Precision Switching in a Discrete Supramolecular Assembly: Alkali Metal Ion‐Carboxylate Selectivities and the Cationic Hofmeister Effect

A cavitand host has been shown to switch from a dimeric assembly to a tetrameric assembly in the presence of cations. Induced by pseudo‐specific cation binding attenuating the net negative charge of each host, switching was shown to be highly cation selective. Thus, the concentration of cation required to induce assembly switching ranged from 2 mM in the case of N(n‐Bu)4+ to ∼80 mM in the case of Na+. Overall cation affinity was found to be essentially the reverse of Collins’ law of matching water affinities, which predicts Na+ to have the strongest affinity for carboxylate groups. Combined with previous data, these results highlight the point that cation affinity for carboxylates are in large part dictated by context. Against the law! Cation binding to a 2 : 1 host‐guest complex induces switching to a tetrameric 4 : 2 complex, but cation affinity does not match Collins’ law of matching water affinities.