Advanced ¹⁹F-NMR studies shed new light on encapsulation of isosteric guests in the hexameric capsules of resorcin[4]arenes and pyrogallol[4]arenes

Abstract

The hexameric capsules of resorcin[4]arenes and pyrogallol[4]arenes are fascinating, catalytically active, and highly accessible structures having large cavities. Despite the apparent similarity between these two types of hexamers, the hexameric capsules of C₁₁-resorcin[4]arenes (1) are much more efficient nanoreactors than the C₁₁-pyrogallol[4]arene (2) capsules. In this study, we investigated the encapsulation of two bulky and structurally related isosteric guests namely adamantane-1-carboxylic acid (3) and 3,5,7-tri-fluoro adamantane-1-carboxylic acid (4) into these hexamers in a competitive (chloroform) and a non-competitive (benzene) solvent. Through the application of NMR spectroscopy, diffusion NMR, and ¹⁹F guest exchange saturation transfer (GEST) applied for the first time on such hexameric capsules, we show that the two apparently similar hexamers behave differently towards these two isosteric guests. We found that in C₆D₆ the hexamers of 1 preferentially encapsulate the non-fluorinated guest 3 over guest 4, while the hexamers of 2 preferentially encapsulate the fluorinated isosteric guest 4. For the hexameric capsule of 2 encapsulating guest 4, ¹⁹F-NMR shows that the disruption of the hexameric capsule by methanol is a more complex process than one would have anticipated revealing, for the first time, three populations of 4 having different exchange rates. The combination of ¹H/¹⁹F diffusion and ¹⁹F-GEST NMR provides new insights into these important and catalytically active capsular systems demonstrating the advantages of using this combination of NMR methods to explore such supramolecular systems in solution.

---

Introduction

During the last decades self-assembled molecular capsules and cages of different natures were prepared.^1–9 Among those, the hexameric capsules of resorcin[4]arenes and pyrogallol[4] arenes are perhaps the most intriguing, highly accessible and fascinating molecular capsules known.^10–18 These self-assembled hexameric capsules having large cavities were observed first in the solid state and then in solution and in the gas phase.^10–18 Shyvaniuk and Rebek showed that by using a suitable guest it is possible to probe, unequivocally, the existence of such hexameric capsules of lipophilic resorcin[4]arene in chloroform.¹³ Diffusion NMR showed that, surprisingly, hexameric capsules are indeed the resting state of lipophilic resorcin[4]arenes and pyrogallol[4]arenes like 1 and 2 (Scheme 1) in common organic solvents.^14–17 These diffusion NMR studies were instrumental in the characterization of the solution structures of these hexamers showing that in chloroform 1 forms (1)₆(H₂O)₈-type capsules, whereas 2 forms 2₆-type capsules.^14–16 Diffusion NMR also enabled us to show that these hexamers self-sort, and that their self-assembly process results exclusively in homo-hexamers when resorcin[4]arenes and pyrogallol[4]arenes are mixed.¹⁷ These results were then reproduced by fluorescence spectroscopy.¹⁹ Very recently, we could demonstrate that 2 forms only hexameric capsules both in chloroform and in non-competitive solvents such as benzene and toluene. Compound 1, however, was found to form hexameric capsules and higher aggregates as a minor product in non-competitive solvents like benzene and toluene.²⁰ This is in contrast to what was recently claimed using small angle neutron and X-ray diffraction (SANS and SAXS, respectively).^21,22 Note, however, that the minor product of 1 observed in benzene and toluene appears to have an octameric structure according to diffusion NMR measurements.²⁰ In addition, the hosting capability of the two types of hexamers was found to differ.^23–31 Catalysis was performed mostly in the resorcin[4]arene capsules, first by Scarso,^32,33 and then by the Tiefenbacher and Neri groups.^34–40 In some of the cases, the acidic water molecules of the hexameric capsule of 1 were said to play a pivotal role in the catalysis.⁴¹ Moreover, it was demonstrated that water molecules that are part of the hexameric capsule of 1, are essential for various reactions performed within the hexameric capsules of the lipophilic resorcin[4]arene.⁴² At least in one case, the lack of water molecules in the structure was blamed for the catalytic incompetence of the hexameric capsules of pyrogallol[4]arene.⁴¹ Note that different computational studies were performed on the peculiar hexamer of 1 ^43–45 and a very recent experimental and computational study suggested that the hexameric capsule of 1 with 14 water molecules is the most active species in the catalysis.⁴⁶ Additionally, following our demonstration that ammonium salts can be encapsulated in the pyrogallol[4]arene hexamers in a non-competitive solvent like benzene,⁴⁷ it was recently shown that these hexameric capsules affect the product distribution and may have some catalytic effects on 1,3 dipolar addition reaction in that solvent.⁴⁸ All the above facts highlight the differences between the seemingly similar hexamers of 1 and 2. Some of the above observations and findings about these capsules are, in fact, difficult to reconcile.

