Self-Assembling Hydrogen-Bonded Capsules
Capsules which self-assemble from two identical, self-complementary subunits by hydrogen bonding have been synthesized in the Rebek group since the early nineties. One example is the softball on the right which has the same topology as a tennis ball or an american softball, the larger brother of a baseball. Often it is not trivial to provide evidence for the existence of such non-covalent architectures in solution. Besides a downfield shift of the NH protons in non-competitive solvents which indicates the formation of hydrogen bonds, the encapsulation of guest molecules serves for this purpose. Often, two sets of signals are observed for the guest: One for the free guest and the other one for the encapsulated guest. However, more complex or unsymmetrical capsules make the analysis of NMR spectra difficult, so that another complementary method had to be developed.
Therefore, a way to measure exact masses of such aggregates, it was desirable to use mass spectrometry for the characterization of these compounds. However, already the ionization of intact capsules is difficult, not to speak of their structural characterization as capsules by mass spectrometric experiments.
Our method employs electrospray ionization as a very soft method to transfer the capsules into the gas phase. Since non-competitive solvents are mandatory, an ion labeling strategy had to be developed. Encapsulation of a positively charged, quaternary ammonium ion instead of neutral guests provides charged capsules even in solution which give rise to intense signals in the mass spectra. It should be noted that the counterion plays a crucial role. Only the use of weakly coordinating anions such as tetrafluoroborate provide sufficient solubility without interfering with the seam of hydrogen bonds.
Tetraurea calixarene dimers with interdigitated urea groups form a cyclic array of 16 hydrogen bonds.
Structural evidence comes from several different experiments: First of all, the capsules dimerize to yield heterodimers, if two different homodimers are mixed that geometrically fit well. This indicates a reversible, non-covalent structure. Then, guests which do not fit into the cavity either due to incongruent shape or due to their size do not give rise to any signals for capsules. Similarly, deformed capsule monomers do not form 2:1 complexes with the guest ion, providing evidence for the importance of size and shape. Consequently, the complexes observed in the mass spectra are not just unspecifically bound.
So far, these results give insight into the solution-phase structure of the capsules. Nevertheless, the gas phase structure might be different from that, if some rearrangement would occur during the ionization process. Therefore, two experiments were carried out. Collisional activation of the softballs results in covalent fragmentation processes, i.e. the guest is still present in the complex, while covalent bonds have been broken. If covalent bond cleavage can to some extend compete with guest release, there must be a substantial barrier for the latter process. This barrier might well come from hydrogen bonds which in part have to be opened in order to release the guest. The second experiment utilized (in collaboration with Michael Freitas and Alan Marshall) infrared multiphoton dissociation for the determination of the barrier for guest release from the calixarenes. This activation energy is significantly higher than known literature values for the cation-pi interaction of an ammonium cation with the aromatic rings of the capsule, again indicating that the guest must be inside a capsule. Thus, the capsules even retain their integrity in the gas phase.
The "flexiballs" were synthesized from modules which can be exchanged easily.
Capsules with a cavity volume of ca. 1000 cubic Angstroms encapsulate even cryptate complexes.
A modular synthesis allows us to prepare capsules such as the flexiballs and the bigball in an easy and straightforward way with very different cavity volumes. These capsules are much larger than the softballs and the calixarenes and thus require larger guest cations. They can be characterized mass spectrometrically with dicationic guests such as strontium cryptates, acridinium cations, or Fe(II) tris-phenanthroline complexes.
In a collaboration with Prof. Kari Rissanen , this program has recently been extended to hexameric pyrogallarene capsules (for details, see here) which have been shown to self-assemble through hydrogen bonding around a ruthenium tris-bipyridine guest. This complex carries two charges and can thus easily be detected by mass spectrometric means.
Hexameric pyrogallarene capsule. The ruthenium tris-bipyridine guest dication is shown in spacefill mode, the pyrogallarenes each with a different color.