Probing²⁷Al–¹³C proximities in metal–organic frameworks using dynamic nuclear polarization enhanced NMR spectroscopy

Abstract

We show how 27 Al-13 C proximities in the microporous metal-organic framework MIL-100(Al) can be probed using advanced 27 Al-13 C NMR methods boosted by Dynamic Nuclear Polarization.Metal-organic frameworks (MOFs) have attracted increasing attention owing to their high surface area and their wide range of three-dimensional (3D) architectures that exhibit different pore apertures and chemical tunabilities. These hybrid materials represent promising systems for gas storage, catalysis, capture of radioactive compounds, and drug delivery. 1 Amongst them, aluminium-based MOFs, such as MIL-100(Al), 2 present many advantages, including low cost, density and toxicity, remarkable thermal stability and high Lewis acidity that should allow many industrial applications. 3 Rational design of MOFs requires a clear understanding of structure-property relationships and hence calls for characterization methods endowed with atomic resolution. In particular, solid-state NMR is well suited to assess structural models of MOF frameworks, to probe the organization of extra-framework entities, 4 and to analyze atomic-level dynamics. 5,6 However, the poor sensitivity of NMR limits the observation of adsorbed species, defects, etc., particularly when the observed nuclei have low gyromagnetic ratios, low natural abundances or long longitudinal relaxation times (T 1 ). Besides, the observation of 27 Al-13 C proximities by NMR in Al-based MOFs has hitherto been hindered by limitations of common NMR probes, which cannot be tuned simultaneously to nearby 27 Al and 13 C Larmor frequencies. We report here the first observation of 27 Al-13 C proximities in MOFs using advanced NMR methods combined with a frequency splitter. 7,8 We show that Dynamic Nuclear Polarization (DNP) can significantly reduce acquisition times for MIL-100(Al) at 100 K. EPR spectroscopy and DNP enhancements prove that the biradical TOTAPOL 9 enters the MIL-100(Al) cavities within tens of minutes, although its apertures are smaller than 0.88 nm. Besides, DNP-enhanced 27 Al-13 C correlation experiments prove that the MIL-100(Al) framework is not altered by impregnation with a TOTAPOL solution. So far, DNP of MOFs has only been demonstrated for 1D spectra under Magic-Angle Spinning (MAS) using 1

Is solid-state NMR enhanced by dynamic nuclear polarization?

“…There, distances between the 119 Sn nuclei and 13 C nuclei could be probed (using natural abundance 13 C{ 119 Sn} REDOR experiments), confirming the surface presence of alkyl tin moieties [61]. In this direction, 13 C- 27 Al distances were also analyzed using the sensitivity enhancement of MAS-DNP (and natural abundance 13 C{ 27 Al} S-RESPDOR experiments) for the MIL-100(Al) MOF [78]. Furthermore, 2D 27 Al-13 C dipolar correlation experiments of this MOF (see Fig.…”

“…To conclude on porous materials, there have been other recent successful studies on systems such as Sn-β zeolites [34,79] and metal-organic frameworks (MOFs) [77,78], which have vast surface areas (4400 m 2 g À 1 ) and few overlapping resonances and are therefore also highly pertinent for MAS-DNP analysis. In fact, for the Sn-β zeolite there are three overlapping 119 Sn resonances but these can be resolved using the improved sensitivity with MAS-DNP in combination with 2D CP magic-angle turning experiments, which correlate an isotropic dimension with the directly detected (anisotropic) dimension [34].…”

“…Furthermore, 2D 27 Al-13 C dipolar correlation experiments of this MOF (see Fig. 11a) could be quickly recorded ( $ 7 h) owing to a timesaving factor of 100 with the MAS-DNP, and validated the 13 C chemical shift assignment [78]. The MIL-100(Al) was impregnated with H 2 O containing 16 mM TOTAPOL for the DNP experiments.…”

“…Enhancing the polarization of the bulk nuclei of a material is difficult as the hyperpolarization needs to be propagated into the bulk [76] and is the reason that predominantly surfaces [29] or materials with a small amount of bulk [34,38,59,[77][78][79] have been employed for MAS-DNP analysis. This is especially true when the bulk is devoid of protons, which can usually transport the hyperpolarization by relatively efficient spin diffusion from the proximity of the polarizing agents into the bulk of a material when the system possesses a sufficiently long 1H T 1n [80].…”

Aluminum metal–organic framework as a new host for preparation of encapsulated metal complex catalysts

“…It revealed typical 13 C spectra of MIL-100(Al) host and CuPhen complex. The overlapped signals in the chemical shift range of 132-140 ppm are assigned to the aromatic carbons from MIL-100(Al) host or the Phen ligand and the signal at 171 ppm is assigned to the carboxylate groups of framework trimesate BTC [26,28,29]. The two weak signals at 126.5 and 148.8 ppm are assigned to the chemical shifts in the Phen ligand [30,31].…”