**4. Application of Schiff bases and its metal complexes**

Schiff bases and their metal complexes find widespread use in various industries and applications, including the food industry, the agrochemical industry, the dye industry, analytical chemistry, catalysis, energy storage, environmental, chemosensing, bio-sensing, nanotechnology, and biomedical applications.

#### **4.1 Catalysis**

Catalytic activity is enhanced in both homogeneous and heterogeneous reactions by Schiff base metal complexes. The ligands, coordination sites, and metal ions employed in a given compound determine its activity. Many different reactions, such as polymerization, ring-opening polymerization, oxidation, epoxidation, allylic alkylation, reduction of ketones, hydrazination of acetophenones, the Michael addition reaction, the decomposition of hydrogen peroxide, the annulation reaction, the Heck reaction, the carbonylation reaction, and the Diels-Alder

*Schiff Bases and Their Metal Complexes: Synthesis, Structural Characteristics and Applications DOI: http://dx.doi.org/10.5772/intechopen.108396*

reaction, have been used to critically evaluate the catalytic activity of metal complexes. There is significant potential for Schiff base ligands to be used as metal complexes in catalysis due to their simple synthesis method and heat stability. The catalytic activity of Schiff base complexes differed greatly depending on the structure and kind of ligands used [66, 67].

#### **4.2 Dye industry**

The dyeing technique employs a wide range of Schiff bases and complexes, many of which have been synthesized, investigated, and employed as mordants [68]. As a dye, transition metal complexes such as iron (III), nickel (II), cobalt (II), and copper (II) complexes, among others, have been prepared from a variety of Schiff bases and employed to produce a variety of transition metal complexes. Aldehyde groups that include azo dyestuff are known to synthesized many azomethine linkages that contain azo dyes due to condensation with primer amines. The textile industry utilizes these dyestuffs to color a variety of materials. Outside the textile sector, the field of photochemistry places a significant emphasis on using azo dyes that include the amine group. The Schiff base on fluorene showed desirable properties including sensitivity to pH, as well as heat and color stability. For making a water-based ink, it showed promise as a functional pigment material [69].

#### **4.3 Food industry**

Various research groups have recently concentrated on producing natural novel and active materials for food packaging applications. Because of their antibacterial action, chitosan-derived Schiff base films developed may not only boost the safety of such foods and hence lengthen their shelf life, but also provide a flavor that is wellaccepted by the consumer. Schiff's base (SB) modified zirconium dioxide reinforced PLA bio-composite film serves as an alternate packing material to replace single waste synthetic manufactured materials that pollute the environment. For active packaging applications, Schiff base (SB) modified polylactic acid (PLA) film can provide improved barrier and antifungal qualities [70–72].

#### **4.4 Agrochemical industry**

Metal complexes with diverse Schiff base ligands have attracted the attention of chemists in recent years due to their agricultural applications, such as pesticidal, nematicidal, and insecticidal. Unsymmetrical Schiff bases glyoxal salicylaldehyde succinic acid dihydrazide and its Ni(II), Co(II), Zn(II), and Cu(II) complexes have been synthesized and studied; at greater concentrations, they display considerable insecticidal action [73]. H2L [2, 2′-[(1E, 2E)-ethane-1,2- diylidenedi (E) azanylylidene] dibenzenethiol] and its new Zn(II), Ni(II) metal complexes have been employed as insect repellent agents [74]. Coumarin-based Schiff base and its earth metal complexes [75] have been used to treat pests (Tribolium castaneu) and worms (Meloidogyne incognita).

#### **4.5 Analytical applications**

Schiff bases have been used as analytical probes or reagents by researchers. These are used to analyze primary amines, carbonyl compounds, and functional groups. In complexes, azomethine bonds are formed through complex formation reactions or changes in their spectroscopic properties caused by pH and solvent variations (pH of solvent polarity indicators). Schiff bases are a great carrier for the selective and efficient extraction of certain metal ions. They are well-known for their effective chelating capabilities. Schiff bases extract metal ions, essential in regulating heavy metal pollution. N, N -bis(3-methylsalicylidene)-ortho-phenylene diamine, Schiff base used in spectrophotometric detection of nickel. The approach has been used successfully to quantify trace quantities of nickel in natural food samples [76]. Schiff bases are renowned for their ability to form complexes and serve as good chelating ligands. They have been widely employed as analytical reagents due to their ligation property. Schiff bases made of salicylaldehyde are employed in gravimetric and spectrophotometric analyses. In addition, the same reagent was recently employed for the spectrophotometric detection of Ni (II) at a trace level. Cu*2*<sup>+</sup> ions have been detected using the fluorescent 4-(1-phenyl-1-methylcyclobutane3-yl)-2-(2-hydroxy-5-bromobenzylidene) aminothiazole Schiff base. This chemical sensor operates in the visible region, has a wide dynamic operating range, and may be used over a wide pH range [77].

