Chelated Boronate–Imine and Boronate–Amine Complexes as Chiral Dopants for Nematic Liquid Crystals

Enantiomerically and diastereomerically pure chelated boronate-imine complexes 5 and a library of boronate-amine complexes 8 have been synthesized by taking advantage of a modular concept based on aromatic aldehydes, amino-ethanols and boronic acids. The configuration of the amine complexes 8 featuring stable stereogenic boron and nitrogen centers are assigned based upon crystal structure analyses of the representative compounds 8aaa and 8acj and the comparison of the CD spectra of all complexes 8. They serve as colorless, stable dopants for nematic liquid crystals and provide high helical twisting power. The interaction of boronates 8 with nematic compounds ZLI-1840 and 5-CB featuring a benzonitrile moiety is studied by 19 F and 1 H NMR spectroscopy. A strong π-stacking between the arylboronate residue and the nematic compound as well as a hydrogen bond are indicated by the spectroscopic data. Based thereupon, a model is proposed that correlates the configuration of the dopant with the sign of the helix formed by twisting the nematic phase.