Chiral Metal-organic Frameworks Research Articles

Sixteen isostructural phosphonate metal-organic frameworks with controlled Lewis acidity and chemical stability for asymmetric catalysis

Heterogeneous catalysts typically lack the specific steric control and rational electronic tuning required for precise asymmetric catalysis. Here we demonstrate that a phosphonate metal–organic framework (MOF) platform that is robust enough to accommodate up to 16 different metal clusters, allowing for systematic tuning of Lewis acidity, catalytic activity and enantioselectivity. A total of 16 chiral porous MOFs, with the framework formula [M3L2(solvent)2] that have the same channel structures but different surface-isolated Lewis acid metal sites, are prepared from a single phosphono-carboxylate ligand of 1,1′-biphenol and 16 different metal ions. The phosphonate MOFs possessing tert-butyl-coated channels exhibited high thermal stability and good tolerances to boiling water, weak acid and base. The MOFs provide a versatile family of heterogeneous catalysts for asymmetric allylboration, propargylation, Friedel–Crafts alkylation and sulfoxidation with good to high enantioselectivity. In contrast, the homogeneous catalyst systems cannot catalyze the test reactions enantioselectively.

A Ni(salen)‐Based Metal–Organic Framework: Synthesis, Structure, and Catalytic Performance for CO2 Cycloaddition with Epoxides

A three‐dimensional (3D) chiral metal–organic framework [Cd2{Ni(salen)}(DMF)3]·4DMF·7H2O (1) based on a new enantiopure tetracarboxyl‐functionalized metallosalen Ni(H4salen) {where H6salen is (R,R)‐N,N′‐bis[3‐tert‐butyl‐5‐(3,5‐dicarboxybenzyl)salicylidene]‐1,2‐diphenylethylenediame} was synthesized and characterized by infrared spectroscopy, thermogravimetric analysis, nitrogen and carbon dioxide adsorption, and powder and single‐crystal X‐ray diffractions. In 1, the dinuclear Cd2 cluster [Cd2(COO)4(DMF)3] as a node is cross‐linked by four isophthalate groups on the salen ligands, forming a 2D lamellar structure, which are further linked by Ni(salen) into the 3D network with a 1D open channel (ca. 7.0 × 8.0 Å2) along the a axis. On account of its porosity, Lewis acid sites, and moderate uptake for CO2, 1 can be used as an efficient heterogeneous catalyst for the CO2 cycloaddition with epoxides under relatively mild conditions. Moreover, the bulky epoxide shows a decrease in activity with an increase in the alkyl chain length of the substrate as a result of the confinement effect of 1, showing size‐dependent selectivity.

Chiral Cadmium(II) Metal-Organic Framework from an Achiral Ligand by Spontaneous Resolution: An Efficient Heterogeneous Catalyst for the Strecker Reaction of Ketones.

A thermally stable cadmium-based chiral metal-organic framework (MOF), {[Cd2(L)(H2O)(DMF)]·3DMF·2H2O}n (1; DMF = N,N-dimethylformamide), has been synthesized from an achiral ligand by spontaneous resolution. The MOF features 1D open channels with a large density of active metal sites and has a 3,6-c binodal net with a rare sit 3,6-conn topology. The metal-bound water and DMF solvents could be easily removed along with the guest molecules in the lattice upon activation to afford the desolvated framework 1'. It exhibits microporous nature, as confirmed by the gas-sorption measurements with carbon dioxide uptake of 43.2 cm3 g-1 at 273 K. The open metal sites in the framework make it an outstanding heterogeneous catalyst in the Strecker reaction for the synthesis of α-aminonitriles in a solvent-free state at room temperature with excellent conversion yields.

