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 Kass = 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 CDCl3/[D6]DMSO mixture. DFT calculations corroborated the NMR observations.