Structures of urea/thiourea 1,3-disubstituted thia[4]calixarenes and corresponding monofunctional receptors and their anion recognition properties

Two thiacalix[4]arenes in 1,3-alternate conformation functionalized by two (CH2)2NH(C=X)NHC6H4-NO2-p groups (X = S,O) as well as two related monofunctional receptors MeO(CH2)2NH(C=X)NHC6H4-NO2-p were prepared and characterized by X-ray crystal structures. The thioureido and ureido derivatives have E,Z and E,E conformations respectively both in monofunctional receptors and thiacalixarenes. The thiacalixarene attached thiourea groups are well separated from each other, but respective urea groups are much closer to each other and have mutual parallel orientation making the bisurea derivative a better preorganized receptor as compared to bisthiourea. Binding of Cl−, F−, H2PO4− and AcO− anions in chloroform and DMSO was studied by spectrophotometric and NMR titrations. In chloroform both bisurea and bisthiourea thiacalix[4]arenes bind anions 3–5 times stronger than corresponding monofunctional compounds in spite of better preorganization of the urea derivative. In DMSO simultaneous deprotonation of ureido NH groups of receptors and hydrogen bonding reactions are observed. Deprotonation by H2PO4− is accompanied by a strong association between liberated H3PO4 and H2PO4− (log K = 3.9). For hydrogen bonding associations the binding constants of H2PO4− and AcO− with bisurea thiacalixarene are up to two orders of magnitude larger than those with corresponding monofunctional receptor, but with bisthiourea thiacalixarene the effect is less than two-fold. Thus in this solvent in contrast to chloroform the preorganization is an important factor.