Cyclopalladated Azo- and Azoxybenzene Mononuclear Complexes Containing a Chiral Chelating Ligand

Abstract

Chiral metallomesogens are materials of great interest for different technological applications. In most of the reported chiral liquid crystals, the chiral centre is located remote from the central core of the molecule. Our goal is to prepare new chiral metallomesogens featuring the stereogenic centre as close to the metal ion as possible. Accordingly, new mononuclear cyclopalladated complexes based on azo or azoxybenzene ligands (4,4′-bis(octyloxybenzoyloxy)azobenzene, HAzoCOO-8, p-azoxyanisole, HAzoxy, 4,4′-bis(hexyloxy)azobenzene, HAzo-6, or 4,4′-bis(hexyloxy)azoxybenzene, HAzoxy-6) containing the chiral chelating ligand (D-(−)-α-Phenylglycinol, H(Phenylgly)) have been synthesized and fully characterized. 1H NMR spectra of the azoxybenzene cyclopalladated complexes show the presence of just one isomer while those of the products obtained from the azobenzene ligands evidence the formation of isomeric mixtures. Nematic mesophases have been observed for complexes [(AzoCOO-8)Pd(Phenylgly)] and [(Azoxy-6)Pd-(Phenylgly)]. Mixtures of HAzoCOO-8 and the corresponding cyclopalladated complex, [(AzoCOO-8)Pd(Phenylgly)], in different molar ratios, have been prepared in order to promote the development of chiral mesophases.