Mesogenic Nickel and Palladium Complexes with Pincer Ligands Derived from Pyridine-2,6-bis(thiocarboxylic) Acid

Ni complexes with 4-substituted pyridine-2,6-bis (thiocarboxylate) ligands acting as S,N,S-pincers are described. They feature diamagnetic square-planar nickel(II), with three coordination sites taken by the pincer ligand and the fourth coordination position occupied by a 4-substituted pyridine or a 4substituted biphenyl isocyanide ligand. Only the latter are liquid crystals. The thermal properties of these Ni complexes differ only moderately from those found for similar Pd derivatives. This difference is more important for the compounds with shorter chains. Other changes in the complexes, such as the use of alkylsulfanyl for alkyloxy, or the length of the chain produce an important influence and induce very similar variations for Pd and Ni. Overall Ni shows a somewhat higher tendency to give less ordered (nematic) phases. The use of the newly synthesized chiral 4-substituted pyridine-2,6-bis(thiocarboxylate) acids (E*)-pdtcH2 (E* = (R)-2-butyloxy, (R)-2-octyloxy) as pincer ligands leads to chiral square planar Pd and Ni complexes. These chiral complexes show an important lowering in their melting point temperatures compared to their nonchiral S,N,S-pincers homologues and give rise to chiral smectic C (SmC*) and chiral nematic (N*) phases.