Solid-State Ion Exchange of Organic Ammonium Cations in Molecular Crystals

Artificial ion channels and transporters designed to recognise and transport specific ions and molecules have been extensively studied. These biomimetic single-crystal materials have gained particular attention for their potential to reveal the complex mechanisms of biological functions and to provide functionalities based on molecular design. In the present study, we found that Li⁺ ions in the ion channels of Li₂([18]crown-6)₃[Ni(dmit)₂]₂(H₂O)₄ (1) crystals exchange with R-NH₃⁺ (R=Me, Et, nPr) ions in aqueous solution. After ion exchange, the resultant (R-NH₃⁺)([18]crown-6)[Ni(dmit)₂]⁻ (1-R, R=Me, Et, nPr) crystals are missing one [18]crown-6 molecule per two cations compared to 1. During ion exchange, some of the [18]crown-6 molecules, which constitute the channels, are released into the aqueous solution along with Li⁺ ions. The ion exchange of 1 in mixed aqueous solutions of organic ammonium cations shows selectivity for MeNH₃⁺, which is attributed to the stability of the 1-Me. These results demonstrate the potential for transporting molecular cations in channel structures composed of crown ethers, which will facilitate the development of transporters for drugs and other substances essential for biological functions.