Square planar vs. pyramidal copper(ii)-complexes with benzylal vs. naphthylal-based Schiff bases

Reaction of copper(ii) acetate with (E)-2-(((2-benzoylphenyl)imino)methyl)phenol (HL1), (E)-2-(((2-benzoyl-5-chlorophenyl)imino)methyl)phenol (HL2) and (E)-1-(((2-benzoyl-5-chlorophenyl)imino)methyl)-naphthalen-2-ol (HL3) provided bis[(E)-2-(((2-benzoylphenyl)imino)methyl)phenolato-κ²N,O]copper(ii) (1), bis[(E)-2-(((2-benzoyl-5-chlorophenyl)imino)methyl)phenolato-κ²N,O]copper(ii) (2) and bis[(E)-1-(((2-benzoyl-5-chlorophenyl)imino)methyl)naphthalen-2-olato-κ²N,O]copper(ii) (3). The molecular structure determinations revealed that the ligands existed as a usual (imine)N⋯H-O(phenol) (enolimine-form) in the solid state, which was further evidenced using ¹H NMR studies in solution (CDCl₃ and DMSO-d₆). Unlike HL1 and HL2, two symmetry-independent molecules (A and B) were present in the unit cell of the HL3 crystal. The molecular structures showed that the two N,O-chelating ligands coordinate to the copper(ii) ion through a square-planar (1), a distorted square-planar (2) and a square-pyramidal geometry (3). Each asymmetric unit of the crystal structure contained one-half of the molecule for 1, a single molecule for 2 and two symmetry-independent molecules for 3 (molecules A and B). Thermal investigations using DSC demonstrated an irreversible phase transition from a crystalline solid to an isotropic liquid (m.p.). Cyclic voltammogram results proved two quasi-reversible one-electron charge transfer process for 1 and 3 in DMF at 25 °C. Complexes 1 and 2 exhibited low and significant antibacterial activity, respectively, against E. coli and S. aureus, while 3 was completely inactive. Among the ligands, only HL2 exhibited medium activity against microorganisms. The electronic and molecular structures correlated well with the computational modeling performed using DFT/TD-DFT calculations.