Selective dimerisation of tetraurea calix[4]arenes

The formation of hydrogen-bonded dimeric capsules from tetraurea calix[4]arenes is controlled by loops, connecting adjacent urea functions, and more or less bulky substituents. The dimerisation is only possible if loops are not overlapping and if the respective residues can pass the loops. A sorting scheme based on small and bulky residues and one to four loops allows reducing the number of possible dimers from 35 to 6 in a stoichiometric mixture of 11 ureas. With three different loop sizes (O-(CH2)n-O chains with n=10, 14, 20 connecting adjacent phenylurea functions via their meta positions), it is possible to distinguish four urea residues of different sizes (small, medium, bulky and giant) ranging from tolyl to 4-[tris-(4-t-butylphenyl)methyl]-phenyl. While the smallest residue can pass all loops, the largest is excluded by all loops.