Dynamics in Flexible Pillar[n]arenes Probed by Solid-State NMR.

Ashlea R Hughes,Frédéric Blanc,Ming Liu,Andrew I Cooper,Subhradip Paul

doi:10.1021/acs.jpcc.1c02046

Abstract

Pillar[n]arenes are supramolecular assemblies that can perform a range of technologically important molecular separations which are enabled by their molecular flexibility. Here, we probe dynamical behavior by performing a range of variable-temperature solid-state NMR experiments on microcrystalline perethylated pillar[n]arene (n = 5, 6) and the corresponding three pillar[6]arene xylene adducts in the 100–350 K range. This was achieved either by measuring site-selective motional averaged 13C 1H heteronuclear dipolar couplings and subsequently accessing order parameters or by determining 1H and 13C spin–lattice relaxation times and extracting correlation times based on dipolar and/or chemical shift anisotropy relaxation mechanisms. We demonstrate fast motional regimes at room temperature and highlight a significant difference in dynamics between the core of the pillar[n]arenes, the protruding flexible ethoxy groups, and the adsorbed xylene guest. Additionally, unexpected and sizable 13C 1H heteronuclear dipolar couplings for a quaternary carbon were observed for p-xylene adsorbed in pillar[6]arene only, indicating a strong host–guest interaction and establishing the p-xylene location inside the host, confirming structural refinements.

Highlights

Host−guest chemistry is an important concept in the field of supramolecular chemistry that is driven by the interactions of molecular assemblies or ions via noncovalent interactions.[1]
These interactions play a vital role in the design of advanced functional materials with exciting physical properties and applications in processes, such as adsorption, catalysis, energy storage, and molecular separations
Due to its large conformational flexibility, EtP6 has been found to adsorb a number of guest molecules,[14,23] and we have recently shown that EtP6-β adapts during adsorption of an o-xylene/m-xylene/p-xylene isomer mixture to efficiently capture pX with a high selectivity of 90% to form pX@EtP6 (Figure 1c),[23] while mX@EtP6 (Figure 1d) and oX@EtP6 (Figure 1e) are obtained by adsorption of the respective xylene isomer into EtP6-β

Summary

INTRODUCTION

Host−guest chemistry is an important concept in the field of supramolecular chemistry that is driven by the interactions of molecular assemblies or ions via noncovalent interactions.[1]. Pillar[n]arenes (n = 5,6) have found the greatest interest, mostly due to their relatively small cavity sizes that enable them to host small molecules,[10] combined with substituted alkyl and branched chains that strongly affect the host−guest properties.[19−22] Perethylated pillar[n]arene (n = 5, EtP5; n = 6, EtP6) are examples of these substituted pillar[n]arenes that contain ethoxy groups (Figure 1a,b) with EtP6 existing as two polymorphs, a metastable EtP6-α phase and a crystalline EtP6β phase.[23] Due to its large conformational flexibility, EtP6 has been found to adsorb a number of guest molecules,[14,23] and we have recently shown that EtP6-β adapts during adsorption of an o-xylene (oX)/m-xylene (mX)/p-xylene (pX) isomer mixture to efficiently capture pX with a high selectivity of 90% to form pX@EtP6 (Figure 1c),[23] while mX@EtP6 (Figure 1d) and oX@EtP6 (Figure 1e) are obtained by adsorption of the respective xylene isomer into EtP6-β This is a step forward for the energy efficient separation of the xylene isomers, which are widely used as chemical feedstocks.[23,24]. We exploit variable temperature spin−lattice relaxation measurements to access dynamics in the MHz regime, which confirm the flexibility of the extruding ethoxy groups of these pillar[n]arenes as opposed to the carbon atoms located in the ring core

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES