Development of N-Benzamidothioureas as a New Generation of Thiourea-Based Receptors for Anion Recognition and Sensing

• Published in: Journal of organic chemistry Vol. 69; no. 19; pp. 6449 - 6454
• Main Authors: Nie, Li, Li, Zhao, Han, Jie, Zhang, Xuan, Yang, Rui, Liu, Wen-Xia, Wu, Fang-Ying, Xie, Jian-Wei, Zhao, Yu-Fen, Jiang, Yun-Bao
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 17.09.2004; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Organic; Exact sciences and technology; Noncondensed benzenic compounds; Organic chemistry; Physical Sciences; Preparations and properties; Science & Technology; SWITCHES; DESIGN; DUAL FLUORESCENCE; CHEMOSENSORS; PHOTOINDUCED ELECTRON-TRANSFER; CHROMOIONOPHORE; SITES; BINDING-PROPERTIES; FLUORESCENT SENSORS; INTRAMOLECULAR CHARGE-TRANSFER; Solvent effect; Substituent effect; Ground state; NMR spectrometry; Thioureas; Protic solvent; Intramolecular charge transfer; Hydrogen 1; Anions; Binding capacity; Measurement sensor; Dihydrogenphosphates; Association constant; Acetate; Fluorides; Experimental study; Complexes; Spectral line shift; Hammett constant; Absorption spectrometry; Acetonitrile; Molecular recognition; Benzamide derivatives; Electron acceptor; Electron donor; Hydrogen bond
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo049088f

A series of neutral N-(substituted-benzamido)-N ‘-phenylthioureas (substituent = p-OC2H5, p-CH3, m-CH3, H, p-Cl, p-Br, m-Cl, and p-NO2) were designed as anion receptors, in which the thiourea binding site was attached to the benzamido moiety via an N−N bond. The absorption spectra of these N-benzamidothioureas in acetonitrile peaked at ca. 270 nm were found to show unprecedented red shifts by 7 373 to 14 325 cm-1 in the presence of anions such as AcO-, F-, and H2PO4 -. Under the same conditions, the classic neutral thiourea receptors, N-(substituted-phenyl)-N‘-phenylthioureas, showed absorption spectral shifts in most cases of less than 800 cm-1 with one exception of 6501 cm-1. Control experiments, effects of protic solvent, and 1H NMR titration confirmed the formation of hydrogen-bonding complexes between the new N-benzamidothiourea receptors and anions. The binding constants with AcO-, for example, are at 105−107 mol-1 L order of magnitude, which are 13 to 590 times those of the corresponding classic N-phenylthioureas in the same solvent. It was found that, whereas the absorption of the N-benzamidothiourea receptors showed essentially no dependence on the substituent, the substantially red-shifted new absorption band of the N-benzamidothiourea−anion binding complex was sensitively subject to the substituent. A linear relationship was found between the absorption energies of the N-benzamidothiourea−acetate binding complexes and the Hammett constants of the substituents with a negative slope of −0.34 eV. This led to the assignment that the substantially red-shifted absorption band was the ground-state intramolecular charge-transfer absorption with the substituent locating in the electron acceptor moiety. It was concluded that anion binding to the thiourea moiety of the N-benzamidothiourea receptors switched on their ground-state charge transfer. An anion-binding induced structural change was suggested to occur around the N−N bond in N-benzamidothioureas, which resulted in a substantially increased electron donating ability of the electron donor in the receptor molecules. As a consequence, the ground-state charge transfer takes place in the N-benzamidothiourea−anion binding complexes, leading to unprecedented red shifts in the absorption spectra and substantially enhanced anion binding affinities than those of the corresponding N-phenylthiourea receptors. N-Benzamido-N ‘-phenylthioureas represent a new generation of neutral thiourea-based anion receptors that show substantially improved anion binding performance important for anion sensing and recognition.