**1. Introduction**

Hugo Schiff, a German chemist, initially reported Schiff base in 1864 [1]. Schiff bases are organic molecules formed via condensation reaction of carbonyl compounds and primary amines [2]. The typical structure can be expressed as R'-CR = N-R", where R, R' and R" might vary. R and R' could be alkyl, aryl and heterocyclic structures with various substituents. The carbonyl group may be a constituent of an aldehyde or a ketone (>C=O)). Because Schiff bases contain an azomethine (>C=N-)) group, they are also known as azomethine or imine. The Schiff bases derived from aldehydes and ketones are known as aldimines and ketimines, respectively. A general

Schiff base condensation reaction involving the amine and the carbonyl functional group could be represented as follows (**Figure 1**).

When synthesizing Schiff bases, nucleophilic amines are used to attack electrophilic carbonyl compounds via a nucleophilic addition process, forming a hemiaminal group, and then the hemi-aminal group is dehydrated to generate imine compounds. In the first phase of the reaction, the amine reacts with the aldehyde or ketone to generate the unstable addition product carbinolamine. Carbinolamine undergoes acid- or base-catalyzed dehydration. Considering that carbinolamine is an alcohol, it undergoes a dehydration reaction when subjected to an acid catalyst (**Figure 2**). Reversible acid or base catalysis or heating often happens during the production of a Schiff base from aldehydes or ketones. When the product is isolated or water is evaporated, or both, the formation is pushed to completion. Hydrolysis of various Schiff bases by an aqueous acid or base yields the corresponding aldehydes, ketones, and amines.

Schiff bases with aryl substituents are significantly more stable and easier to synthesize, but those with alkyl substituents are relatively unstable [3, 4]. Aliphatic aldehyde Schiff bases are highly unstable and easily polymerizable, but aromatic aldehyde Schiff bases with efficient conjugation are more stable. Because aldehydes have less stearic hindrance than ketones, thus react faster. Extra carbon in ketones makes them less electrophilic than aldehydes. Schiff bases have received a great deal of attention due to their simplicity of synthesis, availability, and electronic characteristics. There is great interest in developing a wide range of applications in organic [5], inorganic [6, 7], coordination [8–10], bioinorganic [11, 12] and environmental chemistry [13–16]. Schiff base derivatives have been utilized in medical, pharmaceutical, metal refining, metallurgy, catalysis, food, sensing, filtration, environmental, photography and diagnostic applications.

The discovery of Schiff base was a major step forward for the discipline of coordination chemistry. When combined with a variety of transition metal ions, Schiff base ligands can generate stable metal complexes with a wide range of applications. The Schiff base has remarkable chelating characteristics. The presence of hydroxyl and thio-groups in azomethine groups may result in forming a penta or hexa ring structure with metal ions. Bidentate, tridentate, tetradentate, and polydentate Schiff bases are also possible. Because of the availability of a lone pair on the nitrogen atom, the Schiff base forms complexes with many metals. This lone pair aids in creating monodentate complexes, while adding other groups, such as OH and SH, may result in the formation of bidentate chelates. The azomethine nitrogen atom's lone pair of electrons and sp2 hybridized. As a result, it has substantial biological and chemical significance. Because there are more donor atoms in heterocyclic rings containing Schiff bases, they play a larger role in coordination chemistry [17–19].

Schiff bases show biological activities like nematicidal [20], insecticidal [21], antibacterial [22], antifungal [23], antileukaemia [24], anti-inflammatory [25], anti-HIV activity [26], antimycobacterial activity [27], antioxidant [28],

**Figure 1.** *General scheme of formation of Schiff bases.*

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

**Figure 2.** *Mechanistic explanation of the formation of Schiff base.*

anticancer [29], and plant growth regulatory activity [30] among others. Besides biological applications, Schiff base and their metal complexes have enormous applications in analytical chemistry [31], dye industry [32], and corrosion inhibitors [33]. Schiff base has attracted the attention of many experimental and theoretical researchers due to their particular photo-luminescence [34] in the visible range and at room temperature and so applied in many realms such as microelectronics [35], optoelectronics [36] and biological sensors [37]. Schiff base shows excellent catalytic activity in various reactions such as polymerization reaction and reduction of thionyl chloride, reduction reaction of ketones, oxidation of organic compounds, aldol reaction, epoxidation of alkenes, hydrosilylation of ketones, Henry reaction, synthesis of bis (indolyl) methanes and Diels Alder reaction [38–40]. Synthetic chemists have used Schiff bases and related complexes for a wide variety of processes, including the oxidation of alkenes and the catalytic transformation of hydrocarbons into useful oxygenated derivatives such alcohols, aldehydes, and epoxides. A further area of intense curiosity is the catalysis of alkene oxidation by soluble transition metal complexes. Schiff bases have a wide range of donor sites, resulting in significant transition metal complexes. Schiff base metal complexes have been used in biological applications, resulting in important recent improvements in a variety of chemistry areas. Because of their unique treatment method, metal-containing antibacterial compounds appear to be promising candidates for brand-new antibiotic medications that restrict the growth of bacterial strains. The characteristics of Schiff base metal complexes vary depending on the ligands and the transition metal ion. Schiff bases have attracted a lot of interest because of their various chemical and physical properties, as well as their ease of production.
