N,N '-bis(benzamido)thioureas as anion receptors

A series of N,M-bis(substituted-benzamido)thioureas (3a-e, substituent =p-OEt, p-Me, H, p-Cl, and m-Cl) was designed as anion receptors based on their double hydrogen bonding interaction with anions. NMR and X-ray crystal structural analyses revealed that in these receptor molecules the benzamido moiety serving as a spectral signal reporter and the thiourea moiety as anion binding site were decoupled by the twisted N-N single bond and the two thioureido and amido NH protons experienced differed electronic environment. Yet the absorption spectra of 3a-e in acetonitrile in the presence of anions such as AcO-, F-, and (HPO4_-)-P-2 underwent substantial changes by the appearance of a new band at ca. 325 nm, which is red-shifted by ca. 60 nm, and of a clear isosbestic point at 270 mu, suggesting that anion binding led to the communication between thiourea binding site and benzamido signal reporter via probably a conformational change around the N-N bond. The new band energies were found correlating linearly with the Hammett constant of the substituent with a slope of -0.361 eV, indicative of the charge-transfer character of the absorption band. The binding constants for AcO- and F- of 3a-e were obtained at not less than 107 M-I orders of magnitude, which are much higher than those of the corresponding N,N'-bisarylthioureas. We suggest that anion binding to the thiourea moiety in 3a-e switches on charge transfer in the anion-receptor binding complex, which reinforces anion binding and therefore results in a dramatically enhanced binding affinity of the receptors. The symmetric N,N'-bis(benzamido)thioureas were therefore found better in anion binding and sensing than N-benzamidothioureas.