Encapsulation complexes are assemblies in which a reversibly formed host more or less completely surrounds guest molecules. Host structures held together by hydrogen bonds have lifetimes in organic solvents of milliseconds to hours, long enough to directly observe the encapsulated guest by NMR spectroscopy. We describe here the action of alkyl ammonium compounds as guests that gather up to six molecules of the host module to form encapsulation complexes. The stoichiometry of the complexes-the largest hydrogen-bonded host capsules to date-is determined by the size and concentration of the guest. In 1997 Atwood and MacGillivray (1) reported the structure of resorcinarene 1a (Scheme 1) in the solid state and suggested its expanded possibilities for molecular recognition in solution (for a related structure see ref.2). Such compounds had hardly been ignored before then: As early as 1982, their curvature and functional groups provided the skeleton of the first cavitand (3) and subsequently, from two such units, the carcerands (4, 5) and hydrogen-bonded capsules (6-9) were prepared. The single resorcinarene unit has recognition capabilities in its own right. In 1988 Aoyama (10, 11) described the formation of stoichiometric, 1:1 complexes of 1b with dicarboxylic acids, ribose, and even with terpenes, and steroids in organic solution (12). Later, Aoki (13), Rissanen and coworkers (14), and others (ref. 15; a related structure including disordered solvent was obtained from 1d) found dimeric capsules of 1c with alkyl ammonium guests in the solid state. But when the structure of 1a revealed it to be a spectacular, spherical hexamer, surrounding an enormous cavity, it became apparent that encapsulation of much larger guests and alternate stoichiometries might be possible with this host. To our knowledge, no such complexes have been described in solution, and we introduce them here. Materials and MethodsThe resorcinarenes 1b and 1d were prepared by known procedures (16). The quaternary ammonium and phosphonium guests were purchased from Aldrich and Fluka and used without further purification. The stock solutions of 1b in commercial CDCl 3 were saturated by shaking with H 2 O before the complexation studies. All NMR spectra [one-dimensional 1 H-, 19 F-, 31 P-, two-dimensional rotating-frame Overhauser effect spectroscopy (ROESY)] were recorded with Bruker DRX 600-MHz spectrometer by using the solvent signal as internal reference. Results and DiscussionThe earlier finding that quaternary ammonium guests are useful for the detection of other capsular hosts in the gas phase by electrospray ionization-MS (17) was applied to the characterization of assemblies from Scheme 1. These salts act as ionic labels for MS but show high affinity for deep cavitands even in aqueous solution (18). The bonds that coat the inner surfaces of structures 1 provide a thin layer of negative charge that is complementary to quaternary ammonium salts. It seemed likely that the cation-interactions could be recruited to generate and stabilize assemblies of...

Resorcinarene 1b forms a hexameric assembly in water-saturated CDCl(3) that encapsulates one tetraalkylammonium salt (2(+)Br(-)). The remaining space is occupied by coencapsulated solvent molecules. A maximum of three and minimum of one CHCl(3) molecule were found inside of capsules with tetrapropyl- and tetraheptylammonium bromide, respectively. The encapsulation of the salt is endothermic and entropically favored by the liberation of the solvent molecules. The stabilities of the encapsulation complexes and the rates of guest exchange decrease for larger cations. The higher activation barriers for in/out exchange of the larger guests suggest tight conformational restraints in the transition state. It is likely that complete dissociation of one resorcinarene molecule is necessary for the exchange of the alkylammonium salts.

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Resorcinarenes assemble in wet chloroform or benzene to form hexameric capsules, resembling inflated cubes or volleyballs. NMR methods are used to determine the number of solvent molecules detained inside; eight molecules of benzene are encapsulated.

We introduce here a new form of isomerism-social isomerism-that arises when two different guests are confined to a cylindrical host capsule. The isomerism deals with the orientationof one guest with respect to the other when they are in a container. Specifically, para-substituted toluenes coencapsulated with typical solvent molecules such as benzene, chloroform, and cyclohexane show two different isomeric arrangements that interconvert slowly on the NMR time scale. The dimensions of the capsule prevent the guests from squeezing past one another or tumbling freely.