Benzothiazole appended lower rim 1,3-di-amido-derivative of calix[4]arene: Synthesis, structure, receptor properties towards Cu2+, iodide recognition and computational modeling
Graphical abstract
1,3-Di-derivative of calix[4]arene bearing benzothiazole moiety connected through amide (L) at the lower rim has been synthesized and structurally characterized and demonstrated for its ability to recognize Cu2+.
Introduction
Copper is one of the top three biologically essential elements [1] involved in exhibiting a variety of oxidative functions including electron transfer [2], [3], [4], [5], while iodide is involved only in the thyroid function [6], [7]. Insufficiency or overload of this element results in various neurodegenerative disorders [8], [9] and the insufficiency of iodine results in goiter [10], [11]. This necessitates the development of receptors for selective recognition of Cu2+ and I−. Calix[4]arenes [12] are important class of macrocyclic compounds possessing both hydrophilic and hydrophobic regions and hence are suited for receiving cations and anions selectively. Calix[4]arene derivatives can be synthesized easily by introducing moieties having different functional groups that can bind to metal ions through nitrogen, sulfur or oxygen ligating centers [13], [14], [15], [16], [17], [18], [19]. Calix[4]arene based receptors for the selective recognition of Cu2+ as well as I− are rather limited in the literature [20], [21], [22], [23], [24], [25], [26], [27], [28]. This includes, the report of a 1,3-di-derivative of calix[4]arene bearing anthracene–isoxazolymethyl at the lower rim by Chung et al. [20], which revealed chemosensor properties towards Cu2+ by fluorescence quenching. Other literature reports for Cu2+ sensing by calix[4]arene derivatives includes quinoline [21] or 5-nitro salicylaldehyde [22] group connected to the upper rim of calix[4]arene through an imine moiety. Among the lower rim functionalized derivatives, 3-alkoxy-2-naphthoic acid [23] and coumarin [24] appended calix[4]arenes are a few examples for Cu2+ sensing. Our research group recently demonstrated the selective recognition of some biologically relevant cations, viz., Zn2+ [29], [30], Zn2+ and Ni2+ [31] and Cu2+ [32], as well as environmentally relevant heavy metal ion, viz., Hg2+ [33] in addition to an anion, viz., iodide [34] by 1,3-di-derivatives of calix[4]arene. In this paper we report the synthesis, characterization, and cation and anion binding properties of a new 1,3-di-derivative of calix[4]arene connected to a benzothiazole moiety through amide linkage (L) as a fluorescent sensor for Cu2+ and the corresponding complex as absorption sensor for I−. Fluorescence, absorption and ESI-MS techniques have been used for studying the binding properties. The studies were compared with appropriate control molecules. The coordination and structural features of the Cu2+ complex of L has been demonstrated by computational calculations at DFT level.
Section snippets
General
All the metal salts used in the titrations were as their perchlorate salts (Caution: perchlorate salts may explode under certain conditions) with a formula, M(ClO4)2·xH2O and these were procured from Sigma Aldrich Chemical Co., U.S.A. Sodium salts of anions, viz.; F−, Cl−, Br−, I−, , SCN−, AcO−, , , , , , and have been used for the titrations and were procured from local sources. All the solvents used were of analytical grade and were purified and dried by
Crystal structure of L
Slow evaporation of a solution mixture of CHCl3 and ethanol containing L resulted in good quality single crystals suitable for X-ray diffraction studies and the corresponding crystallographic data fits well with triclinic system with space group (SI 02) and the corresponding crystallographic parameters were given in Table 1. The asymmetric unit cell possesses one molecule of L, two molecules of CHCl3 and one molecule of C2H5OH. The intramolecular hydrogen bonding present at the lower rim of
Conclusions
In summary, an effective molecular receptor, L for the selective recognition of Cu2+ followed by iodide, has been synthesized by appropriate derivatization at the lower rim of calix[4]arene and L has been characterized. The pre-organized binding core present in L is good enough to hold Cu2+ even after the addition of excess amount of other M2+ ions as demonstrated through competitive titrations. Comparison of the fluorescence results of L with those of L1 clearly explains the necessity of
Acknowledgements
CPR acknowledges the financial support from DST, CSIR and DAE-BRNS. RJ thanks UGC and JPC thanks CSIR for their senior research fellowships respectively. We thank SAIF for ESI-MS and DST’s national single crystal XRD facility of IIT Bombay for crystal data.
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