Highly Active Ethylene Oligomerization Catalysts

Reaction of RN(H)PBr(Ph)2N(H)R [R = t-Bu, i-Pr, Ph] with two equivalents of n-BuLi followed by reaction with CrCl2(THF)2 afforded the divalent chromium complexes [(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ2-Cl)2Li(THF)2 (1), [(i-Pr)NP(Ph)2N(i-Pr)]Cr(μ2-Cl)2Li(THF)2 (2), and [{[(Ph)NP(Ph)2N(Ph)]Cr}2(μ2-Cl)3][Li(DME)3] (3). The trivalent analogue of 1, {[(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ2-Cl)3(μ3-Cl)2}{Li (THF)2} (4), was obtained in a similar manner via treatment of the double deprotonated ligand with CrCl3(THF)3. Both reactions of the divalent 1 or trivalent 4 with AlMe3 yielded the trivalent and cationic complex {[(t-Bu)NP(Ph)2N(t-Bu)]2Cr}{(Me3Al)2Cl}·toluene (5). Upon activation with MAO, 1–3 produced unprecedented and potentially useful catalytic systems for nonselective ethylene oligomerization devoid of polymer. Divalent chromium is clearly the species responsible for the catalytic behavior, ruling out that nonselective oligomerization proceeds via a redox metallacycle mechanism. The absence of polymer in combination with the record activity makes 1 competitive with the best performing industrially used systems.