Anion-Binding Properties of Aliphatic Symmetric Squaramide Receptors

Squaramides (SQs), which are very popular for their H-bonding ability, have attracted great interest due to their wide range of applications such as asymmetric synthesis, pharmacology, and anion transportation. In this study, aliphatic symmetric SQs based on cis/trans-1,2-diaminocyclohexane (DACH) substituted with cyclic tertiary amines, synthesized in four steps under simple reaction conditions, were investigated for the first time for their ability to bind Cl^-, Br^-, and I^- anions. The changes in cis/trans geometric isomers and the cyclic ring (pyrrolidine vs piperidine) were found to have a combined effect on the degree of anion binding. The spectroscopic titrations of the SQs with TBA-Cl, TBA-Br, and TBA-I in the range of 0.2 to 20.0 equiv were monitored by ¹H NMR, and the analyses of the magnitude of chemical shift differences in the NH peaks of the SQs in course of titration were performed by DynaFit and BindFit programs for the calculation of their K_a values. All symmetric SQs I-IV were found to selectively bind Cl^- anion more strongly than Br^- anion to varying degrees depending on the SQ derivatives. Especially, SQ IV, which has a symmetric trans-DACH and a pyrrolidine ring, was found to have the highest Cl^- anion-binding ability compared to the other SQs. However, the SQs did not show any change in the chemical shift of the NH proton in ¹H NMR upon successive addition of TBA-I, indicating that they do not interact with I^- anion. The stoichiometries of the complexation behavior of SQs I-IV toward Cl^- and Br^- anions were also analyzed by Job plots.