title: Selective amine recognition driven by host-guest proton transfer and salt bridge formation.
abstract: 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 (1)H NMR spectroscopy and single-crystal X-ray diffraction studies. These carboxylcalix[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 noncompeting unprotonated form. The role of the solvent in the ionization/complexation process is discussed. Structural studies on the n-BuNH(2) complexes with 1a·H and 1b·H provide evidence that binding of the in situ formed n-BuNH(3)(+) 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.
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Messina, viale F. Stagno d'Alcontres 31, 98166 Messina, Italy.
authors.forename: Calogero
authors.initials: C
authors.lastname: Capici
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Gattuso
authors.forename: Anna
authors.initials: A
authors.lastname: Notti
authors.forename: Melchiorre F
authors.initials: MF
authors.lastname: Parisi
authors.forename: Sebastiano
authors.initials: S
authors.lastname: Pappalardo
authors.forename: Giovanna
authors.initials: G
authors.lastname: Brancatelli
authors.forename: Silvano
authors.initials: S
authors.lastname: Geremia
mesh_terms: D000588:Amines / Q000737:chemistry*; D047250:Calixarenes / Q000138:chemical synthesis* / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D007477:Ions / Q000737:chemistry*; D008968:Molecular Conformation; D015394:Molecular Structure; D011522:Protons; D012492:Salts / Q000737:chemistry*; D012965:Sodium Chloride / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D000588:Amines; D007477:Ions; D011522:Protons; D012492:Salts; D047250:Calixarenes; D012965:Sodium Chloride