Recognition of Two Ammonium Ions by Borate‐Containing Crown Ether: Selectivity of the Internal and External Recognition of Distinct Ammonium Ions

This study investigates the recognition properties of (pseudo)rotaxanes comprising bis‐catechol‐borate‐containing crown ether and ammonium‐containing axles. Nuclear magnetic resonance spectroscopy revealed that the borate‐based (pseudo)rotaxanes could bind additional secondary ammonium ions outside the crown ether cavity. The association constant between an acidic ammonium ion and (pseudo)rotaxane was higher than that for the less acidic analogue. When an equimolar mixture of two ammonium ions with different acidities was introduced, the borate‐based crown ether predominantly bound the more acidic ion within its cavity. Furthermore, the recognition of an additional ammonium ion decreased the selectivity toward internal binding of the more acidic ammonium ion.