Supramolecular anion binding by the [ZnCl(HpztBu)3]+cation (HpztBu = 5-tert-butylpyrazole)

Abstract

Reaction of ZnCl₂ with three molar equivalents of 3{5}-tert-butylpyrazole (Hpz^tBu) yields [ZnCl(Hpz^tBu)₃]Cl (1). Treatment of 1 with an appropriate Ag(I) or Tl(I) salt gives [ZnCl(Hpz^tBu)₃]X (X⁻ = BF₄⁻, 4; X⁻ = ClO₄⁻, 5; X⁻ = NO₃⁻, 6; X⁻ = CF₃SO₃⁻, 7; X⁻ = PF₆⁻, 8). Crystal structures of all of the complexes show a [ZnCl(Hpz^tBu)₃]⁺ cation, with (for 1 and 4-7) only small deviations from C_3v symmetry, with the tert-butyl groups forming a bowl-shaped cavity on one face of the complex. The X⁻ anion binds within this cavity, via hydrogen bonding to the three pyrazole N–H groups. For 8, the PF₆⁻ guest only interacts with two N–H donors, the third pyrazole ring being twisted away from the anion. We attribute this to steric repulsions between the third ligand and the PF₆⁻ anion, which is larger than the other guests used in this study. ¹H, ¹³C and ¹⁹F NMR, ESMS and IR studies provide good circumstantial evidence for these structures being retained in solution, although the guest anions undergo rapid intermolecular exchange by NMR that cannot be frozen out above 190 K.