Noninnocent Role of Na+ Ions in the Binding of the N-Phenyl-2-naphthylammonium Cation as a Ditopic Guest with Cucurbit[7]uril

Na+ ions influence the mechanism for the binding of the ditopic guest N-phenyl-2-naphthylammonium cation (Ph-AH+-Np) to cucurbit[7]uril (CB[7]) by facilitating, at increased Na+ concentrations, the formation of a higher-order complex. Binding of the larger naphthyl moiety of Ph-AH+-Np forms the Ph-AH+-Np@CB[7] 1:1 complex (where “@” represents an inclusion complex) at low Na+ ion concentrations (≤5 mM), whereas the inclusion of the smaller phenyl moiety in CB[7] (CB[7]@Ph-AH+-Np) is transient. Ph-AH+-Np@CB[7] is formed by reactions with free CB[7] and CB[7]·Na+ (where “·” represents an exclusion complex) with displacement of the Na+ cation. Because of the latter reaction, the dissociation of Ph-AH+-Np@CB[7] is faster at higher Na+ concentrations. At high Na+ concentrations (≥25 mM), the Na+ ion stabilizes the inclusion of the phenyl moiety in CB[7] by capping the portal of CB[7]. The dynamics of the capped Na+·CB[7]@Ph-AH+-Np 1:1 complex is slower than in the absence of Na+ capping. This stabilization of the phenyl moiety inclusion in CB[7] by Na+ leads to the formation of the Na+·CB[7]@Ph-AH+-Np@CB[7] 2:1 host–guest complex, where each moiety of the ditopic guest is included in a different CB[7]. The opposing roles of Na+ cations in the formation of the two 1:1 complexes are essential for the switch in mechanism with changes in Na+ concentration and provide an example of systems chemistry, where new properties arise in the form of an increased diversity of complexes and altered complexation dynamics that depend on the system’s composition.