Selective amine recognition driven by host-guest proton transfer and salt bridge formation

The stepwise synthesis of ionizable p-tert-butylcalix[5]arenes 1a∙H and 1b∙H, featuring a fixed cone cavity endowed with a carboxyl moiety at the narrow rim, is described. Single crystal X-ray analyses have shown that in the solid state 1a∙H and 1b∙H adopt a cone-out conformation with the carboxylic OH group pointing in, toward the bottom of the aromatic cavity, as a result of a three- or two-center hydrogen bonding pattern between the carboxyl group and the phenolic oxygen atom(s). The affinity of amines for calix[5]arene derivatives 1a∙H and 1b∙H was probed by 1H NMR spectroscopy and single crystal X-ray diffraction studies. These carboxyl-calix[5]arenes are shown to selectively recognize linear primary amines—over branched, secondary and tertiary amines—by a two step process involving a proton transfer from the carboxyl to the amino group to provide the corresponding alkylammonium ion, followed by binding of the latter inside the cavity of the ionized calixarene. Proton transfer occurs only with linear primary amines, that is, when the best size and shape fit between host and substrate is achieved, while the other amines remain in their non-competing unprotonated form. The role of the solvent in the ionization/complexation process is discussed. Structural studies on the n-BuNH2 complexes with 1a∙H and 1b∙H provide evidence that binding of the in situ formed n-BuNH3+ substrate to the cavity of the ionized macrocycle is ultimately secured, in the case of 1a∙H, by the formation of an unprecedented salt-bridge interaction.