Borate Salts of Aluminum-Alkyl Cations Stabilized by P-, O-, and C-Donors: Synthesis, Characterization and Application as Cocatalysts

Well-defined Al-alkyl borate (AAB) salts {[iBu2(L)Al]2(μ-H)}+[B(C6F5)4]- (AlHAl_L) with L = N-donor ligands have been recently reported as promising "complete" cocatalysts for olefin polymerization. Herein, we explore structural variations of AlHAl_L going beyond the class of N-donors like the prototypical N,N-dimethyl aniline (DMA). Thirteen P-, O-, and C-donor ligands were screened, allowing isolation of AAB salts with mono- and bidentate phosphines, alkyl-, aryl-, and silyl-ethers, and a N-heterocyclic carbene. Except for the diphosphine with the longest spacer between the P atoms [bis(diphenylphosphino)hexane, DPPH], all donors gave well-defined tetracoordinate or tricoordinate molecular species, which were characterized in solution (NMR) and solid state (XRD), and tested as cocatalyst in ethylene/1-hexene copolymerization with an ansa-zirconocene catalyst [rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2]. The vast majority of novel AAB salts provided active catalytic systems, further demonstrating the broad tunability of these species. Consistent with previous studies, variability in productivity upon L variation is primarily related to the efficiency of precatalyst activation, determining the fraction of Zr active sites. Variations in polymer molecular weight and comonomer incorporation observed with some P-, O- and C-donor ligands indicate that also interactions between the L donors and the Zr active species might be relevant in determining catalytic performance in some cases.