Organochalcogen (Se/Te) substituted Schiff bases: Syntheses and applications
Introduction
Metal containing organochalcogen substituted compounds have been recognised from the early 19th century, long before the report of the Schiff base ligands [1]. Schiff bases have mostly been introduced as a chelating agent with d block metals and most oftenly used in organic, inorganic, and medicinal chemistry [1]. These compounds can also be used to synthesize metal complex using different kind of metal ions which exhibit binding behaviour in bidentate, tridentate, and even multidentate fashion. Organochalcogen multidentate ligands contain atoms from chalcogen family i.e. O, S, Se, and Te in the compound which exhibit Lewis base property [2]. The compounds bearing hybrid donor atom sets such as ‘hard’ (N, O) and ‘soft’ (Se, Te) facilitate the stabilization of the central metal ion in their lower and higher oxidation states [2], [3], [4], [5]. The main advantage of the availability of hybrid donor sets containing N, P, O, Se, and Te in the Schiff bases is to provide a variety of metal complexes such as soft/hard monometallic system, soft/hard bimetallic system, and hybrid multimetallic homo and hetero nuclear molecular systems [6], [7].
In terms of atmospheric degradation of materials made up of organosulphur or organotellurium compounds, the selenium analogues provide sufficient resistance towards environmental oxygen and also towards the corrosion [8]. As in general, the intermolecular attraction of selenium is higher than sulphur and oxygen towards hydrogen and thus the synthetic applicability of selenium analogues have been given more importance than others [9], [10].
Furthermore, the coordination chemistry of selenium substituted Schiff base ligands are well explored and the chemistry of these compounds with some of the soft transition metals such as Pt [11], Pd [11], Hg [12], Ir [13], Ru, Rh etc. [11], [14] provide wide applications in the field of material or biological science [15].
Selenium and tellurium substituted Schiff bases show unique behaviour in the area of biological studies such as anti-inflammatory [16], [17], [18], analgesic [16], [17], [18], [19], antimicrobial [20], [21], anticonvulsant [22], anti-tubercular [23], anticancer [24], [25], antioxidant [26], and anthelmintic [27]. Among various metal complexes of organochalcogen substituted Schiff bases, the complexes made up of Zn metal are very fascinating [28]. These complexes when synthesized from the substitution of salicylaldimines exhibit numerous optical properties such as fluorescence possessing ultra-short lifetimes and high quantum yield. Therefore, they facilitate effective energy transfer [28]. Furthermore, these organochalcogen substituted Schiff base complexes have been reported to show prominent DNA binding capabilities which inhibit the tumor forming cells; accelerate anticancer effect [28].
Section snippets
General mechanism of Schiff base formation
The general scheme for the synthesis of Schiff bases is given in Fig. 1. Schiff bases containing aromatic ring are generally more stable because of the effective conjugation of the ring while the Schiff base containing aliphatic chain are less stable [4], [29], [30]. The synthesis of Schiff bases using aldehyde or ketone based precursors are reversible (Fig. 2) in nature and usually synthesized under acidic and basic conditions with or without heating depending upon the structure and functional
Antimicrobial activity
In a study published by Malik A. in 2018, a novel organotellurium substituted Schiff base complex (3-APY-{o-VanH}) were synthesized and used for antimicrobial study against gram positive bacteria (S. aureus & S. pyogenes), gram negative bacteria (P. aeruginosa & E. coli) and fungi (C. albicans, A. niger, and A. clavatus) [37]. The study indicates antibacterial properties against all the bacteria used in the study and antifungal properties were observed only against the A. Niger and A. Clavatus
Conclusion
In this review article we have discussed about the structure, synthesis and mechanism of Schiff base in general and synthesis of organochalcogen (Se/Te) substituted Schiff base and their metal complexes with transition metal from various literatures. Furthermore, we have also discussed about biological applications such as antimicrobial, antioxidant and anticancer activity for medical and pharmaceutical purposes.
CRediT authorship contribution statement
Afsar Ali: Investigation, Methodology, Writing – original draft, Resources. Bhaskar Banerjee: Visualization, Validation, Writing – review & editing. Vivek Srivastava: Visualization, Validation, Writing – review & editing. Vinay Kumar Verma: Supervision, Writing – review & editing, Conceptualization, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors of this manuscript are very thankful to the comments and suggestions of the reviewers.
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