Influence of spacer group substituent on mesomorphism in copper complexes of ‘salen’ type Schiff bases bearing long alkoxy arm

Novel four-coordinate copper(II) complexes of the type [CuL], L = N, N′-di-(4-hexadecyloxysalicylidene)-4-substituted(X)l,2-diamino-benzene (X = H, CH3, NO2) were synthesised. The compounds were characterised by elemental analyses, Fourier transform infrared spectroscopy, 1H, 13C nuclear magnetic resonance, ultraviolet–visible spectroscopy and fast atom bombardment mass spectrometry. The liquid crystalline properties were ascertained using polarised optical microscopy, differential scanning calorimetry and powder X-ray diffraction technique. The ligands are non-mesomorphic, but the complexes with methyl (CH3) or no substituent on the aromatic spacer showed rectangular columnar (Colr) mesophase and rectangular plastic columnar (Colrp) phase, respectively. An antiparallel dimeric association of half-disc shaped molecule forming a disc-like arrangement in the mesophase is proposed on the basis of X-ray diffraction study. The complex with nitro (NO2) substituent in the spacer linkage decomposes before melting precluding any mesomorphic study. The density functional theory calculations carried out using Gaussian 09 program at B3LYP level revealed distorted square planar geometry around the metal centre. The natural charges and electronic configuration of the atoms of the complexes and free ligand were evaluated by natural bond orbital analysis.