Polycatenar pyrazole and pyrazolate ligands as building blocks of new columnar Pd(ii) metallomesogens

Abstract

Dicatenar pyridine-functionalised pyrazole ligands [Hpz^R(n,n)py] (R(n,n) = C₆H₃(OC_nH_2n+1)₂, n = 4, 6, 8, 10, 12, 14, 16, 18) have been strategically synthesised to be used as new building blocks for designing discotic liquid crystalline materials. Their coordination to Pd(II) fragments has allowed to achieve two novel families of metallomesogens, [Pd(pz^R(n,n)py)₂] (I) and [PdCl₂(Hpz^R(n,n)py)] (II), in which the ligand is coordinated in the anionic form as pyrazolate or in the neutral form as pyrazole, respectively. Thermal studies showed that the ligands with n = 14 and 16 carbon atoms, as well as all the palladium complexes, display discotic mesophases in the temperature range of 68–141 °C. The results indicate that the coordination environment around the metal is a determining factor which allows control of the supramolecular arrangement of the mesophase. Disc-like molecules from complexes I pack themselves into cylindrical structures that result in hexagonal columnar phases (Col_h), while the half-disc shaped molecules from II self-assemble into a layer with an antiparallel dimeric disposition which generates lamellar columnar phases (Col_L). Schematic models based on X-ray powder diffraction (XRD) experiments are proposed to illustrate the molecular organisation of these Pd metallomesogens in the columnar mesophases.