Co(II), Ni(II), Zn(II) and Cu(II) complexes of (3E)-3-[(2-{(E)-[1-(2,4-dihydroxyphenyl)ethylidene]amino}ethyl)imino]-1-phenylbutan-1-one (DEPH₂) derived from ethylenediamine, 2′,4′-dihydroxyacetophenone and 1-phenylbutane-1,3-dione have been synthesized and characterized by elemental analysis, FTIR, UV-Visible spectroscopy, and screened to establish their potential as antibacterial agents, antioxidants and DPPH radical scavengers. The FTIR spectra showed that the ligand behaves as a dibasic tetradentate ligand with the dioxygen-dinitrogen donor atom system oriented towards the central metal ion. The analytical and spectroscopic data suggest a square planar geometry for Cu(II) and Ni(II) complexes and an octahedral geometry for the Co(II) complex. The ligand and their metal complexes were screened for antibacterial activity against Gram (+) and Gram (−) bacteria by the agar well diffusion method. In addition, the antioxidant activities of the complexes were also investigated through their scavenging effect on DPPH and ABTS radicals. The obtained IC₅₀ value of the DPPH activity for the copper complex (2.08 ± 0.47 µM) and that of the ABTS activity for the copper complex (IC₅₀ = 2.11 + 1.69 µM) were higher than the values obtained for the other compounds.

The huge research on Schiff base coordination complexes in the past few decades has given rise to several new molecules that have been of biological importance. The ease with which the Schiff base ligands are designed and prepared and their pattern is elucidated have made them to be referred to as “fortunate ligands” possessing azomethine derivatives, the C=N linkage that is essential for biological activity, including antibacterial, antifungal, antioxidant, anticancer, and diuretic activities. A variety of Schiff base and its complexes have been studied as model molecules for biological oxygen carrier systems. The uses of Schiff bases as DNA-cleaving agents and its mode of interaction and free-radical scavenging properties are described. The review encapsulates the applications of Schiff bases and their complexes.

Co(II), Ni(II), Cu(II), and Zn(II) complexes of (4E)-4-[(2-{(E)-[1-(2,4-dihydroxyphenyl)ethylidene]amino}ethyl)imino]pentan-2-one have been synthesized and characterized by elemental analyses, molar conductance, electronic and IR spectral studies, and XRD. FTIR confirmed the ligand coordinates the metal ion to form mononuclear complex via the oxygen and nitrogen atoms of the phenolic group and azomethine group, respectively. Tetrahedral geometry is proposed for Co(II) complex and square-planar geometry for Ni(II) and Cu(II) complexes. The antibacterial studies of the compounds were determined and they show that the metal complexes are more active than the free ligands. The antioxidant activity by DPPH and ABTS method was examined and it shows Cu(II); IC₅₀= 2.31 ± 1.54 µM for DPPH and Co(II); IC₅₀= 1.83 ± 1.08 µM for ABTS were the most active.

Ruthenium(III) complexes of three tetradentate Schiff bases with N₂O₂ donors formulated as [RuCl(LL¹)(H₂O)], [RuCl(LL²)(H₂O)] and [RuCl(LL³)(H₂O)] were synthesized and characterized by elemental analyses, molar conductance, FTIR, and electronic spectral measurements. The FTIR data showed that the tetradentate Schiff base ligands coordinate to Ru ions through the azomethine nitrogen and enolic oxygen. The antioxidant activities of the complexes were investigated through scavenging activity on 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2′-azinobis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radicals. The DPPH activity for [RuCl(LL²)(H₂O)] with IC₅₀ = 0.031 mg mL⁻¹ was higher than the values obtained for the other Ru(III) compounds. The study revealed that the synthesized Ru(III) complexes of the tetradentate Schiff base exhibited strong scavenging activities against DPPH and moderate against ABTS radicals. In addition, the antiproliferative studies of the complexes were also tested against human renal cancer cells (TK10), human melanoma cancer cells (UACC62), and human breast cancer cells (MCF7) using the SRB assay. The results indicated that the Ru(III) complexes showed low anticancer activities against the tested human cancer cell lines.

Mononuclear Ru(III) complexes of the type [Ru(LL)Cl₂(H₂O)] (LL = monobasic tridentate Schiff base anion: (1Z)-N′-(2-{(E)-[1-(2,4-dihydroxyphenyl)ethylidene]amino}ethyl)-N-phenylethanimidamide [DAE], 4-[(1E)-N-{2-[(Z)-(4-hydroxy-3-methoxybenzylidene)amino]ethyl}ethanimidoyl]benzene-1,3-diol [HME], 4-[(1E)-N-{2-[(Z)-(3,4-dimethoxybenzylidene)amino]ethyl}ethanimidoyl]benzene-1,3-diol [MBE], andN-(2-{(E)-[1-(2,4-dihydroxyphenyl)ethylidene]amino}ethyl)benzenecarboximidoyl chloride [DEE]) were synthesized and characterized using the microanalytical, conductivity measurements, electronic spectra, and FTIR spectroscopy. IR spectral studies confirmed that the ligands act as tridentate chelate coordinating the metal ion through the azomethine nitrogen and phenolic oxygen atom. An octahedral geometry has been proposed for all Ru(III)-Schiff base complexes.In vitroanticancer studies of the synthesized complexes against renal cancer cells (TK-10), melanoma cancer cells (UACC-62), and breast cancer cells (MCF-7) was investigated using the Sulforhodamine B assay. [Ru(DAE)Cl₂(H₂O)] showed the highest activity with IC₅₀valves of3.57±1.09,6.44±0.38, and9.06±1.18 μM against MCF-7, UACC-62, and TK-10, respectively, order of activity being TK-10 < UACC-62 < MCF-7. The antioxidant activity by DPPH and ABTS inhibition assay was also examined. Scavenging ability of the complexes on DPPH radical can be ranked in the following order: [Ru(DEE)Cl₂(H₂O)] > [Ru(HME)Cl₂(H₂O)] > [Ru(DAE)Cl₂(H₂O)] > [Ru(MBE)Cl₂(H₂O)].

The current study reports the synthesis of copper complexes of a tridentate Schiff base ligand. The compounds of the type [Cu(L)X]∙n(H₂O) (where L = tridentate ONN Schiff base ligand, X = Cl⁻, Br⁻, SCN⁻, NO₃⁻, CH₃COO⁻), were characterized on the basis of elemental analyses, FT-IR, UV-vis, molar conductance, ¹H-NMR, XRD and thermal analyses. The spectra revealed that the Schiff base ligand acts as a tridentate ligand through two azomethine nitrogen atoms and a phenolic oxygen atom. The molar conductance measurements of the complexes in DMF correspond to non-electrolytic nature. TGA and DTA studies results gave insight into the dehydration, thermal stability, and thermal decomposition. Square-planar geometry has been assigned to the prepared complexes as indicated by the electronic spectral measurements. Cu(II) compounds showed antiradical potential against DPPH and ABTS radicals. The antimicrobial potential of the Schiff base ligand and its Cu(II) complexes were evaluated by the rapid p-iodonitrotetrazolium chloride (INT) colorimetric assay against some selected bacteria strains: Staphylococcus aureus and Enterococcus faecalis (Gram +ve); Klebsiella pneumoniae and Pseudomonas aeruginosa (Gram −ve), and fungi (Candida albicans and Cryptococcus neoformans). The compounds showed a broad spectrum of antibacterial and antifungal activities, with MIC values ranging from 48.83 to 3125 μg/mL.