Molecular modeling, spectral investigation and thermal studies of the new asymmetric Schiff base ligand; (E)‐N'‐(1‐(4‐((E)‐2‐hydroxybenzylideneamino)phenyl)ethylidene)morpholine‐4‐carbothiohydrazide and its metal complexes: Evaluation of their antibacterial and anti‐molluscicidal activity

Abstract

A new tetradentate asymmetric Schiff base ligand; (E)‐N'‐(1‐(4‐((E)‐2‐hydroxybenzylideneamino)phenyl)ethylidene)morpholine‐4‐carbothiohydrazide (H2L) was prepared by consecutive condensation reactions between p‐aminoacetophenone, salicylaldehyde and 4‐morpholinethiosemicarbazide. The structures of the obtained ligand along with its Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Cr(III), Fe(III) and Ru(III) complexes were authenticated by different analytical and spectral techniques. The ligand behaves as a monobasic tetradentate one and chelates with metal ions through the deprotonated phenolic oxygen, imine nitrogen, thione sulfur and azomethine nitrogen atoms (ONNS chromophore). All nonelectrolyte metal complexes possess octahedral geometry, except Ni(II) and Cu(II) complexes (2, 3) which have tetra‐coordinate structures. The thermal stability of metal complexes is higher than that of the free ligand and the general thermal degradation pathways were discussed based on thermogravimetric technique. The activation thermodynamic parameters as well as the order of reaction (n) were estimated from DTG curves by using Piloyan method. Thermogravimetric analyses and mass spectra of metal complexes confirm the number of solvents in their inner and outer spheres. Molecular modeling data were used to demonstrate the thermal decomposition pathways of the ligand and its metal complexes, confirming their chemical structures. The ESR spectra of Cu(II) complexes indicate that the unpaired electron is localized mainly in the d(x2‐y2) orbital with axial symmetry. Powder X‐ray diffraction analyses for Co(II), Cu(II) and Ru(III) complexes (1, 3, 7) show that copper complex has a crystalline nature, whereas, cobalt and ruthenium complexes are amorphous. The ligand and its metal complexes were tested in vitro for their anti‐molluscicidal activity against Eobania vermiculata. The results reveal that the investigated compounds have remarkable anti‐molluscicidal activities against the tested land snail with various sensitivity levels. The synthesized compounds were also screened in vitro for their antibacterial activity against Gram‐positive bacteria; Staphylococcus aureus and Gram‐negative bacteria; Escherichia coli and the results were compared with the activity of ceftriaxone. The data show that all investigated compounds display a high antibacterial activity against the tested bacterial strains with different inhibition degrees and the activity is significantly higher against Staphylococcus aureus than Escherichia coli.