Selective colorimetric sensing of CN by dihydropyrazol-3-ol derivative in CH 3 CN/H 2 O medium.

The synthesis, characterisation and anion binding studies of 2-(2,4-dinitrophenyl)-3-ethoxy-5-methyl-3,4-dihydro-2H-pyrazol-3-ol (1) have been described. 1, bearing electron withdrawing 2,4-dinitophenyl group in its structure, recognises CN−  selectively in CH3CN/H2O (4:1, v/v) medium with a binding constant of 440 M− 1 and visible colour change from yellow to red, whereas no colour change is observed in the presence of other anions such as F− , Cl− , Br− , I− , , AcO− , PhCOO− . Interestingly, in CH3CN, 1 is responsive not only to CN−  but also to F− , , AcO−  and PhCOO−  with a binding-induced visible colour change from yellow to reddish-brown. The strongest binding ability of 1 is observed with CN−  in CH3CN, the binding constant being 1.34 × 105 M− 1, whereas the binding constants of F−  and oxyanions ( , AcO− , PhCOO− ) are found to be lower by one order or more in magnitude. 1H NMR titrations of 1 are carried out with CN−  and F− , which not only provide the evidence for the hydrogen-bonding interaction between the alcohol –OH of 1 and anions, but also offer some invaluable information about the structure of the sensor–anion complexes. [ABSTRACT FROM PUBLISHER]