The Power of Heterometalation through Lithium for Helix Chain-Based Noncentrosymmetric Metal–Organic Frameworks with Tunable Second-Harmonic Generation Effects

We demonstrate herein a general strategy for developing acentric or chiral nonlinear optical MOFs from centrosymmetric ligands by shifting the asymmetrical roles to the metal centers through heterometalation. With 1,3,5-benzenetricarboxylic acid (H3BTC), a very common polyfunctional centric ligand, the Li+/Zn2+, Li+/Co2+, or Li+/Cd2+ heterometallic combinations successfully led to five new noncentrosymmetric metal–organic frameworks, namely, {[Zn(HBTC)(H2O)]}n (SNNU-71), {Li4[Cd2(HBTC)(BTC)2]}n (SNNU-72), {CoLi(BTC)(DMA)2}n (SNNU-73), {CdLi2(BTC)4/3(H2O)3}n (SNNU-74), and {Li2[Zn3Li5(BTC)4(MTAZ)(H2O)4]}n (MTAZ = 5-methyl tetrazole, SNNU-75). All these MOFs are acentric or chiral on the base of helical chain building blocks and four colorless members of them show remarkable and tunable SHG effects. Remarkably, more than 1000 metal-BTC frameworks are reported to date and most of them are centrosymmetric due to the C3-symmery of BTC ligands. Clearly, our heterometalation approach greatly increases the possib...

Enantioselective Recognition of Ammonium Carbamates in a Chiral Metal-Organic Framework.

Chiral metal-organic frameworks have attracted interest for enantioselective separations and catalysis because of their high crystallinity and pores with tunable shapes, sizes, and chemical environments. Chiral frameworks of the type M2(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) seem particularly promising for potential applications because of their excellent stability, high internal surface areas, and strongly polarizing open metal coordination sites within the channels, but to date these materials have been isolated only in racemic form. Here, we demonstrate that when appended with the chiral diamine trans-1,2-diaminocyclohexane (dach), Mg2(dobpdc) adsorbs carbon dioxide cooperatively to form ammonium carbamate chains, and the thermodynamics of CO2 capture are strongly influenced by enantioselective interactions within the chiral pores of the framework. We further show that it is possible to access both enantiomers of Mg2(dobpdc) with high enantiopurity (≥90%) via framework synthesis in the presence of varying quantities of d-panthenol, an inexpensive chiral induction agent. Investigation of dach-M2(dobpdc) samples following CO2 adsorption-using single-crystal and powder X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and density functional theory calculations-revealed that the ammonium carbamate chains interact extensively with each other and with the chiral M2(dobpdc) pore walls. Subtle differences in the non-covalent interactions accessible in each diastereomeric phase dramatically impact the thermodynamics of CO2 adsorption.

Synthetic strategies for chiral metal-organic frameworks

In recent years, metal-organic frameworks (MOFs) have attracted great attention owing to their potential applications such as in gas storage and separation, catalysis, luminescence and nonlinear optics. Chirality is widespread in the nature and chiral MOFs can be used in chiral recognition, chiral separation, chiral catalysis and so on. In this review, the synthetic strategies of chiral MOFs are briefly summarized. The advantages and disadvantages of those strategies and their applications are discussed.

Boosting Chemical Stability, Catalytic Activity, and Enantioselectivity of Metal-Organic Frameworks for Batch and Flow Reactions.

A key challenge in heterogeneous catalysis is the design and synthesis of heterogeneous catalysts featuring high catalytic activity, selectivity, and recyclability. Here we demonstrate that high-performance heterogeneous asymmetric catalysts can be engineered from a metal-organic framework (MOF) platform by using a ligand design strategy. Three porous chiral MOFs with the framework formula [Mn2L(H2O)2] are prepared from enantiopure phosphono-carboxylate ligands of 1,1'-biphenol that are functionalized with 3,5-bis(trifluoromethyl)-, bismethyl-, and bisfluoro-phenyl substituents at the 3,3'-position. For the first time, we show that not only chemical stability but also catalytic activity and stereoselectivity of the MOFs can be tuned by modifying the ligand structures. Particularly, the MOF incorporated with -CF3 groups on the pore walls exhibits enhanced tolerance to water, weak acid, and base compared with the MOFs with -F and -Me groups. Under both batch and flow reaction systems, the CF3-containing MOF demonstrated excellent reactivity, selectivity, and recyclability, affording high yields and enantioselectivities for alkylations of indoles and pyrrole with a range of ketoesters or nitroalkenes. In contrast, the corresponding homogeneous catalysts gave low enantioselectivity in catalyzing the tested reactions.

Superficial Chiral Etching on Achiral Metal-Organic Framework for Enantioselective Sorption.

