Associate Professor

Department of Chemistry

S.N.K.P. Govt. College

 Neem kathan, Rajasthan, India 

Abstract

Azomethines (Schiff bases) are protean substances which are containing ‘azomethine as a functional group. They are synthesized from the condensation of ketone and aldehyde with 1⁰amine. Due to their powerful coordinating abilities, Schiff base ligands serve as a scaffold for the synthesis of different metal complexes, and they form incredibly stable complexes with the majority of transition metals.The flexibility, ease of coordinating ability and structural resemblance biological enzymes made them perfect candidates for chemists all over the world to study the importance of Schiff base ligands^. Due to their wide range of uses and structural flexibility, they have become essential compounds that are frequently utilised in homogeneous and heterogeneous catalysis, chemical sensing, and nonlinear optics (NLO). Schiff’s base ligands may contain a variety of substituents with different electron-donating or electron-withdrawing groups, and therefore may have interesting chemical properties. It has been often used as chelating agents (ligands) in the field of coordination chemistry and Schiff’s base metal complexes were of great interest for many years. It is well known that O and N atoms play a key role at the active sites of numerous metallo biomolecules in the coordination with metals. Schiff bases and their complexes show interesting biological properties, e.g. antibacterial, antiviral, antitumor, anticancer, antiflammatory, allergic inhibitors reducing, analgesicand antioxidativeactivities.

Key wordsSchiff bases,Biological enzymes,Chelating agents,Antibacterial, Antiviral

Formation of Schiff bases:-

Schiff bases represent one of the most widely used families of organic compounds and their chemistry is essential material in many organic chemistry textbooks. These compounds which contain the azomethine (imine) group (–RC=N–) are usually prepared by the condensation of a primary amine with an active carbonyl compound.and were first reported by a German chemist, Hugo Schiff in 1864. Frequently they are known as anils, imines or azomethines. It's vital to emphasize that Schiff bases are substances with the formula RR₁C=NR₂ where R and R₁ are aryl, alkyl, cycloalkyl, or heterocyclic groups that may be substituted in numerous ways and R₂ is either an alkyl or aryl group but not hydrogen.

The completion phase of the formation is frequently accelerated by the removing of the product, a loss of H₂O, or both. Aqueous acid or base can hydrolyze several Schiff bases to convert them to their aldehydes, ketones, and amines.

The mechanism of Schiff base synthesis is shown in above reaction which demonstrates that the subsequent generation of azomethine is highly anticipated in an acidic media. To create an unstable addition product known as carbinolamine, the amine consolidated with the carbonyl compounds aldehyde or ketone in the initial step of the reaction. Either an acid or a base can catalyse the removal of water molecules by carbinolamine; it undergoes acid-catalyzed dehydration because it is an alcohol. Since the rate of the reaction is determined by the dehydration of the carbinolamine, acids often catalyze the reaction². However, as amines are basic compounds, a high acid concentration is unacceptable. If the amine protonates and becomes less nucleophilic, the equilibrium shifts backward direction, which inhibits the synthesis of carbinolamine. Thus, it is advisable to do a number of Schiff base syntheses at a slightly acidic pH.

Applications of Schiff bases

1. In coordination chemistry (In synthesis of metal complexes)

During the past two decades, metal complexes with Schiff bases as ligands have been amongst the most widely studied coordination compounds. Schiff base complexes have been studied extensively due to various reasons like manifestation of novel structural features, up-normal magnetic properties and relevance to biological processes. Schiff compounds also include hydrazones, substituted hydrazones, semicarbazones, thiosemicarbazones, etc. formed by the condensation of corresponding ammonia derivatives with a functioning carbonyl group. Due to the availability of a variety of amines and carbonyl compounds, Schiff bases are known to have simple and adaptable synthesis methods, various structural characteristics, and applications as physiologically active and modifiable molecules.

