New Self-Activating Organochromium Catalyst Precursor for Selective Ethylene Trimerization

Two catalytic systems based on anionic ligands with the NPN structural motif, bearing tri- and pentavalent phosphorus, respectively, have been compared vis-à-vis their ability to selectively trimerize ethylene. In the case of the trivalent phosphorus ligands, reaction of the Cr(II) catalyst precursor [(t-Bu)NPN(t-Bu)]2Cr (1) with 2 equiv of MeLi afforded the Cr(III) species [(t-Bu)NP(Me)N(t-Bu)]2CrLi(OEt2) (2). The same reaction with 3 equiv of MeLi yielded instead the Cr(II) species {[(t-Bu)NP(Me)N(t-Bu)]Cr(μ-Me)}2{Li(THF)}2 (3). The P atoms of both 2 and 3 have been methylated. Activation of 2 and 3 with MAO produced an S-F distribution of oligomers. Conversely, activation of 3 with EtAlCl2 exclusively afforded 1-hexene together with a small amount of polymer as a byproduct. Treatment of 2 with EtAlCl2 did not yield an active catalyst. The reaction of CrCl2(THF)2 with [(t-Bu)NP(Ph)2N(t-Bu)]−Li+, containing pentavalent phosphorus, afforded the Cr(II) derivative [(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ-Cl)2Li(THF)2 (4). Its alkylation with EtLi gave the ethyl-bridged dimer {[(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ-Et)}2 (5), which, upon thermolysis, afforded [(t-Bu)NP(Ph)2N(t-Bu)]2Cr (6). The structures of 5 and 3 are closely related, having in common two bridging alkyls and the same metal oxidation state but differing with respect to the P atom oxidation state and the presence/absence of alkali-metal cations. The catalytic behavior instead is remarkably different. Complex 5 is active as a self-activating selective ethylene trimerization catalyst, while complex 3 requires activation.