Co-crystallization of a carboxylcalix[5]arene in the presence of n-butylamine afforded the corresponding host–guest carboxylate–butylammonium salt-bridged complex as a result of a proton transfer process assisted by a second non-ionized calixarene molecule. The crystals isolated consist of a supramolecular pseudo-dimer in which a host–guest inclusion complex is connected to a formally neutral receptor molecule – hosting a solvent molecule in the aromatic cavity – via a strong negative charge-assisted H-bond between the carboxyl/carboxylate substituents present at the narrow rim of the two interacting calixarene molecules.
