The synthesis and structural flexibility of the metal-organic frameworks M 2 (2,6-ndc) 2 (dabco) (DUT-8(M), M ¼ Ni, Co, Cu, Zn; 2,6-ndc ¼ 2,6-naphthalenedicarboxylate, dabco ¼ 1,4-diazabicyclo[2.2.2] octane) as well as their characterization by gas adsorption, 129 Xe NMR and 13 C MAS NMR spectroscopy are described. Depending on the integrated metal atom the compounds show reversible (DUT-8(Ni), DUT-8(Co)), non-reversible (DUT-8(Zn)) or no (DUT-8(Cu)) structural transformation upon solvent removal and/or physisorption of several gases. DUT-8(Co) exhibits a similar structural transformation by solvent removal and adsorption behavior as observed for DUT-8(Ni). DUT-8(Zn) undergoes an irreversible structural change caused by solvent removal. The non-flexible copper containing MOF reveals the best performance concerning porosity and gas storage capacities within the DUT-8 series. Xenon adsorption studies combined with 129 Xe NMR spectroscopy are used to study the flexibility of the DUT-8 compounds. 129 Xe chemical shift and line width strongly depend on the metal atom. Solid-state 13 C NMR spectroscopy has been applied in order to further characterize the organic parts of the DUT-8 frameworks. While DUT-8(Ni) exhibits narrow, well-resolved lines in its ''as made'' state, the signals of are broadened and shifted over an unusually wide chemical shift range (À72 to 717 ppm). No detectable signals are found in DUT-8(Cu) indicating significantly changed internal dynamics compared to and .

Nuclear Magnetic Resonance (NMR) spectroscopy is a well-established method for the investigation of various types of porous materials. During the past decade, metal–organic frameworks have attracted increasing research interest. Solid-state NMR spectroscopy has rapidly evolved into an important tool for the study of the structure, dynamics and flexibility of these materials, as well as for the characterization of host–guest interactions with adsorbed species such as xenon, carbon dioxide, water, and many others. The present review introduces and highlights recent developments in this rapidly growing field.

Recently, we have described the metal-organic framework Ni(2)(2,6-ndc)(2)(dabco), denoted as DUT-8(Ni) (1) (DUT = Dresden University of Technology, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). Upon adsorption of molecules such as nitrogen and xenon, this material exhibits a pronounced gate-pressure effect which is accompanied by a large change of the specific volume. Here, we describe the use of high-pressure in situ (129)Xe NMR spectroscopy, i.e., the NMR spectroscopic measurements of xenon adsorption/desorption isotherms and isobars, to characterize this effect. It appears that the pore system of DUT-8(Ni) takes up xenon until a liquid-like state is reached. Deeper insight into the interactions between the host DUT-8(Ni) and the guest atom xenon is gained from ab initio molecular dynamics (MD) simulations. van der Waals interactions are included for the first time in these calculations on a metal-organic framework compound. MD simulations allow the identification of preferred adsorption sites for xenon as well as insight into the breathing effect at a molecular scale. Grand canonical Monte Carlo (GCMC) simulations have been performed in order to simulate adsorption isotherms. Furthermore, the favorable influence of a sample pretreatment using solvent exchange and drying with supercritical CO(2) as well as the influence of repeated pore opening/closure processes, i.e., the "aging behavior" of the compound, can be visualized by (129)Xe NMR spectroscopy.

The linker functionalization strategy was applied to incorporate proline moieties into a metal−organic framework (MOF). When 4,4′-biphenyldicarboxylic acid was replaced with a Boc-protected proline-functionalized linker (H 2 L) in the synthesis of DUT-32 (DUT = Dresden University of Technology), a highly porous enantiomerically pure MOF (DUT-32-NHProBoc) was obtained, as could be confirmed by enantioselective high-performance liquid chromatography (HPLC) measurements and solid-state NMR experiments. Isotope labeling of the chiral side group proline enabled highly sensitive one-and two-dimensional solid-state 13 C NMR experiments. For samples loaded with (S)-1-phenyl-2,2,2-trifluoroethanol [(S)-TFPE], the proline groups are shown to exhibit a lower mobility than that for (R)-TFPE-loaded samples. This indicates a preferred interaction of the shift agent (S)-TFPE with the chiral moieties. The high porosity of the compound is reflected by an exceptionally high ethyl cinnamate adsorption capacity. However, postsynthetic thermal deprotection of Boc−proline in the MOF leads to racemization of the chiral center, which was verified by stereoselective HPLC experiments and asymmetric catalysis of aldol addition. ■ INTRODUCTIONIt has been widely accepted that enantiomers of the same chiral compound may show quite different bioactivities, as well as different physical, chemical, pharmacological, and toxicological properties. Therefore, their preparation and especially analysis are important in many fields of science. To obtain a chiral heterogeneous selector/catalyst, usually the porous substrates are coated or immobilized with proteins, enzymes, oligosaccharides, or polysaccharides. Because important progress has been made in the field of porous materials through the development of metal−organic frameworks (MOFs), this material class is also considered to be very promising for the design and development of a new generation of chiral porous materials. The incorporation of chiral groups by ligand or cluster functionalization or the synthesis of chiral MOFs from achiral building blocks generates homochiral materials. Inspired by homogeneous chiral catalysis, the implementation of wellestablished chiral catalytically active groups as linkers or as side groups of the framework backbone has been reported for a variety of MOFs. 1−5 One common organocatalytic group for a wide range of asymmetric reactions is proline. Up to now, there are only a few homochiral MOFs with proline functionalities reported. So far, the incorporation of proline could be performed by coordination on open metal sites in MIL-101, 6 postsynthetic click reaction, 7 or postsynthetic amide coupling. 8 Furthermore, the incorporation of proline by chiral functionalization of the linker before MOF synthesis was carried out by Telfer et al. 9 The reported homochiral material could be used as heterogeneous chiral catalysts in asymmetric aldol reactions. The important point in the synthesis of enantiopure (prolinecontaining) MOFs is to avoid the racemization that can ...

Recently, we have described the synthesis of chiral metal-organic frameworks iPr-ChirUMCM-1 and Bn-ChirUMCM-1 and their use in enantioselective separation. Here, we demonstrate for the first time the use of a chiral solvating agent (1-phenyl-2,2,2-trifluoroethanol, TFPE) for chiral recognition in iPr-ChirUMCM-1 and Bn-ChirUMCM-1 by means of solid-state(13)C NMR spectroscopy.