Scheme 1 The structures of compounds 1, 2, 3 and 4.

Results and discussion

In this study, we decided to use a pair of bulky isosteric guests and to investigate their interactions with the capsules of 1 and 2 (Scheme 1) in both competitive and non-competitive solvents. Here we report on the encapsulation of adamantane-1-carboxylic acid (3) and 3,5,7-trifluoro adamantane-1-carboxylic acid ((4), Scheme 1) into the hexameric capsules of 1 and 2 both in chloroform and in benzene.

Fig. 1 shows the ¹H-NMR spectra of solutions of 30 mM of 1 and 10 mM of 3 and 4 in CDCl₃ and C₆D₆. Fig. 2 shows the same data for solutions of 2 and the two isosteric guests, namely 3 and 4. From the high-field ¹H-NMR signals of guest 3 in the solutions with 1 presented in Fig. 1a and b, we could conclude that guest 3 is encapsulated in the hexameric capsules of 1 both in CDCl₃ and in C₆D₆. The isosteric guest 4 shows lower affinity toward the cavity of the hexamers of 1 in C₆D₆ (Fig. 1c). In CDCl₃, we see no clear evidence for encapsulation of 4 into the hexameric capsule of 1 (Fig. 1d). Fig. 2 shows somewhat different behaviour for the hexameric capsules of 2. There we observe some encapsulation of 3 into the hexameric capsules of 2 in C₆D₆ and in CDCl₃ (Fig. 2a and b). The isosteric guest 4 is, however, significantly encapsulated in the hexameric capsule of 2 in C₆D₆, but not at all in the CDCl₃ solution (Fig. 2c and d). The encapsulation of 4 (Fig. 2c) in C₆D₆ is favoured compared to 3 (Fig. 2a), which is opposite to the trend observed for the capsule of 1 (Fig. 1c and a). This was also validated by integration of the relevant ¹H-NMR signals in the C₆D₆ solutions (Table S1 in the ESI†). Table S1† shows that, in C₆D₆, the integration ratios, of the hexameric capsule of 1 and encapsulated guests 3 and 4 are 1 : 0.45 and 1 : 0.28, respectively, showing a higher affinity of the capsules of 1 to 3 compared to 4. However, for the capsule of 2, the integration ratios of the capsule of 2 and the encapsulated guests 3 and 4 are 1 : 0.18 is 1 : 0.40, respectively, indicating a clear preference of the hexameric capsule of 2 to guest 4 over guest 3.

Fig. 1 ¹H-NMR spectra (500 MHz, 298 K) of the solutions of 30 mM of 1 and 10 mM of 3 (a and b) and 4 (c and d) in C₆D₆ (a and c) and CDCl₃ (b and d). The * symbols represent the signals of the higher aggregates of 1 in C₆D₆.

Fig. 2 ¹H-NMR spectra (500 MHz, 298 K) of the solutions of 30 mM of 2 and 10 mM of 3 (a and b) and 4 (c and d) in C₆D₆ (a and c) and CDCl₃ (b and d).

Next, we employed diffusion NMR, a technique that has been extensively used to study such systems,^{14–17,20,49–52} to show that the encapsulation is taking place in the hexameric capsules of 1 and 2. This was concluded based on the results of the diffusion NMR measurements of 1 and 2 in the presence of 3 and 4 across the various CDCl₃ solutions, both before and after the addition of methanol. The results are presented in Fig. 3 and Table S2.† Indeed, the extracted diffusion coefficients are consistent with the formation of hexameric capsules encapsulating guests 3 and 4 that are disrupted to their respective monomers by the addition of methanol. This disruption is manifested by the dramatic increase in the diffusion coefficients of 1 and 2 upon addition of methanol, coupled with the disappearance of the high-field ¹H-NMR signals assigned to the encapsulated guests. Very similar results were also observed when methanol was added to the C₆D₆ solutions of 1 and 2 with 3 and 4.

Fig. 3 Diffusion coefficients of 1 and 2 for the solutions presented in Fig. 1(b, d) and 2(b, d) both before and after addition of CD₃OD. To the 0.5 ml solutions of 1/3 and 2/3, 45 μl and 150 μl of CD₃OD were added, respectively. The diffusion coefficients are averages ± SD of three independent measurements. SD values were equal or smaller than 0.01 × 10⁻⁵ cm² s⁻¹ in all cases.

