N-(Acetamido)thiourea based simple neutral hydrogen-bonding receptors for anions

Abstract

N-(Acetamido)-N′-phenylthioureas (4–6) were found to be efficient anion receptors with higher anion affinity than their N-benzamido-N′-phenylthiourea counterparts (1 and 2). The N′-phenylthiourea moiety in 4–6 was shown to be the chromophore with an absorption maximum at ca. 270 nm. It was found that, in the presence of anions, the absorption at ca. 270 nm of 4–6 (except 5f) in acetonitrile (MeCN) was blue shifted and enhanced while a red-shifted shoulder appeared at ca. 295 nm, together with an isosbestic point at ca. 240 nm. The 1:1 anion binding constants of 4–6, for example at 10⁶–10⁷ M⁻¹ order of magnitude for AcO⁻ in MeCN, were found to be higher than those of 1 and 2, although the acidity of the thioureido -NH protons in 4–6 is lower than that in 1 and 2. ¹H NMR data indicates that the N–N single bond in 4–6 is twisted but less than that in 1 and 2. A conformation change at the N–N single bond of 4–6 was suggested to occur upon anion binding which leads to a planar hydrogen-bonding network in the anion binding complex in which a charge transfer takes place with the N-acyl moiety being the electron acceptor. Variations in the CD signals of a proline derivative 6 bearing a chiral center in the N-amido moiety provide direct evidence for this conformation change upon its binding with anions in MeCN. The amplified effect of substituent X at the N′-phenyl ring of 5 on the anion binding constant supports the conclusion of anion-binding switched charge transfer in the anion binding complex. ¹H NMR and absorption titrations for 5 indicated that the anion–receptor interaction was of a hydrogen-bonding nature until the N′-phenyl substituent X is as electron-withdrawing as m-CF₃ (5e). With X being the more electron-withdrawing p-NO₂ (5f), deprotonation of the thioureido -NH occurs in the presence of anion. Results reported here confirm that N-amidothioureas derived from both N-aliphatic and N-aromatic amides can in general be a family of efficient hydrogen-bonding receptors, with the aliphatic N-amido derivatives being more efficient. This provides a wider structural diversity for designing thiourea-based functional molecules such as anion receptors and organocatalysts. Preliminary experiments confirm that 6 could catalyse efficiently the reduction of nitrostyrene in CH₂Cl₂ and MeCN.