Liquid crystalline oxovanadium(IV) and copper(II) complexes of halogen-substituted salphen ligands: role of metal and spacer substituents

Three new series, each of hemi-discoid oxovanadium(IV) and copper(II) complexes based on halogen-substituted 'salphen' type ligands; H2L = [N,N-Bis(4-n-alkoxysalicylidene)-4-fluoro/chloro/bromo-1,2-diaminobenzene] (n = 12, 14, 16) have been accessed and their mesomorphic properties investigated. The complexes have been characterised by elemental analyses, FT-IR and absorption spectroscopy. The fluoro and the chloro substituted oxovanadium(IV) complexes display thermotropic columnar rectangular mesophases with ap2mmsymmetry. On the contrary, the bulky bromo substituted oxovanadium(IV) complexes decompose at a high temperature precluding any mesomorphic study. The analogous copper(II) complexes also undergo decomposition on heating and are non-mesomorphic. Molecular arrangements in the mesophases have been proposed based on variable temperature powder X-ray diffraction (PXRD) study. The influence of metal's coordination and electronegativity and/or steric hindrance of the spacer's substituent on the mesomorphic properties of the complexes is discussed. Density functional theory (DFT) calculations carried out using the GAUSSIAN 09 program on representative oxovanadium(IV) and copper(II) complexes indicated a distorted square-pyramidal geometry around the VO(IV) centre and a distorted square planar geometry around the Cu(II) centre in their complexes.