Donor atoms' electronegative nature, steric effect, and ability to act as bonding sites all have an impact on the ligand's tendency to form bonds. The azomethine group's nitrogen atom has a lone pair of electrons, which makes the Schiff base more active due to the electron-donor properties of the (>C=N-) double bond and the N atom suitable donor property (>C=N-). When the resulting ring has five or six members, the coordination complex resulting from this process is extremely stable. As a result, the stability is improved by the presence of an azomethine group that has a replaceable hydrogen atom close to >C=N. Aryl groups should be linked to the carbon or nitrogen of the >C=N double bond to prevent rapid decomposition or polymerization. The chiral Schiff base ligands and their metal complexes can induce high stereo-selectivity in organic transformations, it is more important in modern coordination chemistry. Hence there is a continuous interest shown in the studies of Schiff base metal complexes. There are several reports available for the Schiff base metal complexes and their applications, vanadium complexes¹, chromium complexes², manganese complexes^3-5, cobalt (Co+2/Co+3) complexes⁶, iron(Fe+2/Fe+3) complexes⁷, nickel complexes⁸, copper complexes⁹, zinc complexes¹⁰.

2. In catalytic activities

The Schiff compounds combine with d and f-block metals to produce metal complexes, and these complexes are known to function as extremely effective catalysts in a variety of syntheses and other beneficial reactions¹¹. The catalytic activities of chiral Schiff base and metal complexes in several asymmetric synthetic name reactions were reviewed by Barman¹² et al. They described the synthesis of several name reactions as well as their catalytic processes, including their enantio-, diastereo-, regio-, and stereoselectivities of numerous complexes to organic transformation.

In literature reports¹³, the emergence of Schiff base coordinated compounds as catalyst properties in various kinds of reactions is regularly highlighted. Salen (chiral), salophen, and salcyclo type metal complexes are used as catalysts in a variety of reactions, including epoxidation, oxidation, hydrogenation, and hydrogenation of imines, ketone, unsaturated hydrocarbon, ring opening of epoxide, and the interruption of H₂O₂.

3. In microbial activity:-

A number of Schiff base metal complexes were also reported to be a great utility in pharmacological and biological aspects. They demonstrate also powerful physiological functions such as antibacterial¹⁴, antifungal¹⁵, anti-inflammatory¹⁶, antiviral, antioxidant, antiproliferative and anticancer etc.  . Design of strategies for novel antimicrobial cells for the exploration of antimicrobials which fight against the infectious  microbes that exploits the beneficial and commensal microbes in sites where normal micro biota reside. Recently, the Schiff base complexes have acquired special attention in medicinal and pharmaceutical field since they show excellent biological activities. Some of the Schiff base complexes are used in biomedical analysis as MRI contrast agents. Because of special, photo physical and biological properties, complexes can be used as biological probes in the areas of clinical chemistry and molecular biology and antileiotis.

A literature review also confirmed that transition metal complexes exhibit stronger biological reactivity than the parent ligands. Chelate interactions between Schiff bases and ligand complexes reduce the polarity of metal ions, allowing them to cross the lipid barrier of cell membranes and interfere with normal cell function. Lipophilicity, which controls the rate at which molecules enter cells, is similarly affected by coordination.

 A number of articles have appeared on the co-ordination chemistry of polymeric Schiff base complexeswhich exhibit a variety of industrial and biological applications because of several useful properties like paramagnetism, semiconductivity and resistance to high energies. Hence polymeric Schiff base ligands and their metal complexes have been applied as polymer catalysts, metallomesogens, supermolecular metal complexes . While most of polymeric Schiff base ligands are used as drugs due to their excellent antimicrobial activity against various strains of bacteria, fungi and yeast. Along with this they are also found to be active as antiangiogenic and DNA-photoclevage agents. It seemed advantageous to prepare a polymeric-bound chelating ligand, which would be able to form complexes with variety of transition metals and hence increases its applications to many folds.

References

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