Pyrazolium salts as a new class of ionic liquid crystals

Abstract

New alkyloxyphenyl substituted pyrazolium salts [H₂pz^R(n)][A] (R(n) = C₆H₄OC_nH_2n+1; n = 8, 10, 12, 14, 16, 18; [A] = Cl⁻, BF₄⁻, ReO₄⁻, SbF₆⁻, CF₃SO₃⁻, CH₃-p-C₆H₄SO₃⁻) have been synthesised and characterised. Those containing Cl⁻, BF₄⁻, ReO₄⁻ and SbF₆⁻ anions behave as liquid crystal compounds exhibiting smectic A (SmA) mesophases. Because the precursor neutral pyrazoles [Hpz^R(n)] were not liquid crystals, their protonation and strategic choice of the counter-anion was found useful to achieve mesomorphism and to modulate the transition temperatures, so giving rise to a new class of ionic liquid crystals. The X-ray crystal structures of selected derivatives as representative examples of each type of compound have been resolved. In most of the cases, for X-ray purposes, the salts were designed to contain one methoxyphenyl-substituted pyrazolium cation. Different structural features have been found depending on the nature of the anion. Dimeric units held through hydrogen bonds are arranged in chains which pack in a layer-like array for compounds [H₂pz^R(10)][BF₄], [H₂pz^R(1)][OTf] and [H₂pz^R(1)][PTS], but 2D structures were found for [H₂pz^R(1)][Cl] and [H₂pz^R(1)][ReO₄]. Relationships between the solid and the mesophase structures are proposed. The absence of mesomorphism for CF₃SO₃⁻ and CH₃-p-C₆H₄SO₃⁻ derivatives is also explained on the basis of the main structural features.