Next, we decided to collect the ¹⁹F-NMR spectra of the solutions of 1 and 2 with the fluorinated guest 4, which are presented in Fig. 4. These ¹⁹F-NMR spectra reveal that for the hexameric capsule of 1 the signals of encapsulated guest 4 are located at lower field compared to the peak of the free guest (Fig. 4a and b, marked with arrows). Conversely, for the hexameric capsule of 2, we observe, in C₆D₆, a relatively strong signal which, surprisingly, is at a high-field (Fig. 4c, marked with an arrow) compared to the signal of the free guest. In contrast, for the CDCl₃ solution of 2 and 4 we observe no signal of the encapsulated guest (Fig. 4d) which is consistent with the ¹H-NMR spectrum presented in Fig. 2d. The next step was to verify that the signal at high-field in the ¹⁹F-NMR spectrum of the C₆D₆ solution of 2 and 4 is indeed that of 4 encapsulated in the hexamer of 2. For this purpose, we measured the diffusion coefficient of that signal in the ¹⁹F-NMR spectrum (Table S3†) and compared it to the diffusion coefficient of signals of 2 in that solution extracted from ¹H diffusion NMR which was 0.20 ± 0.01 × 10⁻⁵ cm² s⁻¹. As expected, we found the two diffusion coefficients to be similar within the experimental error, corroborating even further that this high-field ¹⁹F-NMR signal represents encapsulated 4 in the hexamer of 2.

Fig. 4 ¹⁹F-NMR spectra (470 MHz, 298 K) of the 30 mM solutions of 1 (a and b) and 2 (c and d) in the presence of 10 mM of 4 in C₆D₆ (a and c) and CDCl₃ (b and d). The * symbols represent minor impurities. Arrows indicate the peaks of encapsulated guest.

Since the fluorinated guest 4 that is encapsulated in the hexameric capsule of 2 gave a distinct and significantly shifted ¹⁹F-NMR signal we thought to use chemical exchange saturation transfer (CEST) NMR experiment⁵³ to glean, for the first time, quantitative information about the in–out exchange rate of a guest in such hexameric capsules. For the quantitative analysis, we chose to use the ¹⁹F-NMR multi power guest exchange saturation transfer (GEST) experiment, recently suggested by the Bar-Shir group.^54–58 To obtain an accurate estimation of the guest exchange rate using the aforementioned methodology, we had to increase even further the ratio between unbound and bound guest. We achieved that by increasing the concentration of 4 in the solutions from 10 mM to 100 mM.

Fig. S1 and S2 in the ESI† show the same data as in Fig. 1 and 2 but for solutions where the guest concentrations are 100 mM. The ¹H-NMR spectra presented in these Figures show essentially the same information as that presented in Fig. 1 and 2. Before performing the ¹⁹F-GEST NMR experiments, we performed a detailed titration of the solution presented in Fig. S2c† using CD₃OD. This titration was followed by ¹H- and ¹⁹F-NMR and the results are presented in Fig. S3† and Fig. 5, respectively. The ¹H-NMR spectra presented in Fig. S3† show small changes upon addition of the first 2 μl of CD₃OD. After addition of 6–8 μl of CD₃OD significant line broadening and some line shape changes are observed. Following the addition of 10–12 μl of CD₃OD, there is almost a complete disappearance of the ¹H-NMR signals of the encapsulated guest, as shown in Fig. S3f and g.† Addition of 18 μl of CD₃OD results in practically complete disappearance of the signals of the encapsulated guest from the ¹H-NMR spectrum (Fig. S3h†). However, the ¹⁹F-NMR spectra presented in Fig. 5 appear to reveal more details about the process. Fig. 5b and c show that addition of 2–4 μl of CD₃OD to the solution results in the splitting of the original signal in the ¹⁹F-NMR spectrum of the encapsulated guest 4, accompanied by the appearance of a new signal just 0.2 ppm away from this signal. Adding an extra 2 μl of CD₃OD to this solution results in a further decrease in the intensity of the original ¹⁹F-NMR signal at −145.7 ppm and a further increase of the new signal (Fig. 5d). After addition of 8 μl of CD₃OD one observes in fact three weak signals, two of similar intensity and a third one which is much weaker in intensity (Fig. 5e). Addition of more CD₃OD results in further broadening and reduction in the intensity of the ¹⁹F-NMR signals of encapsulated 4. After addition of 18μl of CD₃OD these signals appear to vanish from the one pulse ¹⁹F-NMR spectrum. These results suggest that indeed addition of CD₃OD weakens the hydrogen bonds holding the hexamers and that this process is accompanied by the formation of three pools of encapsulated 4.

Fig. 5 ¹⁹F-NMR spectra (470 MHz, 298 K) of the 30 mM solutions of 2 in the presence of 100 mM of 4 in C₆D₆ (a) before and after addition of (b) 2 μl, (c) 4 μl, (d) 6 μl, (e) 8 μl, (f) 10 μl, (g) 12 μl, and (h) 18 μl of CD₃OD.