Chiral recognition and separation is of general research interests in natural product separation and the pharmacy industry. In this work, we develop a novel strategy to modify achiral porous metal-organic framework (MOF) surfaces via a superficial chiral etching process (SCEP), in which reacting a presynthesized achiral MOF with a chiral ligand produces an achiral@chiral MOF core-shell hybrid composition. SCEP creates chiral species on an achiral porous MOF surface but does not change the porosity and pore structure, enabling core-shell composition enantioselective sorption. Reacting (+)-camphoric acid, (+)-Cam, and 1,4-diazabicyclo[2.2.2]octane (Dabco) with [Cu3(Btc)2] microcrystals leads to a chiral MOF of [Cu2((+)-Cam)2Dabco] crystallites attached on the surface of [Cu3(Btc)2] (Btc = 1,3,5-benzenetricarboxylate). The resulting [Cu3(Btc)2]@[Cu2((+)-Cam)2Dabco] core-shell composition displays preferred sorption kinetics toward (S)-limonene against (R)-limonene, with a similar discrimination effect with pure chiral [Cu2((+)-Cam)2Dabco]. Superficial chiral etching of the porous achiral MOF represents an economic and efficient strategy for enantioselective separation.

Optically active derivatives of terephthalic acid: synthesis and crystal structures

New optically active derivatives of terephthalic acid, which are of interest as precursors of chiral metal-organic frameworks (MOFs), were synthesized starting from available dimethyl 2-aminoterephthalate, N-Boc-l-alanine, and N-Boc-l-proline. The crystal structures of three samples were determined by powder X-ray diffraction. In all three structures, the acid molecules exist as zwitterions.

Tunable chiral metal organic frameworks toward visible light-driven asymmetric catalysis.

A simple and effective strategy is developed to realize visible light-driven heterogeneous asymmetric catalysis. A chiral organic molecule, which only has very weak catalytic activity in asymmetric α-alkylation of aldehydes under visible light, is utilized as the ligand to coordinate with different types of metal ions, including Zn2+, Zr4+, and Ti4+, for construction of crystalline metal organic frameworks (MOFs). Impressively, when used as heterogeneous catalysts, all of the synthesized MOFs exhibit markedly enhanced activity. Furthermore, the asymmetric catalytic performance of these MOFs could be easily altered by selecting different metal ions, owing to the tunable electron transfer property between metal ions and chiral ligands. This work will provide a new approach for fabrication of heterogeneous catalysts and trigger more enthusiasm to conduct the asymmetric catalysis driven by visible light.

Chiral [Mo8O26]4- Polyoxoanion-Induced Three-Dimensional Architectures With Homochiral Eight-Fold Interpenetrated Metal-Organic Frameworks.

A pair of novel chiral compounds based on homochiral 8-fold interpenetrated metal-organic frameworks (MOFs) and chiral polyoxometalates (POMs), formulated as d-[Cu(4,4'-bipy)1.5]4[Mo8O26] (d-1) and l-[Cu(4,4'-bipy)1.5]4[Mo8O26] (l-1) (4,4'-bipy = 4,4'-bipyridine), have been successfully synthesized and characterized by single crystal X-ray diffraction, infrared, thermogravimetric analysis, elemental analysis, and solid circular dichroism spectra. Structural analysis indicates that two compounds are enantiomers. The connection of copper cations and 4,4'-bipy ligands generates helical infinite chains, while adjacent chains are further linked by Cu-N bonds to form a three-dimensional interpenetrating framework with a (10,3)-a topology. Interestingly, the chiral [Mo8O26]4- polyanions are encapsulated in the chiral MOFs via sharing the oxygen atoms. Both d- and l-[Cu(4,4'-bipy)1.5]4[Mo8O26] crystallize in chiral space group C2221. Additionally, two compounds represent new examples of chiral self-assembly.

Series of chiral interpenetrating 3d–4f heterometallic MOFs: Luminescent sensors and magnetic properties

Series of chiral 3d-4f heterometallic MOFs based on a multidentate terpyridyl carboxylic acid ligand have been synthesized under the solvothermal conditions, namely, [LnZnL(CO3)2(H2O)]n (Ln = Eu (1), Gd (2), Dy (3), Ho (4), Er (5), Tm (6), Yb (7), Lu (8)) (HL = 4′-(4-carboxyphenyl)−2,2′:6′,2″-terpyridine). Compounds 1–8 were structurally characterized by the elemental analyses, infrared spectra, and single crystal X-ray diffractions. Compounds 1–8 exhibit the chiral interpenetrating 3D frameworks. Interestingly, 1 can serve as the luminescent sensor to detect nitrobenzene molecules with high sensitivity. The investigations on CD spectra of single crystals clearly assigned the Cotton effect, indicating that there exist two chiral enantiomers of 1–8 in the course of crystallization. The magnetic properties of 2 and 7 were exploited, respectively.

