What Anions Do to N−H-Containing Receptors

TLDR

Molecules with polarized N−H fragments act as H‑bond donors to anions and are widely used as receptors for recognition and sensing in aprotic solvents. The authors present examples of pyrrole‑ and urea‑based receptors. They discuss the stability of the H‑bond complexes formed with various anions. The authors demonstrate that 1:1 complex stability is governed by receptor acidity and anion basicity, that highly basic anions—especially fluoride—can deprotonate electron‑withdrawing substituted receptors forming the stable [HF2]− self‑complex, and that excess fluoride or hydroxide can trigger sequential deprotonation of chromogenic urea receptors, producing vivid color changes.

Abstract

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.