The effect of different ligand substituents on the chemistry of a zinc–pyrazole anion host

Abstract

The influence of the pyrazole ligand on the previously reported anion host [ZnCl(HpztBu)3]+ (HpztBu = 5-tert-butylpyrazole) has been investigated. Reaction of ZnCl2 with 3 equiv. of 3{5}-cyclohexylpyrazole (HpzCy) or 3{5}-phenylpyrazole (HpzPh) affords [ZnCl(HpzR)3]Cl (R = Cy or Ph), both of which undergo decomposition to [ZnCl2(HpzR)2] upon recrystallisation. A similar reaction using 3{5}-(thien-2-yl)pyrazole (HpzTn) affords [ZnCl2(HpzTn)2] only. The salts [ZnCl(HpzPh)3]BF4, [ZnCl(HpzCy)3]X (X− = NO3−, ClO4−, CF3SO3− or ½SO42−) and [ZnBr(HpzCy)3]NO3·H2O have been isolated, by performing the above reactions in the presence of 1 equiv. of AgX. The cations in [ZnCl(HpzCy)3]NO3 associate into a dimeric capsule encapsulating two nitrate anions. In contrast, [ZnBr(HpzCy)3]NO3·H2O contains hydrogen-bonded [NO3·H2O]nn− chains enclosed within channels formed by the complex cations, while [ZnCl(HpzCy)3]ClO4 forms a different hydrogen-bonded dimer with the anions occupying two shallow cavities in its surface. These data imply that anion binding by [ZnCl(HpzR)3]+ depends strongly on the steric and inductive properties of the pyrazole ‘R’ substituents.