Chiral Metal-organic Frameworks Research Articles

Chiral two-dimensional metal–organic frameworks based on Zn(salen) ligands: subcomponent self-assembly and circularly polarised luminescence

Solvothermal subcomponent self-assembly of 2-hydroxy-3-(pyridin-4-yl)benzaldehyde, chiral diaminocyclohexane, and Zn(ii) ions offered a pair of two-dimensional chiral metal–organic frameworks, which display circularly polarised luminescence.

Construction of hierarchically chiral metal–organic frameworks for fast and mild asymmetric catalysis

Here, CMOF(ASP) is synthesized as a chiral Zr-fumarate framework functionalized with l-aspartate for highly efficient catalysis of ring opening and nitroaldol reactions and asymmetric synthesis of oxazolidinone in mild and green conditions.

Rational design of mesoporous chiral MOFs as reactive pockets in nanochannels for enzyme-free identification of monosaccharide enantiomers.

Monosaccharides play significant roles in daily metabolism in living organisms. Although various devices have been constructed for monosaccharide identification, most rely on the specificity of the natural enzyme. Herein, inspired by natural ionic channels, an asymmetrical MOF-in-nanochannel architecture is developed to discriminate monosaccharide enantiomers based on cascade reactions by combining oxidase-mimicking and Fenton-like catalysis in homochiral mesoporous CuMOF pockets. The identification performance is remarkably enhanced by the increased oxidase-mimicking activity of Au nanoparticles under a local surface plasmon resonance (LSPR) excitation. The apparent steady-state kinetic parameters and nano-fluidic simulation indicate that the different affinities induced by Au-LSPR excitation and the confinement effect from MOF pockets precipitate the high chiral sensitivity. This study offers a promising strategy for designing an enantiomer discrimination device and helps to gain insight into the origin of stereoselectivity in a natural enzyme.

Adaptive host–guest chiral recognition in nanoarchitectonics with biomimetic MOF mimicking DNA

A biomimetic chiral MOF, ZnBTCHx, was synthesized by mimicking a DNA groove. The adaptability of the local shape of ZnBTCHx can facilitate specific recognition of the chiral guest.

Grafting (S)-2-Phenylpropionic Acid on Coordinatively Unsaturated Metal Centers of MIL-101(Al) Metal-Organic Frameworks for Improved Enantioseparation.

Chiral metal-organic frameworks (cMOFs) are emerging chiral stationary phases for enantioseparation owing to their porosity and designability. However, a great number of cMOF materials show poor separation performance for chiral drugs in high-performance liquid chromatography (HPLC). The possible reasons might be the irregular shapes of MOFs and the low grafting degree of chiral ligands. Herein, MIL-101-Ppa@SiO2 was synthesized by a simple coordination post-synthetic modification method using (S)-(+)-2-Phenylpropionic acid and applied as the chiral stationary phase to separate chiral compounds by HPLC. NH2-MIL-101-Ppa@SiO2 prepared via covalent post-synthetic modification was used for comparison. The results showed that the chiral ligand density of MIL-101-Ppa@SiO2 was higher than that of NH2-MIL-101-Ppa@SiO2, and the MIL-101-Ppa@SiO2 column exhibited better chiral separation performance and structural stability. The binding affinities between MIL-101-Ppa@SiO2 and chiral compounds were simulated to prove the mechanism of the molecular interactions during HPLC. These results revealed that cMOFs prepared by coordination post-synthetic modification could increase the grafting degree and enhance the separation performance. This method can provide ideas for the synthesis of cMOFs.

A Mechanistic Study of Asymmetric Transfer Hydrogenation of Imines on a Chiral Phosphoric Acid Derived Indium Metal-Organic Framework

