What anions do to N-H-containing receptors

Molecules containing polarized N-H fragments behave as H-bond donors toward anions and are widely used as receptors for recognition and sensing purposes in aprotic solvents (CHCl3, MeCN, and DMSO). We present examples of receptors containing pyrrole and urea subunits, and we discuss the stability of their H-bond complexes with a variety of anions. It is demonstrated that the stability of the 1: 1 complexes is strictly related to the acidic tendencies of the receptor and to the basic properties of the anion. It may happen also that more basic anions induce the deprotonation of the receptor, if equipped with electron-withdrawing substituents. This is typically observed on interaction with fluoride, due to the formation of the very stable [HF2](-) self-complex. For urea-based receptors armed with chromogenic substituents, the addition of a large excess of the anion (F-, OH-) may induce the consecutive deprotonation of both N-H fragments, processes signaled by the development of vivid colors.