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 {[<i>i</i>Bu₂(<b>L</b>)Al]₂(μ-H)}⁺[B(C₆F₅)₄]^- (<b>AlHAl_L</b>) with <b>L</b> = N-donor ligands have been recently reported as promising "complete" cocatalysts for olefin polymerization. Herein, we explore structural variations of <b>AlHAl_L</b> going beyond the class of N-donors like the prototypical <i>N,N</i>-dimethyl aniline (<b>DMA</b>). 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, <b>DPPH</b>], 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 <i>ansa</i>-zirconocene catalyst [<i>rac</i>-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 <b>L</b> 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 <b>L</b> donors and the Zr active species might be relevant in determining catalytic performance in some cases.