A density functional theory (DFT) study is reported to examine the asymmetric transfer hydrogenation (ATH) of imines catalyzed by an indium metal-organic framework (In-MOF) derived from a chiral phosphoric acid (CPA). It is revealed that the imine and reducing agent (i.e., thiazoline) are simultaneously adsorbed on the CPA through H-bonding to form an intermediate, subsequently, a proton is transferred from thiazoline to imine. The transition state TS-R and TS-S are stabilized on the CPA via H-bonding. Compared to the TS-S, the TS-R has shorter H-bonding distances and longer C-H···π distances, it is more stable and experiences less steric hindrance. Consequently, the TS-R exhibits a lower activation barrier affording to the (R)-enantiomer within 68.1% ee in toluene. Imines with substituted groups such as -NO2, -F, and -OCH3 are used to investigate the substitution effects on the ATH. In the presence of an electron-withdrawing group like -NO2, the electrophilicity of imine is enhanced and the activation barrier is decreased. The non-covalent interactions and activation-strain model (ASM) analysis reveal that the structural distortions and the differential noncovalent interactions of TSs in a rigid In-MOF provide the inherent driving force for enantioselectivity. For -OCH3 substituted imine, the TS-S has the strongest steric hindrance, leading to the highest enantioselectivity. When the solvent is changed from toluene to dichloromethane, acetonitrile, and dimethylsulfoxide with increasing polarity, the activation energies of transition state increase whereas their difference decreases. This implies the reaction is slowed down and the enantioselectivity becomes lower in a solvent of smaller polarity. Among the four solvents, toluene turns out to be the best for the ATH. The calculated results in this study are in fairly good agreement with experimental observations. This study provides a mechanistic understanding of the reaction mechanism, as well as substitution and solvent effects on the activity and enantioselectivity of the ATH. The microscopic insights are useful for the development of new chiral MOFs toward important asymmetric reactions.

Regulation of Chirality in Metal-Organic Frameworks (MOFs) Based on Achiral Precursors through Substituent Modification.

The generation and regulation of chirality are closely related to the origin of life. Using achiral precursors to spontaneously build chiral MOFs remains a major challenge. Here, a method to synthesize chiral MOFs from achiral precursors by utilizing chiral fragments was achieved. The transformation from chiral fragments of 1 to chiral frameworks of 2 and 3 was realized by modifying the substituents, and the enantiomer resolution of 3-P41212 and 3-P43212 was achieved by d/l camphoric acid. 3 was then further studied in applications.

An electrochemical sensor employing β-cyclodextrin chiral cross-linked metal organic framework and graphene oxide for chiral enantiomer recognition

Chirality is a universal feature of nature, and the effective differentiation of chiral molecules is still a challenge. Here we propose a method for recognizing chiral molecules based on an electrochemical detection method. We use β-cyclodextrin as an organic ligand and coordinate it with K ions to form a metal–organic framework (MOF), then we use diphenyl carbonate to connect MOFs to obtain chiral Cross-Linked MOF (CLMOF). This CLMOF is different from the cyclodextrin MOFs reported in previous literature in that it can maintain a stable structure in the aqueous environment. CLMOF and graphene oxide (GO) are jointly modified on the glassy carbon electrode (GCE) surface to build a GO-CLMOF/GCE electrochemical sensor which is used for the chiral recognition of the enantiomers of mandelic acid (MA). The DL enantiomers of MA exhibit different electrochemical behaviors on the GO-CLMOF/GCE sensor. Their peak current ratio (IL/ID) reaches 1.8, and the peak potential difference ΔE (ED-EL) is 96 mV. It shows good linearity in the range of 0.5–30 mM. The detection limits of D-MA and L-MA are 0.15 mM and 0.09 mM, respectively. The method was successfully applied to the detection of DL-MA enantiomer of unknown concentration in urine. The recognition of DL enantiomers on the electrode surface is possible because DL-MA forms diastereomers with CLMOF. The electrochemical sensor made of CLMOF and GO is simple to prepare and has excellent selectivity for DL enantiomers of MA, and provides a possible idea for the identification and quantification of chiral isomers.

Recent progress in the design, synthesis and applications of chiral metal-organic frameworks.

Chiral Metal-Organic Frameworks (CMOFs) are unique crystalline and porous class of materials which is composed of organic linkers and metal ions. CMOFs surpass traditional organic and inorganic porous materials because of their tunable shape, size, functional diversity, and selectivity. Specific applications of CMOFs may be exploited by introducing desired functional groups. CMOFs have chiral recognition abilities, making them unique for chiral compound synthesis and separation. The CMOFs can be synthesized through different approaches. Two main approaches have been discussed, i.e., direct and indirect synthesis. Synthetic strategies play an essential role in getting desired properties in MOFs. CMOFs find potential applications in adsorption, asymmetric catalysis, luminescence, degradation, and enantioselective separation. The MOFs’ porosity, stability, and reusability make them an attractive material for these applications. The plethora of applications of CMOFs have motivated chemists to synthesize novel MOFs and number of MOFs have been ever-escalating. Herein, the synthetic methods of CMOFs and their various applications have been discussed.

Superficial chiral etching on an achiral metal-organic framework for high-performance liquid chromatography enantioseparations.