#### **4.6 Energy storage**

There is a resurgence of interest in the search for effective, clean, and sustainable energy sources (like wind and solar) as well as cutting-edge energy conversion and storage technologies as a result of the rapid growth of the world economy, the depletion of fossil fuels, and rising environmental pollution. Energy storage technologies are more important in our lives since the sun does not shine at night and the wind does not blow all the time. Currently, there is a lot of interest in electrical energy storage technologies including batteries and electrochemical capacitors (supercapacitors). Recent research has shown that organic oligomeric Schiff bases and electroactive polymeric (linear or hyperbranched) Schiff bases perform satisfactorily as negative electrodes (anodes) in sodium-ion batteries [78]. Lithium-ion batteries have also made use of nitrogen-rich carbon nanosheets produced by the Schiff base reaction in a molten salt solution as anode materials [79]. The linear polymeric Schiff bases developed by Armand et al. [80] as a consequence of the condensation of aromatic dialdehydes with aliphatic and aromatic diamines performed well as anodes for sodium-ion batteries. Polymeric Schiff bases are also produced by combining terephthalic-aldehyde, phenylenediamine, and polyether amine blocks, resulting in polymers with high adhesive qualities that can be used as redox-active binders for sodium-ion anodes. Similarly, Zhang et al. [81] developed another ImCOF (Imine bonds containing *covalent organic frameworks)* that performed again as an anode material for lithium-ion batteries derived from 2,4,6-triaminopyrimidine and terephthalaldehyde.

#### **4.7 Environmental applications**

Most firms worldwide need copper, silver, lead cadmium, aluminum and cobalt. These metals can be present in nearly all dairy products. Their widespread prevalence in industrial processes, chronic metal contamination from occupational contact, and health risks associated with these metals necessitate their identification and control in biological and dietary samples. Metals are used in electroplating, alloy production, and battery manufacturing. As a result, excessive metal concentrations

*Schiff Bases and Their Metal Complexes: Synthesis, Structural Characteristics and Applications DOI: http://dx.doi.org/10.5772/intechopen.108396*

have been discovered in diverse water sources, soil, and plants. Products such as cigarettes, beers, oils, and supplements necessitate metals monitoring and quality control [82].

Metal corrosion has a tremendous impact on the national economy and critical safety and pollution issues. Although many inhibitors have good inhibitory properties, they are insufficient for environmental protection and sustainable development initiatives for various reasons (such as difficulty in degradation, toxicity or hightemperature resistance). Stable, efficient, and ecologically friendly inhibitors are the inhibitors of the future. Many inhibitors, including imidazolines, Mannich bases, and Schiff bases, contain heteroatoms (N, S, O) or chemical interactions with electrons (p bonds). N, O, and S heteroatoms, as well as unsaturated >C=N- bonds, can create strong and durable corrosion-inhibiting adsorption films on metal surfaces, demonstrating outstanding inhibitory effects. At the same time, Schiff base compounds are attractive to researchers due to their inexpensive cost, ease of synthesis and purification, strong water solubility, and low toxicity [83].

#### **4.8 Chemo-sensing applications**

Schiff base-based fluorescent probes have recently been invented for detecting and monitoring numerous hazardous analytes in biological systems. Schiff base compounds with nitrogen-oxygen-rich coordination as a receptor site provide a stable platform for fluorescence sensing with significant, visible color shifts. Detecting metal ions with diverse mechanisms in an accurate sample using Schiff base-based sensors is appealing currently. In the recent decade, Schiff base probes based on fluorescence live-cell imaging have been used to detect metal ions such as Co*2*<sup>+</sup> , Cu*2*<sup>+</sup> , Zn*2*<sup>+</sup> , Hg*2*<sup>+</sup> , Ag+ , Al*3*<sup>+</sup> , and ClO− ions [84–86].

#### **4.9 Bio-sensing applications**

Within cells, Schiff base compounds have been used as biosensors for H2O2, glucose, and Oncomarker CA-125 [87]. Evaluation of the sensitivity and specificity of *the gold Schiff base complex-doped sol gel nano optical sensor* for the detection of CA-125 in ovarian cancer patient samples was performed and compared to results obtained from samples taken from healthy women serving as a control group [88]. Sheta M. Sheta et al. created an ultrasensitive method of detecting human creatinine using a cerium(III)-isatin Schiff base complex as an optical sensor [89].

#### **4.10 Biomedical applications**

Schiff bases and their metal complexes have numerous applications in various biomedical pharmaceuticals such as antimicrobial, anti-malarial, anticancer, antiviral, anti-inflammatory, antioxidant, anticonvulsant, anti-anthelmintic, bioprinting, tissue regenerating, enzyme inhibition and drug delivery. In biological systems, the azomethine nitrogen of Schiff bases serves as a binding site for metal ions to attach to diverse biomolecules such as proteins and amino acids for anti-germ activity. Our bodies' Schiff bases catalyzed many metabolic events in the form of enzymes that are active against certain bacteria. Several studies have been conducted to improve the bio-functions of Schiff bases and their metal complexes. Schiff bases can fight cancer, fungus, germs, ulcers, and viruses, depending on which transition metal ions they contain [90–92].
