Mesogenic palladium complexes with pincer ligands derived from dipicolinic acid.

4-Substituted pyridine-2,6-dicarboxylic acids, (E)-dipicH2, and 4-substituted pyridine-2,6-bis(thiocarboxylic) acids, (E)-pdtcH2, (E = OCnH2n + 1, SCnH2n + 1) have been synthesized and used as O,N,O- and S,N,S-pincer ligands with palladium. In the fourth coordination site the complexes bear 4-decyloxy-4'-stilbazole (L1), 4-decyloxy-N-(4-pyridylmethylene)anilines (L2), decyl 4-pyridinecarboxylate (L3), 4-(4'-decyloxyphenyl)pyridinecarboxylate (L4), 4-(3',4',5'-tridecyloxybenzyl)pyridinecarboxylate (L5), 4-isocyano-1-decyloxybenzene (L6), or 4-isocyano-4'-decyloxybiphenyl (L7). Thermotropic mesomorphism is observed for the (E)-dipic complexes with L5 and n = 12, which display columnar phases. The complexes with S,N,S-pincers show an important depression in the melting point compared to their O,N,O homologues and this change gives rise to mesomorphic materials (SC). However, in the case of L5 the mesomorphic behavior observed for the O,N,O derivative is lost in its S,N,S analogue. The alkylsulfanyl compounds exhibit lower transition temperatures and wider mesophase ranges than their alkoxy analogues.