Hexameric Capsules Studied by Magic Angle Spinning Solid‐State NMR Spectroscopy: Identifying Solvent Molecules in Pyrogallol[4]arene Capsules

Abstract

Powders of pyrogallol [4]arene hexamers were produced by evaporation from organic solvents and were studied, for the first time,b ym agic angle spinning solid-state NMR (MAS ssNMR). Evaporation selectively removed nonencapsulated solvent molecules leaving stable hexameric capsules encapsulating solvent molecules.A fter exposure of the powder to solvent vapors, 1 H/ 13Cheteronuclear correlation MAS ssNMR experiments were used to assign the signals of the external and encapsulated solvent molecules.T he formed capsules were stable for months and the process of solvent encapsulation was reversible.According to the ssNMR experiments,t he encapsulated solvent molecules occupyd ifferent sites and those sites differ in their mobility.T he presented approach paves the way for studying guest exchange,g uest affinity,a nd gas storage in hexamers of this type in the solid state.Self-assembled molecular capsules have attracted much interest over the past two decades. [1,2] Their importance stems, inter alia, from their ability to serve as nanoreactors where new chemistry and catalysis may emerge.O ne intriguing family of self-assembled molecular capsules is resorcin-[4]arenes and pyrogallol[4]arenes hexamers (1 and 2, Figure 1), [3][4][5][6][7] which have been found to be more abundant than previously thought. [8a,b] Resorcin[4]arene hexamers accommodate amuch larger range of guests than pyrogallol-[4]arene hexamers do [8] and several examples of catalysis within the resorcin[4]arene hexamers have been reported. [9] To partially overcome the limited range of guests that can be encapsulated in organic solutions of pyrogallol[4]arene hexamers,Purse and co-workers employed the melting approach; however, this approach is applicable only in certain cases. [10] Thei nformation gathered to date on the hexameric capsules of systems 1 and 2 was obtained in the solid-state by X-ray crystallography [11] or in solution by NMR, diffusion NMR, [12] and small-angle neutron scattering.[13] Schalley and co-workers also studied these hexameric capsules in the gas phase. [14] However,t hese spectroscopic methods afford only limited information on the encapsulated molecules.Solid-state NMR has been used to study macromolecules and supramolecular systems, [15] but magic angle spinning solid-state NMR (MAS ssNMR) has been used only once to study dimeric capsules [15c] and never to study hexameric capsules.MAS ssNMR experiments yield detailed information on the structure and the dynamics of systems that are not amenable to crystallization. In X-ray studies of pyrogallol[4]arene hexamers [4,11] limited information, if any,w as obtained on the encapsulated guest molecules.T herefore in the present study we used MAS ssNMR to study the systems formed by slow evaporation of organic solutions of 2b,f ocusing on solvents which showed peculiar 1 HNMR resonance signals in solution for the encapsulated solvent molecules. [16] Figures 2A and 2B show the 1 Hand extracts of the 13 Ccross-polarization MAS (CPMAS) ssNMR spectra of the system obt...