Host–Guest Complexes of p-Sulfonato-calix[4]arene and Benzamidine: A Matter of Counterion and Aging in the Mother Liquor

p-Sulfonato-calix[4]arene is commercially available either as a sodium salt or as a sulfonic acid. We aimed to clarify the complexation and crystallization pathways of p-sulfonato-calix[4]arene with benzamidine ligand employing these two commercial forms of macrocyclic host. The additional aspect that we considered is what happens when initially formed crystals are kept in the mother solution simply over passing of time, allowing slow evaporation. This seems to be not as innocent as it intuitively appears. The “aging” of multicomponent crystals exposed to concentration changes of the dissolved components during crystal growth and partial evaporation of the mother solution can embrace different scenarios, including dissolution–recrystallization transformations. Our findings highlight the need of taking into account p-sulfonato-calix[4]arene counterion and crystal aging in the mother liquor as important variables for supramolecular and crystal design of host–guest complexes.