The ¹⁹F-GEST NMR experiments were performed on a C₆D₆ solution presented in Fig. 5 both before and after addition of 8, 12 and 18 μl of CD₃OD and the results are presented in Fig. 6. For the samples presented in Fig. 6 we concomitantly measured the diffusion coefficients of all molecular species observed in the ¹H- and ¹⁹F-NMR spectra of those samples. This was performed to monitor the effect of methanol addition on the aggregation mode of the system and the results are presented in Tables S4 and S5.† The results of the multi-B1 ¹⁹F-GEST NMR experiments performed on the C₆D₆ solution of 2 and 4 show that without the addition of methanol, the in–out exchange is too slow to be measured using the aforementioned methodology (Fig. 6a). Addition of up to 4 μl of CD₃OD had a marginal effect on the line shape of the signals at high-field in the ¹H-NMR spectrum, which are attributed to encapsulated guest, and had no effect on the diffusion coefficients of all signals, including those of the encapsulated guest. However, some changes were already observed in the ¹⁹F-NMR spectra for the signals attributed to the encapsulated guest 4 (Fig. 5a and b). Under these experimental conditions, the GEST experiment showed no effect, probably because the release of the guest is still slow in the NMR timescale. The addition of 8 μl of CD₃OD to the C₆D₆ solution of 2 and guest 4 resulted in significant changes of the signals of the encapsulated guest in the ¹H-NMR spectrum (Fig. S3e†), and significant reduction in the intensity and splitting of the ¹⁹F-NMR signal of encapsulated 4 into three signals (Fig. 5e). Note that the addition 8 μl of CD₃OD had no effect on the diffusion coefficient of 2, implying that the hexamers are still intact (Table S4†). For this solution however, a clear GEST effect was observed (Fig. 6b) from which a quantitative analysis revealed three signals exchanging in not equal but similar k_out values in the range of less than 30 s⁻¹.

Fig. 6 ¹⁹F-NMR spectrum (upper part) and the multi-B1 ¹⁹F-NMR GEST spectra (lower part) of the C₆D₆ solution of 30 mM of 2 and 100 mM of 4 (470 MHz, 298 K) (a) before and after addition of (b) 8 μl, (c) 12 μl, and (d) 18 μl of CD₃OD. The points represent the experimental data and solid lines are the results of the fittings based on the Bloch–McConnell equations⁵⁹ as described recently.^58,60–62 The * symbols represent signals of minor impurities.

After addition of 12 μl of CD₃OD to this C₆D₆ solution, we observe further broadening of the ¹H/¹⁹F-NMR signals of the encapsulated guest (Fig. S3g,† and Fig. 5g) but the diffusion coefficient of the system remained unchanged (i.e., 0.18 ± 0.01 × 10⁻⁵ cm² s⁻¹), implying that the hexameric capsules of 2 are still intact. Here, a stronger ¹⁹F-GEST response was observed (Fig. 6c) and slightly higher k_out values of about 81 ± 4, 29 ± 2 and 50 ± 15 s⁻¹ were extracted. After addition of 18 μl of CD₃OD to the C₆D₆ solution, we observe a disappearance of the ¹H/¹⁹F-NMR signals of the encapsulated guest (Fig. S3h† and Fig. 5h) and some increase in the diffusion coefficient of the system (Table S4†). This increase, however, appears to represent mostly some decrease in viscosity as manifested by the increase in the diffusion coefficient of the solvent itself. There, the extracted k_out values were higher and were found to be 731 ± 66, 152 ± 11 and 72 ± 11 s⁻¹ (Fig. 6d).

Clearly, as expected, the release rate of the encapsulated guest increased with the increase in the amount of methanol added to the benzene solution of 2 and 4. Interestingly, even after complete disappearance of the ¹H- and ¹⁹F-NMR signals of the encapsulated guest in the one-pulse experiments, we still monitor relatively slow k_out exchange rates of guest 4 from the capsule of 2. It is important to note that after addition of 18 μl of CD₃OD to the C₆D₆ solution, we observed three distinct k_out values. The exchange rate for the original ¹⁹F NMR signal of encapsulated guest 4 is the one that show the slowest exchange rate. The new ¹⁹F-NMR signals observed near the original signal of the encapsulated fluorinated guest 4 may be attributed to 4 encapsulated in slightly modified hexameric capsules or to a second molecule of 4 within the same capsule. Note that the volume of 4 is less than 200 ų which, according to the 55% packing rule of Rebek,⁶³ implies that, in principle, two molecules of 4 can be encapsulated in each hexamer of 2. Moreover, in the ¹⁹F 2D-NOESY NMR experiment performed on the sample presented in Fig. 2c after addition of 8 μl of CD₃OD, we indeed found cross-peaks between the two strongest signals attributed to encapsulated 4 as shown in Fig. S4.† These cross-peaks imply that these two species are in close vicinity to each other, corroborating the assumption that these two signals arise from two encapsulated species in the same capsule. Despite all these findings, it is still relatively difficult to pinpoint, unequivocally, on the exact structural nature of the specific hexameric capsules involved in the exchange after methanol addition to the solution 2 and 4 in benzene. What is clear is that the high sensitivity of the fluorine chemical shift and the ¹⁹F GEST NMR experiments enabled to demonstrate, for the first time, that the methanol disintegration of these hexameric capsules is a more complex process than one would have anticipated involving some transient capsules.