Calcium l-Lactate Frameworks as Naturally Degradable Carriers for Pesticides.

Two porous, chiral metal-organic frameworks (MOFs), Ca14(l-lactate)20(acetate)8(C2H5OH)(H2O) (MOF-1201) and Ca6(l-lactate)3(acetate)9(H2O) (MOF-1203), are constructed from Ca2+ ions and l-lactate [CH3CH(OH)COO-], where Ca2+ ions are bridged by the carboxylate and hydroxyl groups of lactate and the carboxylate group of acetate to give a three-dimensional arrangement of Ca(-COO, -OH) polyhedra supporting one-dimensional pores with apertures and internal diameters of 7.8 and 9.6 Å (MOF-1201) and 4.6 and 5.6 Å (MOF-1203), respectively. These MOFs represent the first examples of extended porous structures based on Ca2+ and lactate. They show permanent porosity of 430 and 160 m2 g-1, respectively, and can encapsulate an agriculturally important fumigant, cis-1,3-dichloropropene. MOF-1201 shows a 100 times lower release rate compared with liquid cis-1,3-dichloropropene under the same test conditions (25 °C, air flow rate of 1 cm3 min-1). The hydrolysis of MOF-1201 in water makes it the first example of a degradable porous solid carrier for such fumigants.

Postsynthetic Approach for the Rational Design of Chiral Ferroelectric Metal-Organic Frameworks.

Ferroelectrics (FEs) are materials of paramount importance with a wide diversity of applications. Herein, we propose a postsynthetic methodology for the smart implementation of ferroelectricity in chiral metal-organic frameworks (MOFs): following a single-crystal to single-crystal cation metathesis, the Ca2+ counterions of a preformed chiral MOF of formula Ca6II{CuII24[(S,S)-hismox]12(OH2)3}·212H2O (1), where hismox is a chiral ligand derived from the natural amino acid l-histidine, are replaced by CH3NH3+. The resulting compound, (CH3NH3)12{CuII24[(S,S)-hismox]12(OH2)3}·178H2O (2), retains the polar space group of 1 and is ferroelectric below 260 K. These results open a new synthetic avenue to enlarge the limited number of FE MOFs.

Rational Synthesis of Chiral Metal-Organic Frameworks from Preformed Rodlike Secondary Building Units.

The lack of rational design methodologies to obtain chiral rod-based MOFs is a current synthetic limitation that hampers further expansion of MOF chemistry. Here we report a metalloligand design strategy consisting of the use, for the first time, of preformed 1D rodlike SBUs (1) for the rational preparation of a chiral 3D MOF (2) exhibiting a rare eta net topology. The encoded chiral information on the enantiopure ligand is efficiently transmitted first to the preformed helical 1D building block and, in a second stage, to the resulting chiral 3D MOF. These results open new routes for the rational design of chiral rod-based MOFs, expanding the scope of these unique porous materials.

Synthesis of the homochiral metal–organic framework DUT-129 based on a chiral dicarboxylate linker with 6 stereocenters

A dicarboxylate linker with an exceptionally high density of stereogenic centers for the development of new chiral metal–organic frameworks (MOFs) was synthesized. A bicyclic functionality forms the backbone of the linker, creating a linear structural analogue of terephthalic acid. The linker includes two side chains bearing secondary amine functionalities, which could induce catalytic activity of the resulting MOF structures for nucleophilic organocatalysis. The new chiral linker was used to synthesize a zinc-based MOF structure, named DUT-129. Single crystal X-ray analysis identified the porous framework with a sodalite (sod) topology. The bulky side groups of the chiral linker limit the pore accessibility of DUT-129, which was proven by liquid phase adsorption experiments. The calculated pore limiting diameter of 5.2 A indicates pore accessibility exclusive for small molecules, such as linear and branched alkanes. Therefore, the new MOF DUT-129 is expected to show high size selectivity in adsorption and separation applications in combination with high enantiomeric discrimination.

