**1. Introduction**

A Schiff base is a nitrogen analog of a carbonyl compound (aldehyde or ketone) in which the C=O group is replaced by C=N–R group. Generally, Schiff bases are considered as imines bearing a hydrocarbyl group on the nitrogen atom R2C=NR′ (R′ ≠ H). Schiff bases are usually synthesized by the condensation of a carbonyl compound with a primary amine as shown in the following **Figure 1**.

Schiff bases had various applications in different fields, such as medical [1], pharmaceutical [2] and biological [3]. Due to the presence of electron donating groups, such as sulfur and nitrogen atoms they can bind with metal ions in enzymes. Among various Schiff bases, thiosemicarbazones occupy a major role by having electron donating nitrogen and sulfur atoms. Thiosemicarbazones are a group of compounds obtained by condensing thiosemicarbazide with carbonyl compounds in the presence of a few drops of glacial acetic acid. These reagents function as good chelating agents and form complexes with several metal ions, by bonding through thionate sulfur atom and hydrazino nitrogen atom. In the last few years, much interest has been directed towards the use of chelating ligands containing sulfur and nitrogen in analytical studies as well as in structural studies of metal complexes. The wide applications and rapid growth in the popularity of sulfur ligands is due to their remarkable property as potential donors to form stable as well as characterized complexes in which the back

**Figure 1.** *Formation of Schiff base.*

bonding from the metal ion is possible under favorable conditions. In addition, the presence of nitrogen along with sulfur tends to lower the solubility of the complexes, making the isolation of these complexes easier.

Thiosemicarbazones are having great biological activities due to their ability to coordinate to the metal centers in enzymes. A number of studies reveals the biological and pharmacological activities of thiosemicarbazones and their metal complexes, such as anti-bacterial, anti-viral, anti-malarial and antineoplastic [4]. Anticancer activities of thiosemicarbazones were reported by various authors over worldwide [5]. Because of having various applications, in recent years a large number of authors reported the synthesis and characterization studies of different thiosemicarbazone ligands. The importance of thiosemicarbazones both in analytical and biological fields owe to us to synthesize new thiosemicarbazones.

Based on the starting compounds thiosemicarbazones can be classified into four types as shown below:

a.Aldehydes or substituted aldehydes with thiosemicarbazide.

b.Aldehydes or substituted aldehydes with substituted thiosemicarbazide.

c.Ketones or substituted ketones with thiosemicarbazide, and

d.Ketones or substituted ketones with substituted thiosemicarbazide.

Depending on the type of parent aldehyde or ketone used for condensation they can act as unidentate, bidentate or multidentate chelating agents during complexation with metal ions. Thio- and/or phenylthiosemicarbazones are synthesized by the condensation of carbonyl compound (with or without substitutes) with thio (phenylthio-) semicarbazides.