Experimental

Materials

All starting materials, reagents, guests’ molecules, and deuterated solvents were purchased from Sigma-Aldrich and used as received. Compounds 1 and 2 were synthesized according to ref. 14 and 17.

NMR

¹H- and ¹⁹F-NMR experiments were collected on 11.7T Avance III Bruker instrument operating at 500.1 and 470 MHz, respectively.

Diffusion NMR

¹H- and ¹⁹F-NMR diffusion experiments were collected with longitudinal eddy current delay (LED) sequence.⁶⁴ For more details, see ESI.†

¹⁹F GEST

¹⁹F-NMR GEST experiments were collected and analysed according to ref. 57–62 For more details, see ESI.†

Conclusions

In conclusion, we studied the encapsulation of two bulky isosteric guests namely 3 and 4 into the hexameric capsules of 1 and 2, in competitive and non-competitive solvents such as chloroform and benzene, respectively. We found that encapsulation of 4 is more efficient in benzene than in chloroform. Interestingly, we found that while the hexamers of 1 preferentially encapsulate the non-fluorinated guest 3 over guest 4, the hexamers of 2 preferentially encapsulate the fluorinated isosteric guest 4. Surprisingly, only in the case of 2 and only in benzene the fluorinated encapsulated guest 4 had a strong, distinct and high-field shifted signal in the ¹⁹F-NMR spectrum. This signal enabled us to measure, for the first time in such systems, the in–out exchange rate using the ¹⁹F-GEST approach. This measurement together with diffusion NMR show that, after addition of minute amounts of methanol, exchange of this bulky guest out from the hexamers is possible while the hexamers of 2 still prevail in the solution. The ¹⁹F-GEST NMR experiments reveal that the disruption by methanol of the hexamers of 2 encapsulating 4 is a more complex process than one may have anticipated, revealing three populations of 4 having different exchange rates. Before the addition of methanol to the hexamers of 2 in benzene no in–out exchange could be measured for 4 using the ¹⁹F-GEST NMR experiment. All these findings demonstrate on the different characteristics and guest affinity of the two apparently similar resorcin[4]arene and pyrogallol[4]arene hexamers. The present study shows that the combination of advanced NMR methods such as ¹H/¹⁹F diffusion NMR and ¹⁹F-GEST NMR experiments provides new insights on these important and well-studied, catalytically active capsular self-assembled systems.

Author contributions

Y. C. and L. A. conceived the project. T. S. and I. H. performed the synthesis and characterization of the compounds. S. S. performed the ¹⁹F GEST experiments with the supervision of L. A. Y. C. wrote the paper with S. S. and L. A. All authors participated in reviewing the paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Israel Science Foundation (ISF) Grant Number 1006/2019 supported this work.