A series of chiral metal–organic frameworks based on fluorene di- and tetra-carboxylates: syntheses, crystal structures and luminescence properties

A series of optically pure carboxylate ligands based on a fluorene backbone were synthesized and used for the elaboration of four chiral metal–organic frameworks (6, 7, 8, and 9) containing Cu-based paddle-wheels. The positions and numbers of coordinating carboxylate functions on the ligand core govern the resulting MOF topologies, leading to a 1D framework (8), 2D grids with (7) or without interpenetration (6), and a 3D (9) framework and showing calculated porosities ranging from 8 to 47% of the unit-cell volumes. Ligands as well as corresponding MOFs possess luminescence properties with blue emission. The most porous MOF (9), made of a tetracarboxylate ligand, exhibits a BET apparent surface area of 467 m2 g−1 and methane and carbon dioxide storage capacities of 26.5 cm3 g−1 and 129 cm3 g−1, respectively, at 303 K and 30 bar.

A chiral salen-based MOF catalytic material with high thermal, aqueous and chemical stabilities.

A highly stable chiral salen-based metal-organic framework [(Cu4I4)2L4]·20DMF·3CH3CN (1) [L = (R,R)-N,N'-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)-1,2-diphenylethylenediamine nickel(ii)] has been synthesized and characterized by single crystal X-ray diffraction and other physicochemical methods. 1 exhibits a rare 8-fold interpenetrated 3D framework constructed by a 4-connecting Cu4I4 cluster and a 2-coordinating L ligand. Remarkably, in spite of 8-fold interpenetration, 1 still possesses two types of 1D chiral hydrophobic channels with pore window sizes of 6.77 × 8.64 Å2 and 6.09 × 10.96 Å2 along the crystallographic a axis. All Ni(salen) moieties of L lie inside the 1D channels and the empty coordination sites of Ni2+ are oriented to the cavities. PXRD and N2 adsorption measurements confirmed that 1 is extremely stable under high temperature (>400 °C), in water vapor (90% relative humidity), in acid/base aqueous solution (pH 0-14), and in saturated NaOH solution at 100 °C, as well as in 30 wt% H2O2 and 70 wt% tert-butyl hydroperoxide solution. 1 was proved to be an excellent recycled heterogeneous catalyst for the conversion of simulated industrial CO2 (that is, involving tiny amounts of water vapor and other acidic gases) with epoxides into cyclic carbonates under mild conditions for the first time. The synthesis of β-hydroxy-1,2,3-triazoles from the same epoxides, alkyne and sodium azide was also catalyzed by 1 in aqueous solution with high yield. Interestingly, the cycloaddition reaction of CO2 to bulky epoxides shows a decrease in the activity with an increase in the alkyl chain length of the substrate because of confinement of the channel size of 1, showing size-dependent selectivity. The plausible catalytic mechanisms for these two reactions have also been proposed.

Chiral binary metal-organic frameworks for asymmetric sequential reactions.

Two chiral porous MOFs with precise and appropriate spatial arrangements of different metallosalen active sites are synthesized and used as efficient heterogeneous catalysts for asymmetric sequential alkene epoxidation/epoxide ring-opening reactions with enantioselectivity of up to 99%.

A pair of 3D enantiotopic zinc(ii) complexes based on two asymmetric achiral ligands.

Enantiomorphism and enantioselectivity are critical in biology and many other applications. Herein, we report two 3D chiral MOFs {[Zn6(MIDPPA)3(1,2,4-btc)3(NO2)3(H2O)3](H2O)7}n (1L and 1R) based on achiral ligands with high enantiomeric excess and a novel topological type. The internal mechanism of spontaneous chiral symmetry breaking, during the crystallization of chiral MOFs based on achiral ligands, is elucidated for the first time from both structural and theoretical aspects. Hydrogen bonds are found to play a key role in the spontaneous symmetry breaking of chiral MOFs. Also, DFT calculations support our findings from the aspects of total energies and HOMO-LUMO energy gaps. Both 1L and 1R, exhibiting green fluorescence, present a non-centrosymmetric polar packing arrangement, resulting in good ferroelectric behaviors and second-order nonlinear optical effects.