Hexameric capsules

Crystal structures for several of the corresponding pyrogallol[4]arenes 50 (Figure 3.23) have been solved. These also form hexameric capsules, which now contain an array of 72 hydrogen bonds (48 are intramolecular and 24 intermolecular, HB/ (N— 1) =4) and have no structural water molecules [64]. The increase in the ITm HB/(N— 1) value agrees very well with the fact that nanocapsules based on pyrogallol 50a are more stable in polar media than those derived from resorcinarene 49a. 

ESI mass spectra after the addition of 49 (or 50) to 51(PF6)2 show the hexameric clusters 51 (49)6 and 51 (50)6 as the dominating signals (Figure 3.24). These ions were irradiated in an infrared multiphoton dissociation experiment (I RMPD) with IR radiation from a C02 laser at different time intervals. The observed fragmentation behavior was exactly that expected for the hexameric capsule. 

Solid State Formation of the Hexameric Capsule Derived from Pyrogallol[4]arene (50c) [74]... 

In the hexameric capsule of 30b 48 of the 72 potential hydrogen bond donors are used in intramolecular hydrogen bonds to seam the capsule together [70, 71]. The remaining 24 hydrogen bond donors are used in intramolecular hydrogen bonds... 

Figure 9 Stick-bond representation of hexameric capsule with the enclosed space represented in green (see also Plate 21).

As expected, 12 NDI was the most efficient at forming the hexameric capsule with 70% of the material being incorporated, while only 30% of 14 forms the receptor. Figure 28 shows a cartoon representation of the proposed geometry of the C7o receptor including the possible orientations for an N-desymmetrised NDI [17]. 

Fig. 34 Solid-state structure of the mixed-macrocycle hexameric capsule. Diethyl ether guests are shown in dark gray.

Fig. 5.15. Pyrogallarenes and resorcinarenes which form hexameric capsules through hydrogen bonding. The tetramethyl resorcinarene serves as a control compound. At the bottom, three different guest cations are shown, which carry charges to make the capsule MS-detectable. Right Computer model of the Ru(bpy)32 guest encapsulated in the cavity of the hexamer. Each monomer is shown with a different color. It can nicely be seen that the Ru complex not only fits size-wise, but also with respect to its symmetry and shape.

Prompted by its crystal structure [34a] and the fact that some of these aggregates were formed in water-saturated chloroform solutions, the role of the water molecules in the formation of the hexameric capsules of 15c and 16c was also studied. Diffusion NMR was selected since the chemical shift of water protons in the presence of acidic compounds is very sensitive to the amount of water, the concentration of the acidic material, the exchange rate and solvent pH and is therefore not a predictable parameter. In addition, since water molecules are very small relative to the resorcinarene and pyrogallolarene hexamers, the water diffusion coefficient was anticipated to be a rather sensitive parameter to follow water interaction with these supramolecular structures. Indeed, when the water diffusion coefficient was followed as a function ofthe I5C/H2O ratio, a decrease in the diffusion coefficient of the water peak was observed with the increase of the 15c/H2O ratio, as shown in Fig. 6.12. Fig. 6.12a-d shows the signal decay of the water (Fig. 6.12a, c) and the... 

Diffusion on hexameric capsules a) L. Avram, Y. Cohen, Org. Lett. 

Figure 10.48 (a) Space-filling view of the structure of the snub cube (f) formed from six [4]resorcarenes and eight water molecules. (b) Crystal structure of the analogous pyrogaUol[4]arene hexameric capsule containing two molecules of pyrene butyric acid (reproduced with permission from [54]). 

Figure 14 A large hexameric capsule based on resorcin[4]arene can accommodate several solvent molecules or single cationic guests.

On mixing equal amounts of pyrogaUo[4]arene and resorcin[4]arene no evidence of hetero-hexameric capsules was found as a consequence of the extremely high selfrecognition process that caused the formation of only capsules based on the same macrocycle. Guest affinity is also... 

FIGU RE 7.4 Resordn[4]arene in a wet organic solvent self-assembles foimii the hexameric capsule that can host cationic gnests. (Adapted from Scarso A, Borsato G. Self-assembly of organic supramolecular capsules. In Gale PA, Steed JW, editors. Supramolecular chemistry from molecules to nanomaterials, vol. 5. Chichester, UK Wiley 2012, p. 2085-2114.)... 

The first example published in the literature concerning the use of the hexameric capsule in catalysis was actually an example of catalytic modulation through encapsulation. Scarso and coworkers selected the very well known photocatalyst [Ru(bpy)3] (bpy, 2,2 -fetT-pyridine) that, thanks to its pseudospherical shape and its cationic character, perfectly fits the cavity of the capsule with the aim of investigating the effect of reversible binding on the photocatalytic activity of the metal catalyst [25]. 

Aiming at demonstrating the potentialities of the hexameric capsule as a substrate selective nanoreactor, Scarso and coworkers extended the concept to a stoichiometric reaction. In particular, the steric hindrance due to the confined space within the cavity of the capsule was exploited in the substrate selective amide synthesis mediated by the cationic condensing agent 1-ethyl-3-(-3-dimethylaminopropyl) carbodiimide hydrochloride [47]. Similarly to the encapsulation of other cationic species, the cationic condensing agent that in... 

TABLE 7.1 Catalytic Tests for the Competitive Coupling of Pairs of Amines with one Acid and Pairs of Acids with one Amine Mediated by the Carbodiimide Cationic Condensing Agent in the Presence or Absence of the Hexameric Capsule as Supramolecular Nanoreactor at 60°C, Time 18 h. + Presence Absence... 

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