Selective dimerisation of tetraurea calix[4]arenes

Abstract

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.

Keywords:

• calixarene
• dimerisation
• hydrogen bonds
• self-assembly

Acknowledgement

This work was supported by the Deutsche Forschungsgemeinschaft (Bo 523/14 and SFB 625).

Notes

^†Dedicated to the memory of Dmitry Rudkevich.

^1. Their peak area does not correspond to their quantity. The advantage of ESI-MS in comparison to ¹H NMR is that several (charged!) species can be detected simultaneously, although their response is not proportional to their concentration. Disadvantages are the slow formation of complexes with tetraethylammonium (in principle, a good guest) and a low ‘working concentration’ at the limits of ‘quantitative’ formation of dimers.

^2. The sorting scheme was realised with the following residues: ‘bulky’ = 3,5-di-(4-tert-butylphenyl)-4-propoxy-phenyl; ‘small’ = p-tolyl. For the loops, adjacent phenylurea residues were connected by –O–(CH₂)₁₀–O– chains between their m-positions.