Anion receptors containing -NH binding sites: hydrogen-bond formation or neat proton transfer

When the amide-contg. receptor (I) is in a soln. of DMSO (DMSO) in the presence of basic anions (CH3COO-, F-, H2PO4-), it undergoes deprotonation of the -NH fragment to give the corresponding zwitterion, which can be isolated as a cryst. solid. In the presence of less basic anions (Cl-, Br-, NO3-), I establishes true hydrogen-bond interactions of decreasing intensity. The less acidic receptor II undergoes neat proton transfer with only the more basic anions CH3COO- and F-, and establishes hydrogen-bond interactions with H2PO4-. An empirical criterion for discerning neutralization and hydrogen bonding, based on UV/Vis and 1H NMR spectra, is proposed.