Encapsulation of Ion−Molecule Complexes:  Second-Sphere Supramolecular Chemistry

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...

show abstract

“…This suggested encapsulation of the ion pair 2d ϩ X Ϫ occurred in the cases discussed above (21). This was shown to be the case for 2d ϩ .…”

Section: Methodsmentioning

confidence: 72%

Reversible encapsulation by self-assembling resorcinarene subunits

Shivanyuk

Rebek

2001

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

273

146

View full text Add to dashboard Cite

show abstract

“…A triangular 3:3 complex and a 1:1 complex of Pt corners and bipyridine ligands are formed as fragments. , or finally the inclusion of a charged guest molecule (Brody et al, 1999;Lützen et al, 1999;Schalley et al, 1999a,b,c;Hof et al, 2000;O'Leary et al, 2001).…”

Section: Self-assembly Driven By Hydrogen Bondingmentioning

confidence: 99%

“…field to self-assembling second-sphere inclusion complexes that remind one of the Russian doll Matroshka (Lützen et al, 1999).…”

Section: Molecular Recognition and Supramolecular Chemistrymentioning

confidence: 99%

Molecular recognition and supramolecular chemistry in the gas phase

Schalley

2001

Mass Spectrometry Reviews

Self Cite

289

178

View full text Add to dashboard Cite

Supramolecular chemistry, in particular, the fields of molecular recognition and self-assembly, profit much from the development of soft ionization techniques and advanced methods for mass analysis and gas-phase chemistry. Vice versa, weakly bonded architectures and host-guest complexes represent a veritable challenge for the mass spectrometrist, leading to further development of methods and techniques. This review describes the state-of-the-art in this field, and includes topics such as the effects of solvation on meta binding to crown ethers, chiral discrimination of guests by chiral hosts, the elucidation of the secondary structure of self assembled complexes, and the mechanistic pathways of self assembly or the fragmentations of supramolecular complexes in the gas phase.

show abstract

“…[11] Guests typically enjoy molar concentrations in capsules. When more than one guest is accommodated there is a high probability of the guests interacting, thereby generating molecular arrangements that represent a second order of supramolecular chemistry [12] -complexes within complexes-or reacting, [9b] even along pathways that are unlikely in solution.…”

mentioning

confidence: 99%