Tetrahedral and octahedral metallomesogenic Zn(ii) complexes supported by pyridine-functionalised pyrazole ligands

Abstract

Pyridylpyrazoles with long-chained alkyloxyphenyl substituents [Hpz^R(n)py] (R(n) = C₆H₄OC_nH_2n+1, n = 12, 14, 16, 18) are versatile ligands for designing new zinc metallomesogens [Zn(Hpz^R(n)py)_m][X]₂ (m = 2, X = NO₃⁻; m = 3, X = BF₄⁻) with tetrahedral and octahedral coordination geometry, respectively. Molar conductivity measurements reveal the non-bonding character of the nitrate and tetrafluoroborate anions to Zn(II) and confirm the ionic nature of the complexes in solution. Polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) studies show that all of them are enantiotropic liquid crystals, exhibiting SmA mesophases. A layered packing with interdigitation of the alkyl chains is proposed on the basis of XRD results to explain the organisation in the SmA mesophase. TG experiments indicate that the zinc complexes are stable up to ca. 175 °C, the temperature at which mass loss is observed. On the basis of computational models using hyperchem-7 program the molecular geometry has been examined. The semi-empirical calculations suggest that, in the solid state, the counteranions could be involved in hydrogen bonds, which would contribute to obtaining the stable electronic structure.