Spontaneous Formation of Hexameric Resorcinarene Capsule in Chloroform Solution as Detected by Diffusion NMR

Avram, Liat; Cohen, Yoram

doi:10.1021/ja0272686

Cited by 264 publications

(175 citation statements)

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“…The resorcin [4]arene and pyrogallol [4]arene macrocycles have been shown to have a great deal of robustness in forming large spheroid hexameric cage assemblies in both solution and solid state (26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38). Indeed, this property has inspired and contributed to numerous theoretical and experimental studies of its hostguest relationships.…”

Section: Resultsmentioning

confidence: 99%

Supramolecular blueprint approach to metal-coordinated capsules

McKinlay

Cave

Atwood

2005

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

201

182

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An important problem in designing any large network is the assembly of systems that are resilient to change. From a chemical point of view, an analogy can be used where one requires supramolecular assemblies to maintain their dimensionality combined with limited structural perturbation in response to variation in its intermolecular framework. The identification of hydrogenbonded framework patterns within experimentally known supramolecular assemblies that are structurally robust to disruption and selective hydrogen substitution are envisioned to act as a supramolecular blueprint or template for metal-ion retroinsertion. Here, we report the formation of a large neutral discrete pseudo-spherical coordination capsule assembled from 6 pyrogallol[4]arene ligands and 24 Cu(II) metal ions. Amazingly, this coordination capsule is structurally analogous to its hydrogenbonded counterpart. This result shows a robust ability of pyrogallol[4]arene molecules to self-assemble into large hexameric cage structures from either the hydrogen-bonding or metal-ligand coordination process. The identification of robust supramolecular assemblies that conserve their structure in response to interchangeability between hydrogen-bonded networks for metal coordination, or inversely, represents an important advancement in supramolecular design.coordination ͉ self-assembly ͉ hexamer I nspired by the architectural beauty and robustness found within nature, the design of large multicomponent spheroid cage assemblies on the nanometer scale is becoming increasing feasible. These large supramolecular architectures often resemble and conform to the simple geometrical shapes described by Platonic or Archimedean solids (1). Indeed, such structural morphologies have attracted tremendous interest because of their practical applications for encapsulating various guest molecules (2). Synthetic chemists have principally relied on various self-assembly methods in their construction, such as hydrogen bonds (3) or metal-ligand coordination (4), although some notable covalent methods have recently been reported (5).A popular strategy for designing large multicomponent cage assemblies is the metal-directed self-assembly approach, advocated chiefly by Fujita and coworkers (6-9) and Stang and coworkers (10-12). This method has mainly focused on the geometrical permutation of various triangular pyridine-like bridging ligands driven by coordination to cis-protected Pd metal centers. Nevertheless, for the construction of discrete assemblies with large molecular cavities, the molecular paneling approach by Fujita and coworkers (13) has received the most success. Indeed, an amazingly large discrete functionalized spherical coordination network assembled from 12 metal ions and 24 exo-multidentate ligands was recently reported (14). However, an inherent complication of using n-dimensional pyridine-like spacer multidentate ligands in coordination is the resultant high positive charge combined with porous surfaces within these supramolecular assemblies. The counterions us...

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Section: Resultsmentioning

confidence: 99%

Supramolecular blueprint approach to metal-coordinated capsules

McKinlay

Cave

Atwood

2005

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

201

182

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“…Typical solvents that compete for hydrogen bonds (e.g., methanol or DMSO) disrupt the assemblies and reduce them to their constituent monomers (12,14). Many other polar solvents also interfere with capsule formation.…”

Section: Resultsmentioning

confidence: 99%

Resorcinarenes are hexameric capsules in solution

Evan-Salem

Baruch

Avram

et al. 2006

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

Self Cite

143

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The host-guest complexes of resorcin[4]arenes with small molecules in organic solutions are examined using modern NMR spectroscopic methods. The complexation of glutaric acid and ␤-methyl D-glucopyranoside in chloroform were investigated through 2D COSY, 2D NOESY, 1D NOE, and diffusion-ordered NMR spectroscopy (DOSY) techniques. These methods indicate that the complex is a self-assembled capsule composed of six resorcinarenes that surround six guest molecules of glutaric acid or three molecules of ␤-methyl D-glucopyranoside inside. The multiplicity of guest proton signals shows that the capsule provides an asymmetric magnetic environment that persists on the 1 H NMR time scale. The encapsulation of these guests and common solvents suggests that the phenomenon of reversible encapsulation in chemistry may be a century old. 2). They provided the modules from which open-ended container molecules, the cavitands, were elaborated and led to the first covalent structures that completely surrounded other molecules, the carcerands (3). These shallow, bowl-shaped structures have intrinsic properties as receptors for molecular recognition as introduced by Aoyama and coworkers (4, 5). They described complexes of 1 in organic solvents with a number of guests, small diacids, alcohols, and even steroids, featuring a 1:1 stoichiometry. Intermolecular hydrogen bonding was proposed to hold the guest in the concavity of the resorcinarene host, as shown for glutaric acid in Fig. 2. The NMR spectra confirmed that the signals of the guest were upfield-shifted, as expected for their positions above the four aromatic units of the resorcinarene. However, the same spectra showed an unexpected feature: guest exchange took place slowly on the NMR time scale, with separate signals for free and bound guests. The broad range of molecules recognized and the slow on͞off dynamics appeared at odds with the simple structures proposed for the complexes, and raised some questions as to the extent of the intermolecular forces involved. We have now examined the glutaric acid, ␤-methyl D-glucopyranoside, and related complexes of 1 in solution through modern NMR techniques and report on them here. We find that the unprecedented behavior can be reconciled through encapsulation complexes involving the self-assembly of six resorcinarenes in a capsular host. The host surrounds six glutaric acid or three ␤-methyl D-glucopyranoside guests.A blueprint to the solution structure came from the crystallographic studies of Atwood and MacGillivray (6). They found that, unlike the many previous solid-state structures of resorcinarenes and guests that involve layered arrays of the molecules (7-10), resorcin[4]arene 1Ј crystallized from hot nitrobenzene as a rather large closed shell, a hexameric capsule surrounding a space of nearly 1,400 Å 3 (6). The six resorcin [4]arene molecules appear at the sides of a notional cube, and one molecule of water is at each of the eight corners (Fig. 1). An unknown number of highly disordered solvent molecules are inside the cavity. S...

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“…In contrast, the self-assembled capsule consisting of six resorcin [4]arene units and eight water molecules in the form of an Archimedean snub cube, [20] which is capable of housing up to eight benzene molecules, utilizes only 60 hydrogen bonds, of which 36 are intermolecular (only 2.6 intermolecular hydrogen bonds per bound entity). [10,17] The increased number of hydrogen bonds coupled with the fact that nanocapsules 1-3 each consist of only six building blocks is a factor in stabilizing the nanocapsules in polar media. [14] Indeed, this remarkable stability is of great significance for the possible exploitation of such assemblies for biological applications.…”

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confidence: 99%