**1. Introduction**

Schiff base ligands (SBLs) are known as "privileged ligands" [1] due to their facile synthesis by the condensation of aldehydes with imines. In 1864, Hugo Schiff first introduced the condensation of an aldehyde and an amine, which leads to the formation of a Schiff base (SB) [2]. SBLs are capable to form coordination bonds with metals *via* imine nitrogen and group, which is generally connected to the aldehyde. In recent days, Schiff bases (SBs) are prepared where efficient and well-synthesized SBLs are labeled as "privileged ligands." SBs can stabilize various metals with different oxidation states and control the behavior of metals in diverse catalytic transformations,

which are beneficial to most industries [3]. SBLs are easily synthesized and can combine with practically any metal ion to form complexes [4]. At high temperatures (>100°C) and with moisture, several Schiff base complexes (SBCs) exhibit strong catalytic activity in various organic reactions. There have been numerous studies in recent years explaining application in catalysis; therefore, this chapter highlights the catalytic activity of SBCs. Almost all SBCs exhibit strong catalytic activity [4]. SBs are often bi- or tridentate ligands that can bind to transition metals to produce incredibly stable complexes. In some cases, liquid crystals are also employed for this purpose as most liquid crystals have an azomethine group. The formation of carbon-nitrogen bonds can be accomplished through Schiff base reactions in organic synthesis [5]. Coordination compounds were once thought to be an infrequent and unique group, and later they were acknowledged as the most important compounds, which are capable to transform straightforward inorganic molecules to form organic matter. To give a good example of a coordination component, chlorophyll (the magnesium porphyrin complex found in plants) converts CO2 and H2O into carbohydrates.

The basic structural formula for SBs is RRC=N-R, where R, R', here R is labeled as hydroxyalkyl, alkyl, cyclohexyl, hydroxyaryl, etc. SBs consist of azomethine moiety (C=N) an azomethine group. In some cases, -SH or –OH functional groups also participate in complexation, when they are enough close to azomethine moiety. This is due to the lone pair of electron's being donated to the electron deficient metal ion. Overall, this facilitates the development of a metal complex with good stability. With salicylideneimine, Ettling created the copper (II) complex in 1840. Since then, there has been a significant quantity of material published on SBs and related metal complexes (MCs) [6].

### **2. Synthetic routes to Schiff base complexes**

From the past two decades, there is significant progress in the formation of SB transition MCs, with an increased focus on synthesis and structure. The characteristic of the SB and metal ion determines the synthesis, characteristics, and structure of the SBCs. Mostly, the following techniques have been employed to create MCs of SBs [7].

Aldehydes and amines can condense with one another under various reaction circumstances in the presence of various solvents. Methanol and ethanol are the typical solvents used to prepare the SBs, either at room temperature or under refluxing conditions. SBs are typically more likely to form when dehydrating chemicals such as magnesium sulfate are present. If the syntheses are conducted in organic solvents, the water generated during the reaction can be extracted easily. During the purification process, the SBs may deteriorate. It is preferable in these circumstances to crystallize the SBs and purify them. Here, in a suitable solvent, SB is permitted to interact with metal ions. The use of a binary azeotropic mixture of water and an organic solvent has been documented in some situations, and using an organic solvent is recommended to prevent the hydrolysis of the azomethine group.

#### **2.1 Direct synthesis**

Here, a suitable solvent is used to facilitate the reaction between SB and metal ions. Despite the fact that there have been instances of mixing organic solvent and water, the use of organic solvent is recommended to prevent the hydrolysis of the azomethine group [8].
