Synthesis and anion binding properties of new dihomooxacalix[4]arene diurea and dithiourea receptors

Functionalization of the lower rim of p -tert -butyldihomooxacalix[4]arene with two (thio)ureido moieties provided new diurea (n -propyl 5a, tert -butyl 5b and phenyl 5c) and dithiourea (phenyl 5d) derivatives, all in the cone conformation, as shown by NMR studies. The X-ray crystal structure of 5c is reported. The binding ability of these neutral receptors towards a large variety of anions was assessed by 1H NMR titrations. The structures and complexation energies of some complexes were also studied using DFT methods. The data showed that, in general, the association constants decrease with decrease of anion basicity and they are strongly dependent on the nature of the substituent at the urea moiety. Phenyl-(thio)urea derivatives 5c and 5d are the best anion receptors, showing the strongest complexation for F− (log K ass=2.70 and 2.75, respectively) and also for the oxoanions AcO−, BzO− and −H24POH2PO4−. These results were corroborated by DFT calculations.