Synthesis, Characterization and Applications of Transition Metal Complexes of Symmetric N2O2 Donor Schiff Bases Derived from 2- hydroxy-6-isopropyl-3-methyl benzaldehyde

Abstract

During the past few decades, the importance of the Schiff bases and their metal complexes have been considerably increased because of multipurpose applications for the diverse fields including the biochemical, medicinal, pharmaceutical, catalytic and many more. A large number of Schiff bases and their transition metal complexes have been dynamically investigated and further performed for their biological activities along with their interesting coordination chemistry. Largely Schiff bases with multiple and different coordination environment and their metal complexes with metals in different oxidation states have been reported. Schiff bases were named after the German chemist Hugo Schiff and are produced by reacting aldehydes or ketones with primary amines under different conditions and in different solvents upon elimination of water molecule. The Schiff base is synonymous with an azomethine and can be used as reactive intermediates for the synthesis of many natural products. Schiff bases of aliphatic aldehydes or ketones are relatively unstable and readily polymerisable, while those of aromatic aldehydes are more stable due to the presence of an effective conjugation. In general, aldehydes react faster than ketones in a condensation reaction, leading to the formation of Schiff bases, as the reaction centre of aldehyde is sterically less hindered than that of the ketone. Furthermore, the extra carbon of ketone donates electron density of the azomethine carbon and thus makes the ketone less electrophilic compared to aldehydes Presence of a lone pair of electrons in the sp2 hybridized orbital of nitrogen atoms of the azomethine group is of significant chemical importance and offers the good chelating ability to Schiff bases especially when combined with one or more donor atoms close to the azomethine group. During the last few decades, considerable efforts have been made for the development of metal-free macrocyclic Schiff bases, as macrocyclic Schiff bases are also considered important in the supramolecular chemistry.