There is growing interest in the development of molecular containers dedicated to gas trapping in water. Cucurbit[5]uril (CB[5]), the smallest member of a glycoluril macrocycles family (Figure 1), is able to interact with a large array of guest molecules, among which noble gases. The interaction of cucurbit[6]uril (CB[6]) with xenon, a promising noble gas for molecular imaging applications, was investigated in aqueous solution containing acids or salts designed to increase the solubility of the host molecule. However, it was shown that cations interact with the partial charges on oxygen atoms, while apolar molecules are encapsulated in the hydrophobic cavity of cucurbiturils. We already evidenced that the presence of these acids or salts alters the xenon in/out exchange for CB[6] (Figure S1 of the Supporting Information), confirming results obtained with CB*[6] (Figure 1a). In the literature, observation of xenon binding by cucurbituril derivatives in pure water was previously rendered possible only by grafting substituents, giving MeCB[5] and CB*[6] (Figure 1a). In order to understand the thermodynamics and kinetics of xenon binding in CB[5] , it seemed thus useful to study samples in deionized water. Although xenon binding in a crystalline form of native CB[5] was observed, no quantitative kinetics and thermodynamics data were reported for CB[5] or a derivative in aqueous solution. Herein, taking advantage of a solubility not so low in pure water, we show that CB[5] spontaneously incorporates xenon at 316 K with a quite high binding constant and a low in/out exchange rate. This exchange rate is even measurable at 293 K. In the presence of xenon, the H NMR spectrum of a D2O solution of CB[5] exhibits two series of signals characteristic of a slow exchange between two environments, one with and the other without xenon (Figure 1c). The Xe spectrum shows two signals (Figure 1d) also denoting a slow exchange. The signal at 196 ppm is assigned to free xenon in water and the signal at 225 ppm to xenon encapsulated in CB[5] (Xe@CB[5]), at a chemical shift close to that already observed in a crystal form of the molecule. The second signal exhibits a linear dependence of chemical shift with temperature between 277 and 334 K, with a slope of 117 3 ppbK 1 (Figure S2, Supporting Information). This may easily be explained by a deeper exploration of the CB[5] cavity by xenon in areas affected by the magnetic anisotropy effect of the carbonyl groups at higher temperatures. The xenon in/out exchange is also slow on the xenon longitudinal relaxation (T1) timescale. The T1 value of trapped Xe increases from 15 3 s to 28 5 s when temperature varies from 277 K to 315 K. The rather long T1 confirms previous results on CB*[6] and reflects the weakness of the proton– xenon dipolar interaction in a system where protons point outwards. The observed T1 value cannot be entirely explained by this mechanism, and other relaxation mechanisms such as chemical shift modulation must also be efficient. To our knowledge, CB[5] is the first example of a host molecule with which the xenon in/out exchange is slow on the relaxation timescale. This kinetics was characterized in more detail through gas-release experiments. Proton NMR spectra were recorded at repeated time intervals, in a situation where dissolved xenon tends to equilibrate with gaseous xenon. Figure 1. a) General formula of the cucurbiturils cited in the text. CB[5]: n=5, R=R’=H; CB[6]: n=6, R=R’=H. MeCB[5]: n=5, R=R’=CH3; CB*[6]: n=6, RR’= (CH2)4. b) Space-fill representation of a crystal structure of CB[5] , with both facing glycoluril units and attached methylene groups removed for clarity. A xenon atom is superimposed in the center of the molecule showing the geometry of the complex. c) H NMR spectrum of a 5.4 mm solution in D2O of CB[5] under 2.5 atm of xenon at 316 K. Signals assigned to CB[5] entrapping a xenon atom are denoted by *, while those standing for xenon-free cages are denoted by *. d) Xe NMR spectrum of a 0.25 mm D2O solution of CB[5] under 6 atm of xenon at 316 K at thermodynamic equilibrium, 20500 scans with a repetition rate of 3.8 s, Fourier transformed with a 6 Hz line broadening.
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Oct 29, 2022
Qiang Yu + 8
Open Access