Metal–Organic Frameworks from Edible Natural Products

Abstract

Metal-organic frameworks (MOFs) represent [1] an extensive class of porous crystals in which organic struts link metalcontaining clusters. The success in controlling the functionality and structure of MOFs has led to numerous applications, [2] most notably gas adsorption, [3] storage of clean gas fuels, [4] catalysis, [5] separations, [6] and drug delivery. [7] However, the vast majority of MOFs described to date are composed of organic struts derived from non-renewable petrochemical feedstocks and transition metals. The challenge in preparing MOFs from natural products lies in the inherent asymmetry of the building units, which are not amenable to crystallization in the form of highly porous frameworks. Herein, we report a strategy to overcome this problem using g-cyclodextrin (g-CD), a symmetrical cyclic oligosaccharide that is mass-produced enzymatically from starch [8] and comprised of eight asymmetric a-1,4-linked dglucopyranosyl residues. These g-CD building units are then linked by potassium ions, in aqueous media at ambient temperature and pressure, to form a body-centered cubic structure, termed CD-MOF-1, which has the empirical formula [(C 48 H 80 O 40 )(KOH) 2 ] n . CD-MOFs can be prepared entirely from edible ingredients: combining food-grade g-CD with salt substitute (KCl) or potassium benzoate (food additive E212) in bottled water and Everclear grain spirit (EtOH) yields porous frameworks which constitute edible MOFs.While there have been a few reports of MOFs assembled from amino acids, [9] nucleobases, [7a, 10] peptides, [11] magnesium formates, [12] and metal glutarates, [13] examples of these materials are not common despite the rapidly growing desire to fabricate MOFs from naturally available building blocks. We suspect that the key to our success in assembling CD-MOFs lies in the symmetric arrangement (C 8 ) within the g-CD torus of eight asymmetric (C 1 ) a-1,4-linked d-glucopyranosyl residues and the ready availability of g-CD as a chiral molecular building block (Figure 1). CD-MOF-1 was prepared by combining 1.0 equiv of g-CD with 8.0 equiv of KOH in aqueous solution, followed by vapor diffusion of MeOH into the solution during 2-7 days, resulting in colorless, cubic, single crystals, suitable for X-ray crystallography, in approximately 70 % yield. Other CD-MOFs were readily obtained using salts of Na + , Rb + , and Cs + , giving rise to an extensive new family of porous materials. A complete list of metal salts employed to form CD-MOFs and the full synthesis of CDMOFs are provided in Section S2 of the Supporting Information.The X-ray crystal structure of CD-MOF-1 [14] reveals that eight-coordinate K + ions not only assist in the assembly of (g-CD) 6 cubes (Figure 2 a,b), wherein six g-CD units occupy the faces of a cube, but they also serve to link these cubes together in a three-dimensional array which extends throughout the crystal (Figure 2 c). The (g-CD) 6 repeating motifs adopt a body-centered cubic packing arrangement wherein each symmetrically equivalent K + ion links two co...

Carbohydrate-Mediated Purification of Petrochemicals

“…CD-MOF-1 contains potassium while CD-MOF-2 incorporates rubidium. CD-MOF-2 crystals were grown 30 by vapor diffusion of MeOH into an aqueous solution of γ-CD and RbOH.…”

“…CD-MOF-1, having particles in four different micrometer-size ranges, was synthesized using a modified literature proceedure. 30 γ-CD (8.15 g, 6.2 mmol) and KOH (2.8 g, 49.7 mmol) were dissolved in H 2 O (250 mL). The solution was filtered through a 45-μm syringe filter and decanted into separate vials (5 mL in each vial).…”

“…The extended metal−organic frameworks, CD-MOF-1 and CD-MOF-2, were prepared according to literature procedures. 30,32 CD-MOF-1. γ-CD (1.30 g, 1 mmol) and KOH (0.45 g, 8 mmol) were dissolved in H 2 O (20 mL).…”

Biologically derived metal organic frameworks

“…For example, the synthesis of γ-CD can be synthesized using a salt substitute (KCl) in bottled water and Everclear grain spirit. [150] γ-CD is mass produced enzymatically from starch, and comprised of eight asymmetric α-1,4-linked Dglucopyranosyl residues. [150] These bio-ligands are linked by potassium ions to form a symmetrical cyclic oligosaccharide, which forms a body-centered cubic structure.…”

Coordination Compounds Based on Metacyclophane Derivatives

“…[3] Coordination networks based on the use of porphyrines, [4] cyclodextrines, [5] calixarenes [6] or the combination of porphyrines with calixarenes [1] macrocycles have been reported. The coordination abilities of unsubstituted calixarenes [8] in order to obtain multinuclear complexes or cages, [9] have also been extensively explored.…”