Crystal Structure and Thermodynamics of Reversible Molecular Capsules

Abstract

The reversible encapsulation of small molecules by two rigid hemispherical molecules is described. The "bowls" are linked by four charged hydrogen bonds. The complex is highly guest-selective. A crystal structure of one complex and a thermodynamic analysis of several complexes provide an unusually vivid description of the structure and dynamics of these complexes. The relevance of these complexes as transition state models for the formation of carceplexes is also presented.

Bis-Capsules: Cooperative Reversible Encapsulation of Two Molecules in Adjacent Separate Chambers

“…Indeed, neither empty mono-capsules nor empty mono-carcerands have ever been observed in the tetrol system. [5,6,9] These results are also consistent with the broad NMR signals obtained in [D 5 ]nitrobenzene. In the absence of a suitable guest, no well-defined capsules can form, and only aggregates are observed.…”

“…[5] In the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), two tetrols were linked by charged hydrogen bonds to yield a reversible capsule. [6] Under similar conditions, tetramer 1 should be predisposed to form bis-capsule 32 guest (Scheme 1). The 1 H NMR spectra of tetramer 1 and DBU are broad in [D 5 ]nitrobenzene at 373 K, but addition of pyrazine yields a new set of sharp host signals as well as a new signal at d 5.10, which we assign to encapsulated pyrazine.…”

“…Similar results have been obtained for the tetrol system. [6] Unlike nitrobenzene, chloroform can fit inside the tetrolbased capsules (Figure 1). [6] Addition of MeOAc to a 1 H NMR sample containing tetramer 1 and DBU in CDCl 3 gave rise to two sets of complexed MeOAc signals (d À 0.57/ À 2.13 and d À 0.41/ À 1.97).…”

“…[6] Unlike nitrobenzene, chloroform can fit inside the tetrolbased capsules (Figure 1). [6] Addition of MeOAc to a 1 H NMR sample containing tetramer 1 and DBU in CDCl 3 gave rise to two sets of complexed MeOAc signals (d À 0.57/ À 2.13 and d À 0.41/ À 1.97). With excess MeOAc, exclusively the latter set of signals is observed, and integration yields 2 MeOAc:1 host; thus we assign this latter signal set to bis-capsule 3´2 MeOAc and the former signal set to biscapsule 3´MeOAc´CDCl 3 .…”

Supramolecular Isomerism in Caviplexes

“…[2] In complexes held together by hydrogen bonds such as self-assembling capsules, [3] related types of diastereomerism can also be observed. [4] In open-ended cavity-containing structures, such as cyclodextrins, cyclophanes, and cavitands, this kind of stereoisomerism is more difficult to detect. [5,6] The kinetic stabilities of the complexes are low and guest exchange is generally rapid with respect to the timescale of the spectrometer.…”