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Calix[4]arenes substituted by four urea functions are self-complementary molecules that spontaneouslycombine in apolar solvents in the presence of an ammonium salt to form dimeric capsules held togetherby a belt of hydrogen bonds. In the presence of tetraethylammonium salts, the Et
4N
+ cation is includedas a guest. The sorting between dimeric capsules formed in a mixture of calix[4]arenes directly dependson the steric crowding of the substituents grafted on the urea groups whether aromatic derivatives oraliphatic chains linking urea functions in mono-, di-, or tetraloop structures. Simple rules allow one toanticipate which capsules will be exclusively formed when calix[4]arenes are mixed in different proportions.The stabilization of the dimeric structures by hydrogen bonds is thwarted by the overlaps of aliphaticloops and/or by bulky groups that cannot pass through these loops. Despite the structural similarity ofthe calixarenes, the exclusive formation of dimers of well-defined compositions and clear titration breaksare observed by electrospray mass spectrometry. This technique yields reliable information onstoichiometries and composition despite measurements in the gas phase rather than in solution and itdoes not suffer from excessive peak overlaps in contrast with NMR.
