Novel non-discoid chiral copper(II)-salen type [N₂O₂] donor Schiff base complexes with a cyclohexane diamine spacer: synthesis, electrochemistry, columnar mesomorphism and DFT study

Abstract

A series of new non-discoid chiral copper(ii)-salen type complexes, [Cu((4-C _n H_2n+1O)₂salen)], n = 14, 16, 18 and salen = N,N′-cyclohexane bis (salicylideneiminato), containing 4-substituted alkoxy tails in the side aromatic rings and a cyclohexane spacer, have been prepared and their mesogenic properties investigated. The compounds were characterised by Fourier transform infra-red spectroscopy, ¹H and ¹³C nuclear magnetic resonance spectroscopy, UV-vis spectroscopy and fast atom bombardment mass spectrometry. Mesomorphic properties of these compounds were studied by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction (XRD). The ligands are non-mesogenic but the complexes exhibited enantiotropic rectangular columnar mesophases (Col_r) with extended temperature range (111–220°C). Molecular models for the organisation in the mesophases are proposed on the basis of the small-angle XRD diffraction patterns. Molecules with their planes tilted are stacked in columns which self-organise in a 2D-rectangular lattice with C2mm symmetry. Cyclic voltammetry carried out in CH₂Cl₂ solution revealed a single electron quasireversible response for the Cu(i)/Cu(ii) redox couple (E _1/2 = –0.226V, ΔE _p = 453 mV). The more negative redox potentials indicated a planar coordination environment. Density functional theory calculations performed using the DMol3 program at the BLYP/DNP level revealed a near-square-planar arrangement around the metal centre with the cyclohexane ring tilted relative to the plane of the [N₂O₂] core of the Schiff base ligand.

Keywords:

• copper
• Schiff base
• columnar mesophase
• XRD
• DFT

Acknowledgements

The authors thank SAIF, NEHU and CDRI, Lucknow for analytical and spectral data. GD acknowledges financial support from DST and UGC, Government of India. Dr R.C. Deka, Tezpur University, India, is acknowledged for computational facility.