforded {µ-[(t-Bu)NP(i-Bu)N(t-Bu)]Al(i-Bu) 2 } 2 Cr (2) and [{(i-Bu)P{µ-N(t-Bu)} 2 Al(i-Bu) 2 }Cr(µ-H)] 2 (3) as part of the same reaction sequence. Complex 2 arises from association of alane and the two ligands via alkylation of the P atom and retention of the AlR 2 unit. Complex 3 appears to be generated from subsequent dissociation of one alkylated ligand and possible transfer of an i-Bu group to Cr followed by β-H elimination or transfer of a hydride originating from isobutyl elimination of the aluminumcontaining residue. Both species are potent ethylene polymerization catalysts with no need for further activation. It is assumed that 2 is transformed in situ into 3 under the influence of ethylene. Reactions with ethylene carried out in the presence of excess of Al(i-Bu) 3 switch the selectivity completely toward selective trimerization.

Reaction of [(Ar)NPN(t-Bu)]2Cr (2) with Me3Al afforded {[(Ar)NP(Me)N(t-Bu)]AlMe2}Cr{[(Ar)NP(Me)(AlMe3)N(t-Bu)]AlMe(μ-Me)} (3), in which the ligandʼs P atom has been alkylated and one AlR2 residue retained by the chelating framework. One of the two ligands also retained an additional Me3Al unit via coordination to the alkylated P atom. Complex 3 provides the first case of a selective ethylene trimerization catalyst with high activity and excellent selectivity, producing 1-hexene upon exposure to ethylene at 80 °C. Furthermore, upon treatment with MAO, complex 3 acts as a nonselective catalyst, producing a statistical mixture of oligomers with the highest ever observed activity. In addition, upon treatment with [(i-Bu)2Al]2O, the complex acts as a highly active polymerization catalyst.