Crystal structure of a two-dimensional metal–organic framework assembled from lithium(I) and γ-cyclodextrin

The first metal–organic framework (MOF) formed from lithium(I) and γ-cyclodextrin is reported. The structure is characterized by an unusually low metal/ligand ratio.

The crystal structure of the polymeric title compound, catena-poly [[[diaqualithium]---cyclodextrin(1À)-[aqualithium]---cyclodextrin(1À)] pentadecahydrate], {[Li 2 (C 48 H 79 O 40 ) 2 (H 2 O) 3 ]Á15H 2 O} n , consists of deprotonated -cyclodextrin (CD) molecules assembled by lithium ions into metalorganic ribbons that are cross-linked by multiple O-HÁ Á ÁO hydrogen bonds into sheets extending parallel to (011). Within a ribbon, one Li + ion is coordinated by one deprotonated hydroxyl group of the first -CD torus and by one hydroxyl group of the second -CD torus as well as by two water molecules. The other Li + ion is coordinated by one deprotonated hydroxyl and by one hydroxyl group of the second -CD torus, by one hydroxyl group of the first -CD torus as well as by one water molecule. The coordination spheres of both Li + cations are distorted tetrahedral. The packing of the structure constitute channels along the a axis. Parts of the hydroxymethyl groups in cyclodextrin molecules as well as water molecules show two-component disorder. Electron density associated with additional disordered solvent molecules inside the cavities was removed with the SQUEEZE [Spek (2015). Acta Cryst. C71, [9][10][11][12][13][14][15][16][17][18] routine in PLATON. These solvent molecules are not considered in the given chemical formula and other crystal data. Five out of the sixteen hydroxymethyl groups and one water molecule are disordered over two sets of sites.

Chemical context
Metal-organic frameworks (MOFs) based on cyclodextrin were developed by the Stoddart group and have been known for almost ten years (Smaldone et al., 2010). Many cyclodextrin MOFs with various alkali metal ions have been obtained so far (Patel et al., 2017;Bagabas et al., 2013). Exceptions are lithium ion-based MOFs because all of the compounds obtained that have been reported in the literature contain two different metal ions in the crystal structure (Bagabas et al., 2013;Patel et al., 2017). Lithiumbased MOFs are among the best candidates for electrode materials for lithium-ion batteries because of their high porosity and structural control (Baumann et al., 2019;Sharma et al., 2019). Another potential application of lithium-cyclodextrin MOFs is based on their excellent biocompatibility and low toxicity. Analogous materials with sodium and potassium ions have been studied in the pharmaceutical and biomedicine fields (Han et al., 2018). In view of the importance of the properties of such MOFs, we have successfully synthesized the lithium-based title compound, and report herein its crystal structure.

Supramolecular features
In the crystal structure, the deprotonated -CD molecules are linked by the lithium cations into infinite ribbons (Fig. 2) running parallel to [011] and consolidated by O-HÁ Á ÁO hydrogen bonds into sheets extending parallel to (011). Therefore the crystal structure can be described as that of a two-dimensional MOF. It should be noted that in the asymmetric unit, the top and bottom of the CD-IP torus have inverted positions relative to the top and bottom of the CD-AH torus. The crystal packing shows that in the sheets there are additional 'bottom-to-bottom' intermolecular hydrogenbonding interactions between adjacent tori. However, not all the hydroxyl groups participate in these interactions. For instance, oxygen atoms O2C, O3F, O2H and O2K do not form intermolecular hydrogen bonds. On the other hand, oxygen atom O2G takes part in two intermolecular hydrogen bonds. The asymmetric unit of the title compound drawn with displacement ellipsoids at the 50% probability level. Except for the two Li and coordinating O sites, atomic labels are not shown for clarity.

Figure 2
Ribbons of -CD tori and lithium ions consolidated by O-HÁ Á ÁO hydrogen bonds (not shown) into sheets extending parallel to (011). Channels formed by -CD rings along the a axis. Table 1 Hydrogen-bond geometry (Å , ). (70 mg, 0.054 mmol) were suspended in water (1.0 ml). Lithium hydroxide (31 mg, 1.29 mmol) and -cyclodextrin (70 mg, 0.054 mmol) were dissolved in water (0.5 ml) and added to the suspension. After stirring for several minutes, the solid oxidovanadium(IV) sulfate dissolved to yield a green solution. The flask containing this solution was placed into a sealable container filled with acetone, and crystals were obtained by the vapour diffusion method. The precipitate contained crystals of two different forms. Whereas the large colourless cuboid crystals were not suitable for X-ray diffraction studies since their diffraction intensities were limited to 2 Å , the smaller plate-like colourless crystals were of good quality and were subjected to single-crystal X-ray diffraction analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The atom numbering scheme is as follows: The atoms in the d-glucopyranoside units are numbered according to the rules for sugars, and a suffix from A to P is added at the end of the label to distinguish sixteen different glucose units. Labels of water molecules are marked with a letter W at the end. Several disordered atomic fragments and solvent molecules, as well as a large number of water molecules are present in the crystal structure. To make the refinement stable, it was necessary to apply restraints for the bond lengths (DFIX, SADI), bond angles (DANG), and displacement parameters (SIMU, ISOR) of the disordered moieties. Hydrogen-atom positions of the hydroxyl groups were calculated geometrically and refined using the riding-model approximation, with U iso (H) = 1.5U eq (O). Five out of sixteen hydroxymethyl groups in the two -CD moieties were found to be disordered over two sets of sites. Eighteen oxygen atoms belonging to water molecules were localized from difference-Fourier maps in the space outside the lithium -CD ribbons. Water oxygen atoms O12W and O13W represent two-component positional disorder of a water molecule with refined occupancy factors of 0.578 (12) and 0.422 (12), respectively. Hydrogen atoms were reliably assigned for only eleven of the water molecules. For the other water molecules, modelling of hydrogen atoms lead to unstable refinements, and therefore these oxygen atoms were left as isolated.
Electron density associated with additional disordered solvent molecules inside the cavities was removed by means of the SQUEEZE procedure of PLATON program (Spek, 2015). The solvent-accessible volume is 845 Å 3 , the number of electrons in the cavities being 237. Since the solvent did not contain exclusively water but was a mixture of water and acetone, it was not possible to determine its content from these numbers. Therefore the chemical formula and crystal data given in Table 2 do not take into account these solvent molecules. Mercury (Macrae et al., 2020); software used to prepare material for publication: publCIF (Westrip, 2010).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.