Anion Binding of N‐(o‐Methoxybenzamido)thioureas: Contribution of the Intramolecular Hydrogen Bond in the N‐Benzamide Moiety

Abstract

N-(o-methoxybenzamido)thioureas (2 X/2 Y) are found to show an enhanced anion binding affinity with binding constants over 10(7) mol(-1) L orders of magnitude for AcO(-) and a redshifted absorption of the anion binding complexes in acetonitrile (MeCN) relative to those of N-benzamidothioureas (1) that bear no o-OMe in the N-benzamide moiety, despite the electron-donating character of o-OMe. Absorption of the anion-2 X/2 Y complex was shown to be of the same charge-transfer nature as that of the anion-1 complex, but its dependence on substituent X is interestingly influenced by the o-MeOHNC=O six-membered-ring intramolecular hydrogen bond identified in 2 X/2 Y. Such an intramolecular hydrogen bond is suggested to be responsible for the enhanced anion binding affinity. In the presence of this intramolecular hydrogen bond, the anion binding constant of 2 X was found to be independent of substituent X at the N-phenyl ring, as in the case of 1, whereas that of 2 Y showed an amplified dependence on substituent Y at the N'-phenyl ring, but to a lower extent than that of 1. A similar ring intramolecular hydrogen bond was purported to exist in 2 Za, 2 Zd, and 2 Ze, which bear NHMe, F, and Cl as the ortho substituent in the N-benzamide moiety. In terms of the current roles of thiourea in not only anion recognition and sensing but also organocatalysis and crystal engineering, the present finding would be of significance for a wider structural diversity of smart thiourea derivatives with predesigned functions.