X-ray crystallographic study of 3-Oxo-2-{[4-(thiazol-2-ylsulfamoyl)-phenyl]-hydrazono}-butyric acid ethyl ester and its application in the solvent assisted naked eye sensing of Hg(II)

The 3-Oxo-2-{[4-(thiazol-2-ylsulfamoyl)-phenyl]-hydrazono}-butyric acid ethyl ester (OSPBE) was studied through single crystal structure analysis revealing some interesting supramolecular architectural patterns. The N(3)–N(4) bond length of OSPBE was found to be 1.36 Å matching well with reported N–N bond length in the literature and hence clearly proved that it is the keto form of OSPBE which is stable. Full structural optimization of OSPBE using density functional theory (DFT) at the HCTH407/6-31G∗∗ level also proved that the keto form of OSPBE is stable. The UV–Vis absorption peaks for OSPBE predicted by the time dependent DFT at B3LYP/6-311G∗∗ level matched quite well with the experimentally observed UV–Vis bands for OSPBE. The OSPBE was successfully tested as the naked eye sensor for Hg(II) as its chloride salt at the millimolar level in dimethylsulfoxide. A color change from red orange to olive green was observed on addition of 1.0 equiv. of Hg(II) to the 1.0 × 10−3 M DMSO solution of the chemosensor. The role of DMSO in the sensing process appears to be the crucial one because the intramolecular charge transfer (ICT) band of OSPBE in DMSO observed at 489 nm did not appear in the UV–Vis spectrum of OSPBE in nujol. The UV–Vis and 1H NMR titrations revealed that formation of six membered 1:1 chelate between OSPBE and Hg(II) along with reversible supramolecular association of DMSO with NH at N-2 position in OSPBE may be responsible for its Hg(II) sensing. No sensing for other d10 metal ions like Zn(II) and Cd(II) were observed with OSPBE under similar conditions. Besides DMSO, some other polar aprotic solvents like DMF and acetone having XO (where X = C) also produced similar type of color change on the addition of 1.0 equiv. of Hg(II) to their respective 1.0 × 10−3 M OSPBE solutions. Nevertheless, polar aprotic solvent like acetonitrile not having XO or non-polar aprotic solvent like chloroform no color change was observed under similar conditions.