N,N′-Bis(benzamido)thioureas as anion receptors

A series of N,N′-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 H2PO4− 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 nm, 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−1 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.