**1. Introduction**

Schiff bases derivatives are novel approaches to researchers for designing heterocyclic/aryl compounds for emergence of new-fangled nature-friendly technology [1]. Schiff bases have been employed as synthons in the generation of various industrial and biological active compounds such as formazans, 4-thiazolidines, benzoxazines, and so on, namely ring closure, cycloaddition, and replacement reactions [2]. It has played an important role in the development of coordination chemistry and was included as key point in the progression of inorganic biochemistry and optical materials [3].

Basically, Schiff bases are the novel approach compounds that possess imine or azomethine (–C=N–) functional group. Hugo Schiff et al. had first reported that Schiff bases are the condensed product of primary amines and carbonyl compounds [4–7]. In coordination chemistry, Schiff bases are most important class of the widely used organic compounds and have widely application in several fields including inorganic, biological, and analytical chemistry. Schiff bases are important in the therapeutic purposes in medicinal and pharmaceutical field due to its broadly biological activity such as analgesic [8–10], anti-inflammatory [11], anticancer [12, 13], antioxidant [14], anthelmintic [15], antimicrobial [16, 17], anticonvulsant [18], antitubercular [19], and so on. The presence of N atom of azomethine that involved to form of H-bond with active residue of the protein in the cell and influences cellular mechanisms [20, 21]. Moreover, Schiff bases also play an intermediately role in synthesis of organic compounds, pigments, dyes, polymer stabilizers [7], and for corrosion inhibitors [22]. However, studies revealed that metal complexes of Schiff bases depict more biological activity than free organic compounds [23]. Abdel-Rahman et al. [24] have testified numerous transition metal complexes of Schiff bases as ligands that must be different biological activities, likewise antifungal, anticancer, antibacterial, and so on. As example, Fe (II) complexes have been designed, developed, and synthesized using different Schiff bases ligands that derived from 5- bromosalicylaldehyde and variety of alpha-amino acids such as L-arginine, L-histidine, L-phenylalanine, L-aspartic acid, and L-alanine. Although, these complexes are tested for their antimicrobial activity against various bacterial species such as Escherichia coli, Pseudomonas aeruginosa, and Bacillus cereus. Therefore, it is found that Fe (II) complexes unveiled strong antibacterial activities as compared with amino acid Schiff bases ligands. Moreover, Fe (II) complexes were well interacted with calf thymus DNA using UV-visible spectroscopy, and agarose gel electrophoresis measurement at pH = 7.2. As a result, these complexes showed constant binding with different DNA [25]. However, complexes of Schiff base ligand 2-[(2-Hydroxy-3-methoxy-benzylidene)-amino]pyridin-3-ol obtained from 2-amino-3-hydroxy-pyridine and 3- methoxysalicylaldehyde with nanosized Fe(II), Cd(II), and Zn(II) metals have been synthesized by sonication method, and all complexes examined for antimicrobial activity against various bacterial species [26]. Recent study revealed that some metal complexes of Schiff bases have greater cytotoxic activity against colon cancerous cell (HCT-116 cell line). Since the middle of the nineteenth century, metal Schiff base complexes have been recognized. Their usage as Schiff base ligands, which are typically monodentate, bidentate, tridentate, tetradentate, etc., and depend on the presence of donor atoms, has been noted in a significant amount of literature. Because they can create stable compounds with transition metals, they are widely used in coordination chemistry [27].

Researchers have focused their research on the chemistry of metal Schiff base complexes with nitrogen and other donor atoms because of their numerous applications in dyes, polymers, enzyme preparation, as well as used as catalyst in various biological systems. This is due to the stability of Schiff base metal complexes as well as biological activity [28], electrochemistry [29], and potential applications in oxidation catalysis [30]. Due to their selectivity, sensitivity, and synthetic flexibility to the central metal atom and the presence of an azomethine group, which aids in elucidating the biological transformation reaction's mechanism, Schiff bases are widely investigated [31].

Some Cu (II), Co (II), and Ni (II) complexes have been described employing Schiff base ligands generated from 2-amino-3-hydroxypyridine and 3-methoxysalicylaldehyde, and their in vitro antibacterial properties against many bacteria and fungi have been examined. These complexes were further examined for DNA binding, and

it was discovered that they could attach to DNA in an intercalative way. However, when compared with the clinically used vinblastine standard, the cytotoxicity of these metal Schiff base complexes on different cell lines such as human colon carcinoma cells (HCT-116 cell line) and breast carcinoma cells (MCF-7 cell line) demonstrated effective cytotoxicity against the growth of carcinoma cells [32]. Che et al. [33] stated that complexes of Pt (II) with Schiff base ligands N,N0-bis(salicylidene)-1,2 ethylenediamine (L1), N,N0- bis(salicylidene)-1,3-propanediamine (L2), and N,N0 bis(salicylidene)- 1,1,2,2-tetramethylethylenediamine (L3) revealed the use of vapordeposited Pt (II)-salen (11) triplet emitters as for efficient electrophosphorescent dyes in multilayer organic light-emitting diode (OLED) devices with a maximum luminous efficiency of 31 Cd A<sup>1</sup> . They discovered that the performance of OLEDs utilizing the Schiff base dopant L3 outperforms that of previously reported Pt (II) emitters.

The majority of Schiff bases are created by condensing salicylaldehyde with both aliphatic and aromatic amines. By using the condensation process of salicylaldehyde with substituted anilines and other aromatic amines, Calvin and Bailes [34] described a number of imines.

These compounds' intriguing electronic characteristics were discovered through spectroscopic research. According to reports, the existence of a lone pair of electrons in these compounds may explain why there is a stronger ligation with metal ions. In the most recent years, several such azomethines and their complexes with various transition metals have been recorded in review papers [35, 36].

Similarly, Gao and Zheung [37] synthesized Schiff base ligands by condensation of 2-hydroxyacetophenone with various chiral diamines such as 1,2-diaminocyclohexane, 1,2-diphenylethylenediamine, and 2,20-diamino- 1,10-binaphthalene, respectively, to investigate the steric, electronic, and geometric effect of a methyl (-CH3) group on an azomethine carbon in asymmetric catalytic reaction. Because of the impact of p-conjugation in such luminous complexes, More et al. [37] identified Ni (II) and Zn (II) salophen complexes as potential nonlinear optical materials. A considerable number of Schiff base metal complexes have been shown to be relatively good biological compound models [38].

Schiff bases have played an important part in the advancement of contemporary coordination chemistry, as well as in the advancement of inorganic biochemistry, catalysis, and valuable materials due to optical and magnetic characteristics [39]. In recent years, light emission or charge transport capacity technology has piqued the interest of electronic devices such as solar cells and active components for image and data treatment storage [40]. Metal complexes of Schiff bases contain distinct metals (paramagnetic) groups ascribed to magnetism [41].
