Al-alkyl borate salt cocatalysts for olefin polymerization: exploration of N-donor ligand variations

The well-defined Al-alkyl borate (AAB) salt {[iBu2(PhNMe2)Al]2(μ-H)}+[B(C6F5)4]− (AlHAl) has been recently identified as a promising “complete” cocatalyst for olefin polymerization. Herein, we explore structural variations of AlHAl obtained by replacing the PhNMe2 (DMA) donor with a variety of anilines, amines, and N-heterocycles. Of the 18 investigated N-donors, twelve provided stable AAB salts; these were tested as cocatalysts in ethylene/1-hexene copolymerization with an archetypical metallocene catalyst. In the other six cases, the side reactions were thoroughly analyzed by NMR spectroscopy. For instance, addition of an o-Me substituent on the DMA ligand triggers C-H activation leading to five-membered cyclometalated species; increasing the steric bulk directly at the N-donor atom leads to tricoordinate mononuclear Al-alkyl cations, which could be isolated, fully characterized and tested in polymerization when using PhNEt2 (DEA). The cocatalytic performance of aniline- and amine-based systems varies only marginally with respect to the benchmark AlHAl_DMA. N-heterocyclic AAB salts perform worse; only the two least electron donating donors, namely difluoropyridine (Py-3,5-F) and quinoline (QUI), provide noticeable productivity. A simple quantitative structure-activity relationship, correlating the steric bulk and stabilizing ability of the N-donor with productivity (R2 = 0.88), has been identified.