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, <b>IO</b>. Spherical cations and anions have been employed. The structure of <b>IO</b> 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 <b>IS</b> and <b>TeO</b>, 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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi><mml:msubsup><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mrow><mml:mn>IO</mml:mn></mml:mrow><mml:mrow><mml:mi>t</mml:mi><mml:mi>o</mml:mi><mml:mi>t</mml:mi></mml:mrow></mml:msubsup></mml:math> = -198.2, -175.1, and -653.4 kcal·mol^-1, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi><mml:msubsup><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mrow><mml:mn>IS</mml:mn></mml:mrow><mml:mrow><mml:mi>t</mml:mi><mml:mi>o</mml:mi><mml:mi>t</mml:mi></mml:mrow></mml:msubsup></mml:math>= -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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi><mml:msubsup><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mrow><mml:mn>IO</mml:mn></mml:mrow><mml:mrow><mml:mi>t</mml:mi><mml:mi>o</mml:mi><mml:mi>t</mml:mi></mml:mrow></mml:msubsup></mml:math> = -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 <i>size match rule</i>.