Palladium(II) metallomesogens of crown ether derivatized imines, and their sodium adducts

A series of imines HLn (1) (n stands for the alkoxy chain length: 4, 6, 8, 10, and 12) has been synthesized by condensation of 4-aminobenzo-15-crown-5 and p-alkoxybenzaldehyde. Orthopalladation afforded the dinuclear derivatives [Pd(μ-OAc)Ln]2 (2) which, by substitution reactions, led to dinuclear and mononuclear derivatives [Pd(μ-Cl)Ln]2 (3) and [Pd(β-diket)Ln] (4) (β-diket = H25C12OH4C6–C(O)–CH–C(O)–C6H4OC12H25). The imine ligands and the acetato bridged complexes are non-mesogenic whilst the chloro-bridged and the diketonato complexes are enantiotropic liquid crystals. They display smectic A mesophases. Complexation with sodium perchlorate afforded the corresponding derivatives [(O2ClO2)NaCrown] (Crown = crown derivatives 1–4 for n = 4). None of the sodium adducts is mesogenic. The acetato bridged complexes consist of a mixture of syn and anti isomers and a dramatic increase in the syn:anti ratio is produced upon complexation of NaClO4 (syn:anti = 4:96 for complexes 2 versus 54:46 for the corresponding [(O2ClO2)NaCrown2]). The crown ether derivatives extract sodium picrate from aqueous solutions and the presence of the palladium centers improves clearly the extraction, [Pd(μ-Cl)Ln]2 being the best and the fastest extractor. A model for the extraction and transport process in these complexes is proposed.