Square-planar nickel/copper(II) complexes of (E)-2-(((2-ethylphenyl)imino)(phenyl)methyl)phenol: Supramolecular analysis, keto-enol tautomerism, polymorphism and antibacterial activities

Abstract

A new Schiff base proligand, (E)-2-(((2-ethylphenyl)imino)(phenyl)methyl)phenol (HL), was synthesized from 2-hydroxybenzophenone and 2-ethylaniline and its coordination chemistry with nickel(II) and copper(II) acetate explored. In solution, the proligand HL undergoes ketoimine-enolimine tautomerism which settles exclusively into an enolimine form in the solid-state. Interestingly, while a single crystalline habit was obtained for the nickel complex 1 (block-shaped single crystals), the copper species 2 was found to solidify in two detectable polymorphs: block-shaped crystals (2a, monoclinic space group P21/c) and rhombic crystals (2b, triclinic space group P-1). Single-crystal X-ray diffraction studies reveal that the N,O Schiff base ligand chelates the divalent metal ions with square-planar geometry in the isostructural 1 and 2a and in a distorted square-planar geometry with a dihedral chelate-plane angle of 27.9° in 2b. The copper complex 2 exhibits significant antibacterial activity against the E. coli and medium activity against S. aureus bacteria. The physical properties of the complexes were further probed using thermal techniques, while their electronic structure was investigated by electrochemistry and electronic absorption spectroscopy.