Organization of the interior of molecular capsules by hydrogen bonding

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Abstract
The enclosure of functional entities within a protective boundary is an essential feature of biological systems. On a molecular scale, free-standing capsules with an internal volume sufficiently large to house molecular species have been synthesized and studied for more than a decade. These capsules have been prepared by either covalent synthesis or self-assembly, and the internal volumes have ranged from 200 to 1,500 Å3. Although biological systems possess a remarkable degree of order within the protective boundaries, to date only steric constraints have been used to order the guests within molecular capsules. In this article we describe the synthesis and characterization of hexameric molecular capsules held together by hydrogen bonding. These capsules possess internal order of the guests brought about by hydrogen bond donors within, but not used by, the framework of the capsule. The basic building blocks of the hexameric capsules are tetrameric macrocycles related to resorcin[4]arenes and pyrogallol[4]arenes. The former contain four 1,3-dihydroxybenzene rings bridged together by -CHR- units, whereas the latter contain four 1,2,3-trihydroxybenzene rings bridged together. We now report the synthesis of related mixed macrocycles, and the main focus is on the macrocycle composed of three 1,2,3-trihydroxybenzene rings and one 1,3-dihydroxybenzene ring bridged together. The mixed macrocycles self-assemble from a mixture of closely related compounds to form the hexameric capsule with internally ordered guests.