Synthesis, structural elucidation, molecular modeling and antimicrobial studies of Mn(II), Co(II), Ni(II), and Cu(II) complexes containing NO donor bidentate Schiff base

Solid complexes (1-4) of Schiff base ((o-tolylimino)methyl)phenol (HL) with manganese(II), cobalt(II), nickel(II), and copper(II) were synthesized in methanol under reflux. Schiff base (HL) was synthesized through condensation reaction of 2-hydroxybenzaldehyde with o-toluidine. Ligand (HL) and synthesized complexes (1-4) were characterized by using different spectroscopic techniques (UV-visible, FTIR, H-1 NMR, mass spectrometry, powder X-ray diffractometer [XRD]), magnetic measurements, and thermal gravimetric analysis. Spectroscopic analysis demonstrated that ligand being bidentate, in all the complexes, coordinates with metals through the N atom of azomethine group and O atom of the hydroxyl group. Molecular ion peaks (m/z) appear in the mass spectra of complexes confirm the proposed stoichiometry. TGA data of complexes 1 and 2 exhibited water molecules coordinated with the central metal ions (Mn2+ and Co2+). Magnetic Susceptibility analysis of metal complexes proposed octahedral geometry for Mn(II) and Co(II) complexes. Conductivity analysis was in good agreement with non-electrolyte nature of metal complexes. Global reactivity data exhibited that metal(II) complexes are soft as compared to HL. Moreover, frontier molecular orbital (FMO) findings revealed that among the metal complexes, [Mn(L)(2)(H2O)(2)] 1 was having large energy gap showing greater stability and less reactivity, while [Ni(L)(2)] 3 was found most reactive among all of them. Values of chemical reactivity descriptors obtained denoted that synthesized metal complexes can be proved as efficient biological candidates owing to their reactivity patterns. In vitro antibacterial and antifungal activity of complexes and ligand were evaluated against different strains of bacteria (Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, and Bacillus subtilis) and fungi (Candida albicans, Aspergillus niger, Trichoderma harzianum, and Aspergillus flavus) via disc diffusion method. The bacterial and fungal growth more efficiently inhibited by the use of metal complexes (1-4) than ((o-tolylimino)methyl)phenol (HL).