Solvent-Assisted Naked Eye Sensing of Hg²⁺ by a Chemoreceptor Derived from Diazocoupling of Sulfathiazole with Diethyl Malonate

Abstract

2-{[4-(Thiazol-2-ylsulfamoyl)-phenyl]-hydrazono}-malonic acid diethyl ester (R1) and 2-{[4-(5-methyl-isoxazol-3-ylsulfamoyl)-phenyl]-hydrazono}-malonic acid diethyl ester (R2) were synthesized through diazocoupling of sulfathiazole and sulfamethoxazole, respectively, with diethyl malonate. They were characterized through various spectroscopic and mass spectral studies. R2 was also characterized through a single crystal X-ray diffraction (XRD) study. Only R1 selectively recognized Hg²⁺ from a wide range of metal ions through naked-eye change. A color change from orange to olive green was observed upon addition of 1.0 equivalent of Hg²⁺ as its chloride salt to the 1 × 10⁻³ M DMSO solution of R1. The role of DMSO in the sensing process appears to be the crucial one, because the solvent-assisted band of R1 at 482 nm observed in its UV-Vis spectrum in DMSO did not appear in its spectra recorded in nujol or in a polar aprotic solvent. The UV-Vis and ¹H NMR titrations revealed that the formation of six-membered 1:1 chelate between R1 and Hg²⁺ triggering the desolvation of R1

as the key step towards its sensing activity. The determination of binding stoichiometry between R1 and Hg²⁺ along with binding constant has also been discussed.

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GRAPHICAL ABSTRACT

Keywords:

• Binding constant
• diazo derivatives
• Hg²⁺ sensing
• naked eye sensing
• titrations

Acknowledgments

The authors are thankful to the coordinator SAP for enabling us to perform DFT calculations using Gaussian-03. S. U. is thankful to CSIR, New Delhi, for providing financial assistance in the form of a fellowship.