References

1. F. Hof, S. L. Craig, C. Nuckolls and J. Rebek, Molecular Encapsulation, Angew. Chem., Int. Ed., 2002, 41, 1488.
2. J. Rebek, Molecular Behavior in Small Spaces, Acc. Chem. Res., 2009, 42, 1660.
3. D. Ajami and J. Rebek, More Chemistry in Small Spaces, Acc. Chem. Res., 2012, 46, 990.
4. T. S. Koblenz, J. Wassenaar and J. N. H. Reek, Reactivity Within a Confined Self-Assembled Nanospace, Chem. Soc. Rev., 2008, 37, 247.
5. M. Yoshizawa, J. K. Klosterman and M. Fujita, Functional Molecular Flasks: New Properties and Reactions Within Discrete, Self-Assembled Hosts, Angew. Chem., Int. Ed., 2009, 48, 3418.
6. L. Avram, Y. Cohen and J. Rebek, Recent Advances in Hydrogen-Bonded Hexameric Encapsulation Complexes, Chem. Commun., 2011, 47, 5368.
7. C. J. Brown, F. D. Toste, R. G. Bergman and K. N. Raymond, Supramolecular Catalysis in Metal–Ligand Cluster Hosts, Chem. Rev., 2015, 115, 3012.
8. C. M. Hong, R. G. Bergman, K. N. Raymond and F. D. Toste, Self-Assembled Tetrahedral Hosts as Supramolecular Catalysts, Acc. Chem. Res., 2018, 51, 2447.
9. Q. Zhang, L. Catti and K. Tiefenbacher, Catalysis Inside the Hexameric Resorcinarene Capsule, Acc. Chem. Res., 2018, 51, 2107.
10. L. R. MacGillivray and J. L. Atwood, Chiral Spherical Molecular Assembly Held Together by 60 Hydrogen Bonds, Nature, 1997, 389, 469.
11. J. L. Atwood, L. J. Barbour and A. Jerga, Organization of the Interior of Molecular Capsules by Hydrogen Bonding, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4837.
12. T. Gerkensmeier, W. Iwanek, C. Agena, R. Fröhlich, S. Kotila, C. Näther and J. Mattay, Self-Assembly of 2,8,14,20 -Tetraisobutyl-5,11,17,23-tetrahydroxyresorc[4]-arene, Eur. J. Org. Chem., 1999, 2257.
13. A. Shyvaniuk and J. Rebek, Reversible Encapsulation by Self-Assembling Resorcinarene Subunits, Proc. Natl. Acad. Sci. U. S. A., 2001, 98, 7662.
14. L. Avram and Y. Cohen, Spontaneous Formation of Hexameric Resorcinarene Capsule in Chloroform Solution as Detected by Diffusion NMR, J. Am. Chem. Soc., 2002, 124, 15148.
15. L. Avram and Y. Cohen, The Role of Water Molecules in a Resorcinarene Capsule as Probed by NMR Diffusion Measurements, Org. Lett., 2002, 4, 4365.
16. L. Avram and Y. Cohen, Hexameric Capsules of Lipophilic Pyrogallolarene and Resorcinarene in Solutions as Probed by Diffusion NMR: One Hydroxyl Makes the Difference, Org. Lett., 2003, 5, 3329.
17. L. Avram and Y. Cohen, Self-Recognition, Structure, Stability, and Guest Affinity of Pyrogallol[4]arene and Resorcin[4]arene Capsules in Solution, J. Am. Chem. Soc., 2004, 126, 11556.
18. N. K. Beyeh, M. Kogej, A. Ahman, K. Rissanen and C. A. Schalley, Flying Capsules: Mass Spectrometric Detection of Pyrogallarene and Resocinarene Hexamers, Angew. Chem., Int. Ed., 2006, 45, 5214.
19. E. S. Barett, T. J. Dale and J. Rebek, Stability, Dynamics, and Selectivity in the Assembly of Hydrogen Bonded Hexameric Capsules, J. Am. Chem. Soc., 2008, 130, 2344.
20. I. Horin, S. Slovak and Y. Cohen, Diffusion NMR Reveals the Structures of the Molecular Aggregates of Resorcin[4]arenes and Pyrogallol[4]arenes in Aromatic and Chlorinated Solvents, J. Phys. Chem. Lett., 2022, 13, 10666.
21. S. Fujii, R. Miyake, L. de Campo, J. H. Lee, R. Takahashi and K. Sakurai, Structural Polymorphism of Resorcinarene Assemblies, Langmuir, 2020, 36, 6222.
22. S. Fujii and K. Sakurai, Structural Analysis of an Ocatmeric Resorcinarene Self-Assembly in Toluene and its Morphological Transition by Temperature, J. Phys. Chem. Lett., 2021, 12, 6464.
23. L. Avram and Y. Cohen, Discrimination of Guests Encapsulation in Large Hexameric Molecular Capsules in Solution: Pyrogallol[4]arene versus Resorcin[4]arene Capsules, J. Am. Chem. Soc., 2003, 125, 16180.
24. I. E. Philip and A. E. Kaifer, Electrochemically Driven Formation of Molecular Capsule Around the Ferrocenium Ion, J. Am. Chem. Soc., 2002, 124, 12678.
