Shedding Light on the Capabilities of Heteroditopic Mechanically Interlocked Molecules in Ion-Pair Sensing

Heterotopic mechanically interlocked molecules contain different binding sites within their structure, allowing them to recognize specific ion pairs (cations and anions) with a high affinity. The employment of heteroditopic receptors offers advantages over monotopic analogues, in general, being composed of both cation and anion binding sites. The present study elucidates the electronic structure-based recognition of anions and cations of a heteroditopic [2]catenane, IO. Spherical cations and anions have been employed. The structure of IO was modified by replacing its original oxygen atoms of the crown-ether moiety by sulfur atoms and σ-hole donor iodines by -Te-CH₃ groups leading to the modified [2]catenanes IS and TeO, respectively. Energy decomposition analysis (EDA) and natural orbital for chemical valence reveals that the cations exhibit the strongest interaction with the binding pockets of all structures, with Cu⁺, Li⁺, and Ni²⁺ presenting the most stabilizing values, $\Delta E_{\mathrm{IO}}^{tot}$ = -198.2, -175.1, and -653.4 kcal·mol^-1, and $\Delta E_{\mathrm{IS}}^{tot}$= -226.4, -154.0, -702.5 kcal·mol^-1, respectively. In contrast, anion recognition presented to be significantly lower, being purely dependent on the strength of the σ-hole donors and the size of the applied anion, with Cl^- exhibiting the most stable interaction, where $\Delta E_{\mathrm{IO}}^{tot}$ = -109.9 kcal·mol^-1. It was also found that the anion recognition for this particular molecule does not affect the cation recognition, significantly. The EDA results confirm that changing from a harder (O) to a softer (S) interactive environment will have considerable impact on cation recognition, thereby demonstrating the pivotal role, following the size match rule.