Anion Recognition by Partial Cone Dihomooxacalix[4]arene-Based Receptors Bearing Urea Groups: Remarkable Affinity for Benzoate Ion

Tetraureido-dihomooxacalix[4]arenes (tert-butyl 4a and phenyl 4b) were synthesised and obtained in a partial cone conformation in solution and in the solid state, as confirmed by NMR and X-ray diffraction studies. Their complexation ability towards halides, pseudo-halides and oxoanions was assessed by H-1 NMR and UV/Vis titrations. Structural and energetic insights of phenylurea 4b complexes were also obtained using molecular dynamics (MD) simulations. The binding data showed that the association constants are strongly dependent on the nature of the substituent (alkyl/aryl) at the urea unit. tert-Butyl urea 4a is a much weaker receptor than phenylurea 4b, and showed association constants that decrease with decreasing of anion basicity. Phenylurea 4b is a highly efficient anion receptor, exhibiting a remarkable binding ability towards BzO(-) ion (log K-ass = 4.81). In comparison to the phenylurea analogue bearing a butyl spacer and a cone conformation, receptor 4b containing a shorter spacer (three carbon atoms) and a partial cone conformation is more pre-organized, displaying a strong enhancement of its binding efficiency. MD simulations have shown that the anions are preferentially bound to the urea moieties of the macrocycle lower rim, in agreement with the ROESY spectrum carried out with phenylurea 4b and BzO(-) anion.