The synthesis of new weakly coordinating diborate anions: anion stability as a function of linker structure and steric bulk

Abstract

The successive addition of KCN and Ph₃CCl to B(C₆F₄-C₆F₅-2)₃ (PBB) affords triphenylmethyl salts of the [NC-PBB]⁻ anion. By contrast, the analogous reaction with sodium dicyanamide followed by treatment with Ph₃CCl leads to the zwitterionic aminoborane H₂NB(C₁₂F₉)₂C₁₂F₈, via nucleophilic attack on an o-F atom, together with CPh₃[F-PBB]. Whereas treatment of [NC-PBB]⁻ with either PBB or B(C₆F₅)₃ fails to give isolable cyano-bridged diborates, the reaction of Me₃SiNC-B(C₆F₅)₃ with PBB in the presence of Ph₃CCl affords [Ph₃C][PBB-NC-B(C₆F₅)₃]. Due to steric hindrance this anion is prone to borane dissociation. The longer linking group N(CN)₂⁻ gives the very voluminous anions [N{CNB(C₆F₅)₃}₂]⁻ and [N(CN-PBB)₂]⁻. A comparison of propylene polymerisations with rac-Me₂Si(Ind)₂ZrMe₂ activated with the various boranes or trityl borates gives an anion-dependent activity sequence, in the order [NC-PBB]⁻ < [MeB(C₆F₅)₃]⁻ < [MePBB]⁻ ≈ [PBB-NCB(C₆F₅)₃]⁻ ≈ [N{CNB(C₆F₅)₃}₂]⁻ < [F-PBB]⁻ ≪ [B(C₆F₅)₄]⁻ < [N(CN-PBB)₂]⁻. The anion [N(CN-PBB)₂]⁻ gives a catalyst productivity about 2500 times higher than that of [NC-PBB]⁻ and exceeds that of [B(C₆F₅)₄]⁻ based catalysts. The van der Waals volumes and surface areas of the anions have been calculated and provide a rationale for the observed reactivity trends in polymerisation reactions.