Anion Receptors Containing ‐NH Binding Sites: Hydrogen‐Bond Formation or Neat Proton Transfer?

When the amide‐containing receptor 1 + is in a solution of dimethyl sulfoxide (DMSO) in the presence of basic anions (CH 3 COO − , F − , H 2 PO 4 − ), it undergoes deprotonation of the ‐NH fragment to give the corresponding zwitterion, which can be isolated as a crystalline solid. In the presence of less basic anions (Cl − , Br − , NO 3 − ), 1 + establishes true hydrogen‐bond interactions of decreasing intensity. The less acidic receptor 2 + undergoes neat proton transfer with only the more basic anions CH 3 COO − and F − , and establishes hydrogen‐bond interactions with H 2 PO 4 − . An empirical criterion for discerning neutralisation and hydrogen bonding, based on UV/Vis and 1 H NMR spectra, is proposed.