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 Ph(3)CCl leads to the zwitterionic aminoborane H2NB(C12F9)2C12F8, via nucleophilic attack on an o-F atom, together with CPh3F-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 NCNB(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]- approximately PBB-NCB(C6F5)3]- approximately NCNB(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.