25. M. Yamanaka, A. Shivanyuk and J. Rebek, Kinetics and Thermodynamics of Hexameric Capsule Formation, J. Am. Chem. Soc., 2004, 126, 2939.
26. S. J. Delgarno, S. A. Tucker, D. B. Bassil and J. L. Atwood, Fluorescent Guest Molecules Report Ordered Inner Phase of Host Capsules in Solution, Science, 2005, 309, 2037.
27. T. Evan-Salem, I. Baruch, L. Avram, Y. Cohen, L. C. Palmer and J. Rebek, Resorcinarenes are Hexameric Capsules in Solution, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 12296.
28. L. Avram and Y. Cohen, Molecules at Close Range: Encapsulated Solvent Molecules in the Pyrogallol[4]arene Hexameric Capsules, Org. Lett., 2006, 8, 219.
29. L. Avram and Y. Cohen, Self-assembly of Resorcin[4]arene in the Presence of Small Alkylammonium Guests in Solution, Org. Lett., 2008, 10, 1505.
30. V. Guralnik, L. Avram and Y. Cohen, Unique Organization of Solvent Molecules Within the Hexameric Capsules of Pyrogallol[4]arene in Solution, Org. Lett., 2014, 16, 5592.
31. L. Avram, A. Goldburt and Y. Cohen, Hexameric Capsules Studied by Magic Angle Spinning Solid State NMR Spectroscopy: Identifying Solvent Molecules in Pyrogallol[4]arene Capsules, Angew. Chem., Int. Ed., 2016, 55, 904.
32. A. Cavarzan, A. Scarso, P. Sgarbossa, G. Strukul and J. N. H. Reek, Supramolecular Control on Chemo- and Regioselectivity via Encapsulation of (NHC)-Au Catalyst Within a Hexameric Self-Assembled Host, J. Am. Chem. Soc., 2011, 133, 2848.
33. G. Bianchini, G. La Sorella, N. Canever, A. Scarso and G. Strukul, Efficient Isonitrile Hydration Through Encapsulation Within a Hexameric Self-assembled Capsule and Selective Inhibition by a Photo-controllable Competitive Guest, Chem. Commun., 2013, 49, 5322.
34. Q. Zhang and K. Tiefenbacher, Hexameric Resorcinarene Capsule is a Brønsted Acid: Investigation and Application to Synthesis and Catalysis, J. Am. Chem. Soc., 2013, 135, 16213.
35. Q. Zhang and K. Tiefenbacher, Terpene Cyclization Catalyzed Inside Self-Assembled Cavity, Nat. Chem., 2015, 7, 197.
36. T. M. Bräuer, Q. Zhang and K. Tiefenbacher, Iminium Catalysis Inside a Self-Assembled Supramolecular Capsule: Scope and Mechanistic Studies, J. Am. Chem. Soc., 2017, 139, 17500.
37. I. Némethová, L.-D. Syntrivanis and K. Tiefenbacher, Molecular Capsule Catalysis: Ready to Address Current Challenges in Synthetic Organic Chemistry?, Chimia, 2020, 74, 561–568.
38. P. La Manna, C. Talotta, G. Floresta, M. De Rosa, A. Soriente, A. Rescifina, C. Gaeta and P. Neri, Mild Friedel-Crafts Reactions inside a Hexameric Resorcinarene Capsule: C-Cl Bond Activation through Hydrogen Bonding to Bridging Water Molecules, Angew. Chem., Int. Ed., 2018, 57, 5423.
39. P. La Manna, M. De Rosa, C. Talotta, A. Rescifina, G. Floresta, A. Sorinete, C. Gaeta and P. Neri, Synergic Interplay Between Halogen Bonding and Hydrogen Bonding in the Activation of a Neutral Substrate in a Nano-confined Space, Angew. Chem., Int. Ed., 2020, 59, 811.
40. S. Gambaro, M. De Rosa, A. Soriente, C. Talotta, G. Floresta, A. Rescifina, C. Gaeta and P. Neri, A Hexameric Resorcinarene Capsule as a Hydrogen Bonding Catalyst in the Conjugate Addition of Pyrroles and Indoles to Nitroalkenes, Org. Chem. Front., 2019, 6, 2339.
41. Q. Zhang, L. Catti, V. R. I. Kaila and K. Tiefenbacher, To Catalyze or not to Catalyze: Elucidation of the Subtle Differences Between the Hexameric Capsules of Pyrogallolarene and Resorcinarene, Chem. Sci., 2017, 8, 1653.
42. S. Merget, L. Catti, G. M. Piccini and K. Tiefenbacher, Requirements for Terpene Cyclization Inside the Supramolecular Resorcinarene Capsule: Bound Water and its Protonation Determine the Catalytic Activity, J. Am. Chem. Soc., 2020, 142, 4400.
43. A. Katlyar, J. C. F. Sovierzoski, P. B. Ciao, A. A. Vartia and W. H. Thompson, Water Plays a Diverse Role in a Hydrogen-Bonded, Hexameric Supramolecular Assemble, Chem. Commun., 2019, 55, 6591.
