**Abstract**

1998 onwards, a span reporting thousands of research articles describes the ever-increasing applicability of Schiff bases and their metallic complexes; this chapter comprehensively examines the literature of the last 20 years. The structural diversity of these molecules made them available for a very wide range of biological and abiological applications. Schiff bases are excellent chelators and due to this unique property have found their place in qualitative and quantitative determination of metals in aqueous media. The structural diversity of metal chelates proved these to be outstanding catalysts and displayed interesting fluorescence effect. Finally, Schiff base moieties have found a unique position during the in vitro and in vivo experiments for drug development against a huge number of biological entities including bacteria, fungi, cancer cells, viruses, parasites, etc.

**Keywords:** synthesis, isomerization, electronic and vibrational spectra, catalyst, antitumor

#### **1. Introduction**

Even Hugo Schiff might not know while publishing his research findings as a scientific communication some 155 years ago that these organic compounds would become an independent area of immense interest not only in chemistry but also in other areas of scientific domains including physical, materials, biological, and engineering [1, 2]. Simply, Schiff bases can be defined as the products of chemical reaction of a primary amine with an aldehyde or a ketone under a given set of conditions. Structurally, oxygen of the carbonyl functionality (C═O) in an aldehyde and/or a ketone is replaced with nitrogen leading to the formation of imine or azomethine or anilino or azimethine (C═N) functionality with the liberation of a water molecule which is characteristic of Schiff bases [3]. There has been a substantial increase to study the coordination of the Schiff bases to the transition metals as well as main group metals and main group organometallics. The aforementioned metallic derivatives of Schiff bases have shown a rich diversity of structural motifs based upon coordinating ability of the ligand under investigation [4–7].

Schiff bases and their metallic derivatives have got outstanding applicability in biological systems acting as antibacterial, antifungal, antiviral, antitubercular, antitumor, insecticidal, bacteriostatic, in vitro cytotoxic, anti-inflammatory, analgesic, and antipyretic agents [8–15].

On the other hand, abiological applications of Schiff bases and their metallic derivatives are also very diverse, for example, in polymer chemistry, mechanochemical treatment often leads to the destruction of polymer under investigation, while very recently, the concept of mechanochemical polymerization has seen remarkable improvements which was initially reported with limited success [16]. Schiff base reaction is responsible for visual process in animals; the process starts with excitation of retinaldehyde leading to the formation of Schiff linkage with lysine and change in the membrane electrical potential with eventual transmission of signal to brain [17]. Schiff bases have been exploited very efficiently to monitor hazardous materials in the environment; for example, Cr3+ and organophosphates in the environmental samples were recognized, quantified, and removed with high accuracy and precision [18]. Electroanalytical techniques are very helpful for studying laboratory, clinical, and environmental samples as these are versatile and economical; Schiff bases have been known as ionophores and when fabricated with organic polymers can be transformed into membranes as ion-selective potentiometric sensors [19]. Schiff base ZnO complexes have been investigated as semiconductors by fabricating field-effect transistors to electronic performance [20]. Azo dye-based Schiff bases have been reported efficient chemosensors for detection and quantification of S2<sup>−</sup> ions [21]. Photovoltaic characteristics of pyridine Schiff bases have been reported by illumination-dependent current–voltage measurements in solar cell applications [22]. Nickel Schiff base complexes have been studied for molecular docking experiments and their interaction with β-lactamase [23]. Schiff bases have successfully employed in vitro as well as in vivo for probing real-time sensing and analysis of Al3+ in a variety of diseases in human beings [24]. Polymeric Schiff bases strongly influence the electrochemical properties of the fuel cells and subsequent modification leading to an improved maximum power density in comparison to standard materials [25]. Metal–organic coordination polymers are widely used as conducting/semiconducting materials because of their accessible band gap 1–5 eV [26]. Schiff base palladium complexes immobilized on the mesoporous materials have been used as a heterogeneous catalyst for the Heck-Mizoroki coupling reaction and exhibited excellent catalytic activities for a wide range of alkenes [27, 28]. Schiff bases derived from cyclohexanediamine exhibited unusual structures, and these chiral molecules exhibited interesting photoluminescent properties [29].
