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

## TL;DR

This paper explores how special molecules can detect specific ion pairs by using different binding sites for cations and anions.

## Contribution

The study introduces a modified heteroditopic [2]catenane and shows how its structure affects ion recognition.

## Key findings

- Cations like Cu+, Li+, and Ni2+ show strong interactions with the modified catenane structures.
- Anion recognition is weaker and depends on σ–hole donors and anion size.
- Changing the interactive environment from oxygen to sulfur significantly impacts cation recognition.

## Abstract

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,
ΔEIOtot
= −198.2, −175.1, and −653.4
kcal·mol–1, and
ΔEIStot
= −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
ΔEIOtot
= −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.