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(Hpz^tBu)₃]⁺ (Hpz^tBu = 5-tert-butylpyrazole) has been investigated. Reaction of ZnCl₂ with 3 equiv. of 3{5}-cyclohexylpyrazole (Hpz^Cy) or 3{5}-phenylpyrazole (Hpz^Ph) affords [ZnCl(Hpz^R)₃]Cl (R = Cy or Ph), both of which undergo decomposition to [ZnCl₂(Hpz^R)₂] upon recrystallisation. A similar reaction using 3{5}-(thien-2-yl)pyrazole (Hpz^Tn) affords [ZnCl₂(Hpz^Tn)₂] only. The salts [ZnCl(Hpz^Ph)₃]BF₄, [ZnCl(Hpz^Cy)₃]X (X⁻ = NO₃⁻, ClO₄⁻, CF₃SO₃⁻ or ½SO₄²⁻) and [ZnBr(Hpz^Cy)₃]NO₃·H₂O have been isolated, by performing the above reactions in the presence of 1 equiv. of AgX. The cations in [ZnCl(Hpz^Cy)₃]NO₃ associate into a dimeric capsule encapsulating two nitrate anions. In contrast, [ZnBr(Hpz^Cy)₃]NO₃·H₂O contains hydrogen-bonded [NO₃·H₂O]_nⁿ⁻ chains enclosed within channels formed by the complex cations, while [ZnCl(Hpz^Cy)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(Hpz^R)₃]⁺ depends strongly on the steric and inductive properties of the pyrazole ‘R’ substituents.