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

Hannant, Mark H., Wright, Joseph A., Lancaster, Simon J., Hughes, David L., Horton, Peter N. and Bochmann, Manfred (2006) The synthesis of new weakly coordinating diborate anions: anion stability as a function of linker structure and steric bulk. Dalton Transactions, (20), 2415-2426. (doi:10.1039/b515548g).

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)(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.

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Contributors

Author: Peter N. Horton

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