Abstract
Four model thiosemicarbazide anion chemosensors containing three N-H bonds, substituted with phenyl and/or 4-nitrophenyl units, were synthesised and studied for their anion binding abilities with hydroxide, fluoride, acetate, dihydrogen phosphate and chloride. The anion binding properties were studied in DMSO and 9 : 1 DMSO-H2O by UV-visible absorption and (1)H/(13)C/(15)N NMR spectroscopic techniques and corroborated with DFT studies. Significant changes were observed in the UV-visible absorption spectra with all anions, except for chloride, accompanied by dramatic colour changes visible to the naked eye. These changes were determined to be due to the deprotonation of the central N-H proton and not due to hydrogen bonding based on (1)H/(15)N NMR titration studies with acetate in DMSO-d6-0.5% water. Direct evidence for deprotonation was confirmed by the disappearance of the central thiourea proton and the formation of acetic acid. DFT and charge distribution calculations suggest that for all four compounds the central N-H proton is the most acidic. Hence, the anion chemosensors operate by a deprotonation mechanism of the central N-H proton rather than by hydrogen bonding as is often reported.
Highlights
Molecular chemosensors for the detection of anions have gained significant interest in the last two decades.[1,2] The topic has become an established area of research in the field of supramolecular chemistry.[3,4,5,6,7,8,9] This increased interest is owing to the significant role that anionic species play in biological systems and environmental ecosystems.[10,11] The standard paradigm has Hydrogen bonding between receptors and anions is correlated with the acidity of the receptor protons.[1]
UV-visible absorption spectroscopy revealed that upon addition of these more basic anions, a significant red shift is observed in the internal charge transfer (ICT) absorption band, which is accompanied by dramatic colour changes visible to the naked eye. 1H/15N NMR studies revealed a significant chemical shift in the 15N NMR spectrum with formation of acetic acid at 11.95 ppm in the proton NMR spectra on titration with the acetate
These results suggest that the mechanism of action of 3 and 4 in DMSO-d6–0.5% water with AcO− involves deprotonation of the thiosemicarbazide moiety, in particular at the central thiourea N–H proton, in agreement with the DFT calculations
Summary
Molecular chemosensors for the detection of anions have gained significant interest in the last two decades.[1,2] The topic has become an established area of research in the field of supramolecular chemistry.[3,4,5,6,7,8,9] This increased interest is owing to the significant role that anionic species play in biological systems and environmental ecosystems.[10,11] The standard paradigm has Hydrogen bonding between receptors and anions is correlated with the acidity of the receptor protons.[1]. 15N chemical shifts for the three thiourea N–H nitrogen atoms were determined indirectly by 2D correlation studies. 1H NMR titrations with 1.0 × 10−2 M of 3 and 4 in the presence of AcO−, added as the TBA salt, were performed in DMSO-d6–0.5% water and the anion–receptor interactions of the thiosemicarbazides protons were closely monitored.
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