44. A. Katlyar, J. C. F. Sovierzoski, P. B. Ciao, A. A. Vartia and W. H. Thompson, Water Plays a Dynamical Role in Hydrogen-Bonded, Hexameric Supramolecular Assembly, Phys. Chem. Chem. Phys., 2020, 22, 6167.
45. E. Pahima, Q. Zhang, K. Tiefenbacher and D. T. Major, Discovering Monoterpene Catalysis Inside Nano-capsules with Multiscale Modelling and Experiments, J. Am. Chem. Soc., 2019, 141, 6234.
46. D. A. Pole III, S. Mathew and J. N. Reek, Just Add Water: Modulating the Structure-Derived Acidity of Catalytic Hexameric Resorcinarene Capsules, J. Am. Chem. Soc., 2021, 143, 16419.
47. S. Yariv-Shoushan and Y. Cohen, Encapsulated or Not Encapsulated? Ammonium Salts Can Be Encapsulated in Hexameric Capsules of Pyrogallol[4]arene, Org. Lett., 2016, 18, 936.
48. P. La Manna, C. Talotta, C. Gaeta, Y. Cohen, S. Slovak, A. Rescifina, P. Della Sala, M. De Rosa, A. Sorinete and P. Neri, Supramolecular Catalysis in Confined Space: Making the Pyrogallol[4]arene Capsules Catalytically Active in Non-Competitive Solvent, Org. Chem. Front., 2022, 9, 2453.
49. Y. Cohen, L. Avram and L. Frish, Diffusion NMR in Supramolecular and Combinatorial Chemistry: An Old Parameter New Insights, Angew. Chem., Int. Ed., 2005, 44, 520.
50. L. Avram and Y. Cohen, Diffusion NMR of Molecular Cages and Capsules, Chem. Soc. Rev., 2015, 44, 586.
51. Y. Cohen and S. Slovak, Diffusion NMR for the Characterization, in Solution, of Supramolecular Systems Based on Calixarenes, Resorcinarenes and other Macrocyclic Arenes, Org. Chem. Front., 2019, 6, 1705.
52. Y. Cohen, S. Slovak and L. Avram, Solution NMR of Synthetic Cavity Containing Supramolecular Systems: What We Have Learned on and From?, Chem. Commun., 2021, 57, 8856.
53. S. Frosén and R. A. Hoffmann, Study of Moderately Rapid Chemical Exchange Reactions by Means of Nuclear Magnetic Double Resonance, J. Chem. Phys., 1963, 39, 2892.
54. L. Avram, M. A. Iron and A. Bar-Shir, Amplifying Undetectable NMR Signals to Study Host-Guest Interactions and Exchange, Chem. Sci., 2016, 7, 6905.
55. L. Avram and A. Bar-Shir, ¹⁹F-GEST NMR: Studying Dynamic Interactions in Host–Guest Systems, Org. Chem. Front., 2019, 6, 1503.
56. L. Avram, A. D. Wishard, B. C. Gibb and A. Bar-Shir, Quantifying Guest Exchange in Supramolecular Systems, Angew. Chem., Int. Ed., 2017, 56, 15314.
57. L. Avram, V. Havel, R. Shusterman-Krush, M. A. Iron, M. Zaiss, V. Sindelar and A. Bar-Shir, Dynamic Interactions in Synthetic Receptors: A Guest Exchange Saturation Transfer Study, Chem. – Eur. J., 2019, 25, 1687.
58. R. Shusterman-Krush, L. Grimm, L. Avram, F. Biedermann and A. Bar-Shir, Elucidating Dissociation Activation Energies in Host–Guest Assemblies Featuring Fast Exchange Dynamics, Chem. Sci., 2021, 12, 865.
59. H. M. McConnell, Reaction Rates by Nuclear Magnetic Resonance, J. Chem. Phys., 1958, 28, 430.
60. M. Zaiss, Z. Zu, J. Xu, P. Schuenke, D. F. Gochberg, J. C. Gore, M. E. Ladd and P. Bachert, A Combined Analytical Solution for Chemical Exchange Saturation Transfer and Semi-Solid Magnetization Transfer, NMR Biomed., 2015, 28, 217.
61. M. Zaiss, G. Angelovski, E. Demetriou, M. T. McMahon, X. Golay and K. Scheffler, QUESP and QUEST Revisited – Fast and Accurate Quantitative CEST Experiments, Magn. Reson. Med., 2018, 79, 1708.
62. M. Zaiss and P. Bachert, Exchange-Dependent Relaxation in the Rotating Frame for Slow and Intermediate Exchange – Modeling Off-Resonant Spin-Lock and Chemical Exchange Saturation Transfer, NMR Biomed., 2013, 26, 507.
63. S. Mecozzi and J. Rebek, The 55% Solution: A Formula for Molecular Recognition in Solution State, Chem. – Eur. J., 1998, 4, 1016.
64. S. J. Gibbs and C. S. Johnson, A PFG NMR experiment for Accurate Flow Studies in the Presence of Eddy Currents, J. Magn. Reson., 1991, 93, 395.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details, ¹H and ¹⁹F-NMR spectra of the different samples including ¹H and ¹⁹F diffusion NMR data. See DOI: https://doi.org/10.1039/d4qo00261j

---