Nitro group as a redox switch in urea-based receptors of anions

Abstract

• NO 2 group was used as an electrochemical binding/release switch in anion receptors. • Binding enhancement factor of the receptor is dependent on NO 2 position. • Reduction processes were elucidated by electrochemical/spectroelectrochemical methods. • Complexation of dihydrogen phosphate changes the receptor reduction mechanism. A nitro group was used as an electrochemical binding/release switch in urea-based receptors for anions. The oxidized and reduced forms of the receptors were thoroughly studied, evaluating the impact of structural features on the binding affinity upon redox modification. In the case of a nitro group in the p- position to the urea moiety, the association constant of the oxidized form is up to 80 times higher than the value measured for the reduced form. The possible application of such an anion binding affinity switch was tested electrochemically in reduction of the nitro group up to the hydroxylamine stage. The presence of acidic urea protons induces a three-step auto-protonation mechanism, however, the complexation of dihydrogen phosphate prevents the urea proton dissociation, which leads to a standard two-step reduction process. Taking advantage of redox-sensitive NO 2 chromophore, the whole process was also monitored using spectroelectrochemical measurements.