Cold and Liquid Crystal Properties of Square‐Planar Copper(II) Versus Nickel(II)‐ Iminophenolate/Naphthalen‐2‐Olate Schiff Base Complexes

Reaction of the phenolate or naphthalen-2-olate based Schiff base ligands, (E)-1-((2-ethylphenylimino)methyl)phenol (HL1) or (E)-1-((2-ethylphenylimino)methyl)naphthalen-2-ol (HL2) with nickel(II) and copper(II) acetate provides the complexes bis[(E)-1-((2-ethylphenylimino)methyl)phenolato-ĸ²N,O]Ni/Cu(II), [Ni(L1)₂] (1) and [Cu(L1)₂] (2), or bis[(E)-1-((2-ethylphenylimino)methyl)naphthalen-2-olato-ĸ²N,O]Ni/Cu(II), [Ni(L2)₂] (3) and [Cu(L2)₂] (4), respectively. Single crystal X-ray structure determinations for 1, 3 and 4 reveal N₂,O₂-metal coordination of two chelating Schiff base ligands in a square-planar geometry. Powder X-ray diffractograms confirm the phase purity of the bulk microcrystalline samples. Thermal analyses by differential scanning calorimetry (DSC) and polarized light microscopic (PLM) indicate the copper(II) complexes to exhibit cold crystal (2) and liquid crystal (4) property. Cyclic voltammograms suggest an irreversible electrochemical process with two one electron charge transfer processes in N,N-dimethylformamide. Variable temperature magnetic measurements at the solid-state prove the diamagnetic nature of the low-spin Ni²⁺ centres in 1 or 3, as expected from the square-planar coordination geometry with rather strong ligands. The complexes expose medium level of antioxidant activity in methanol. Optimized geometry and excited state property by DFT/TD-DFT correspond well to the experimental results of the electronic and molecular structure at the ground state.