Ion-pair recognition of amidinium salts by partially hydrogen-bonded heteroditopic cyclo[6]aramide

Abstract

Convergent heteroditopic cyclo[6]aramide demonstrates efficient ion-pair recognition of amidinium salts in 10% methanolic chloroform (>10⁴ M⁻¹) as confirmed by NMR and conductivity experiments. As predicted by density functional theory (DFT) method simulations, cyclo[6]aramide 1a is able to bind amidinium salts G1–G5 with varying binding affinity in 1 : 1 stoichiometry through hydrogen bonding interactions involving both anion-recognizing amide H-atoms and cation-binding amide carbonyl O-atoms. Particularly, the binding affinities for G1, G2 and G3 are found to decrease with increasing the size of substituents in the amidinium ion in the order of G1 > G2 > G3. Moreover, the association ability for simultaneous binding of cationic and anionic guest species depends considerably on the counterions. Among the four formamidinium salts (Cl⁻, Br⁻, I⁻ and BPh₄⁻) examined, formamidinium chloride is best encapsulated as a contact ion-pair species in the macrocycle. The reduced association constants with increasing the size of counterions in the order of G1 > G6 > G7 > G4 underscore the importance of ion paring in effecting the host–guest interaction. Comparative conductivity studies provide a convenient approach to differentiate between contact and loose ion pairs for these amidinium complexes. This work provides a rare example of binding biologically important types of amidinium cations as ion pairs by a synthetic receptor.