Recognition of Anions, Monoamine Neurotransmitter and Trace Amine Hydrochlorides by Ureido-Hexahomotrioxacalix[3]arene Ditopic Receptors

Hexahomotrioxacalix[3]arene-based receptors (phenylurea 4a, phenylthiourea 4b and tert-butylurea 4c) were synthesised and obtained in the partial cone conformation in solution. Phenylurea 4a was also obtained in the cone conformation and its X-ray crystal structure is reported. Their binding properties towards several anions of different geometries were studied by ¹H NMR titrations. The data showed that phenylurea cone 4a is the best anion receptor, displaying a very high affinity for the carboxylates AcO ^– and BzO ^– anions (log K _ass = 4.12 and 4.00, respectively). Phenylurea partial cone 4a and phenylthiourea 4b, although weaker, are still reasonably good receptors, showing the same trend as cone 4a. tert-Butylurea 4c exhibited very low affinity for all the anions, confirming that the association constants strongly depend on the nature of the substituent (aryl/alkyl) at the urea moiety. In addition, cone and partial cone 4a derivatives were also tested in the recognition of n-alkylammonium salts and cone 4a also as a ditopic receptor for biogenic amine hydrochlorides by ¹H NMR titrations. Cone 4a showed an outstanding affinity for the guest alkylammonium and amines, even at 298 K and in a more competitive solvent such as CDCl ₃/[D ₆]DMSO mixture. DFT calculations corroborated the NMR observations.