Liquid crystal behaviour of ionic allylpalladium complexes containing 2-pyrazolylpyridine as bidentate N,N′-ligand

Two types of Pd-complexes containing the new N,N′-ligands 2-[3-(4-alkyloxyphenyl)pyrazol-1-yl]pyridine (pzRpy; R = C6H4OCnH2n+1, n = 6 (hp), 10 (dp), 12 (ddp), 14 (tdp), 16 (hdp), 18 (odp)) (1–6), namely c-[Pd(Cl)2(pzRpy)] (7–10) and c-[Pd(η3-C3H5)(pzRpy)]BF4 (11–16), have been synthesised and characterised by different spectroscopic techniques. Those members of the second type containing the largest chains (R = ddp 13, tdp 14, hdp 15, odp 16) have been found to have liquid crystal properties showing smectic A mesophases. By contrast, neither the free ligands pzRpy nor their related c-[Pd(Cl)2(pzRpy)] complexes exhibited mesomorphism. The new synthesised metallomesogens are mononuclear complexes with an unsymmetrical molecular shape as deduced from the X-ray structures of c-[Pd(η3-C3H5)(pzRpy)]BF4 (R = hp, 11; dp, 12). Both compounds, which are isostructural, show a distorted square-planar environment on the palladium centres defined by the allyl and the bidentate pzRpy ligands. The crystal structure reveals that both the counteranion and the pzRpy ligand function as a source of hydrogen-bonding and intermolecular π⋯π contacts resulting in a 2D supramolecular assembly.