Tuning the recognition properties of urea based receptor towards cyanide in aqueous medium: Effect of fluorine substitution on strength and selectivity of the anion receptor.

In this report, we have successfully tuned the selectivity and sensitivity of an anion receptor L by substituting electron withdrawing natured fluorine atoms directly on to the rim of the phenyl ring. Despite the fact that, we have two electron withdrawing natured nitro substituents on the other side of receptor L; two fluorine substitutions made dramatic change in the sensing ability as well as the selectivity of the receptor L towards anions. The acetonitrile solution contains L with tetrabutyl ammonium salts of fluoride, cyanide, acetate and dihydrogen phosphate results sudden color changes from yellow to brown; almost negligible spectral/color change for azide and bifluoride, where as there is no color change observed with any other anionic guests with L. Solution state binding studies of L are carried out by UV-Vis spectrometry titration in 100% acetonitrile and it is found to be the receptor L selectively binds cyanide, phosphate and fluoride stronger than acetate; it is also found that receptor binds fluoride 100 times stronger than that of the receptor L and L has almost similar efficiency in binding towards acetate ion (AcO). The strength of the receptor L towards fluoride, cyanide, acetate and dihydrogen phosphate bindings are found to be in the order of 1.271 × 10 M > 1.245 × 10 M > 1.368 × 10 M, 1.23 × 10 M respectively. When we used aqueous environment (10% of water in acetonitrile) as testing solvent system, the receptor L selectively sense cyanide ion. Over all, strength of the receptor is increased towards anions with an increasing the number of fluorine atom onto the receptors. Here in we have also prepared a reference compound L in which the receptor molecule is substituted with only one fluoride atom. The acetonitrile solution of reference receptor L with help of naked-eye colorimetric experiment and spectrometric ammonium salts of fluoride, cyanide, acetate and bifluoride results sudden color changes from faint-yellow to brown in color. Unlike receptor L, receptor L does not recognize HPO, but instead of HPO, it recognizes bifluoride as evidenced from UV-Vis spectroscopic and naked-eye colorimetric studies.