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

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)(2)C12F8, 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)(2)ZrMe2 activated with the various boranes or trityl borates gives an anion-dependent activity sequence, in the order [NC-PBB](-) < [MeB(C6F5)(3)](-) < [MePBB](-) approximate to [PBB-NCB(C6F5)(3)](-) approximate to [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.