Chiral metal-organic frameworks have shown great potential in enantioselective separation and asymmetric catalysis due to their diverse and adjustable structures with abundant chiral recognition sites. Herein, a new chiral post-synthetic modification was used for preparing an achiral@chiral metal-organic frameworks core-shell composite [Cu3 (Btc)2 ]@[Cu2 ((+)-Cam)2 Dabco] by a superficial chiral etching method. The [Cu3 (Btc)2 ]@[Cu2 ((+)-Cam)2 Dabco] composite was utilized as a novel chiral stationary phase for HPLC enantioseparation. Various racemates were separated on the [Cu3 (Btc)2 ]@[Cu2 ((+)-Cam)2 Dabco]-packed column (column A). It exhibited good chiral resolving ability toward many different kinds of racemates, especially chiral drugs. Among them, the highest resolution value for 1,2-diphenyl-1,2-ethanediol reaches 2.70. The relative standard deviations of retention time and peak area for repeated separation of 1,2-diphenyl-1,2-ethanol were 0.45% and 0.81%, respectively. Compared with the resolution ability of [Cu2 ((+)-Cam)2 Dabco]-packed column (column B), column A shows higher column efficiency and better separation performance than those of column B. The results indicated that the [Cu3 (Btc)2 ]@[Cu2 ((+)-Cam)2 Dabco] as a stationary phase can greatly improve the column efficiency and chiral resolution ability of chiral metal-organic frameworks, which demonstrated that the superficial chiral etching as an economic and efficient strategy opens up a new way for the application of metal-organic frameworks.

Inside Back Cover: Directing the Morphology, Packing, and Properties of Chiral Metal–Organic Frameworks by Cation Exchange (Angew. Chem. Int. Ed. 34/2022)

Predicting crystal morphologies is challenging because of the many factors influencing their growth. Dan Oron, Michal Lahav, Milko E. van der Boom, and co-workers demonstrate in their Research Article (e202205238) how metal cation exchange in metal–organic frameworks (MOFs) results in a series of chiral crystals having a similar morphology, packing, and a rare space group (P622). The fact that the crystals maintain their morphology facilitates a quantitative comparison of their optical and magnetic properties. The metal cation exchange is likely to be mediated by differently sized nanochannels.

Innenrücktitelbild: Directing the Morphology, Packing, and Properties of Chiral Metal–Organic Frameworks by Cation Exchange (Angew. Chem. 34/2022)

Innenrücktitelbild: Directing the Morphology, Packing, and Properties of Chiral Metal–Organic Frameworks by Cation Exchange (Angew. Chem. 34/2022)

A chiral sensing platform based on chiral metal-organic framework for enantiodiscrimination of the isomers of tyrosine and tryptophan

Chiral metal–organic framework (ZIF-8) was successfully prepared through a two-step electrodepostion method by using l-histidine (l-His) as the chiral source, and the resultant chiral ZIF-8 (l-His-ZIF-8) was used for enantiodiscrimination of the isomers of tyrosine (Tyr) and tryptophan (Trp). Although the isomers of both Tyr and Trp could be well discriminated by the l-His-ZIF-8 modified electrode, they displayed opposite voltammetric behaviors on the differential pulse voltammograms (ID-Tyr > IL-Tyr; ID-Trp < IL-Trp), which could be explained by density functional theory (DFT). Finally, both the pH and the electrodeposition time were optimized to obtain the highest recognition efficiency of the l-His-ZIF-8 modified electrode toward the isomers of Tyr and Trp.

Green and Large-Scale Preparation of Chiral Metal-Organic Frameworks via Mechanochemistry.

It is a significant challenge to construct chiral metal-organic frameworks (CMOFs) by developing a facile and green preparation strategy. In this work, CMOFs were first synthesized via a mechanochemical process by combining a truncated mixed ligand strategy and defect engineering theory. The simple, green, and rapid construction strategy could solvent-freely harvest gram-scale CMOFs with a hierarchical micro/mesoporous structure. The as-synthesized CMOFs were evaluated by Aldol asymmetric catalysis and exhibited excellent catalytic performance (conversion was up to 97.1%, the ee value was 44.3%, and the activity was still good after 5 cycles).

Directing the Morphology, Packing, and Properties of Chiral Metal–Organic Frameworks by Cation Exchange**

AbstractWe show that metal–organic frameworks, based on tetrahedral pyridyl ligands, can be used as a morphological and structural template to form a series of isostructural crystals having different metal ions and properties. An iterative crystal‐to‐crystal conversion has been demonstrated by consecutive cation exchanges. The primary manganese‐based crystals are characterized by an uncommon space group (P622). The packing includes chiral channels that can mediate the cation exchange, as indicated by energy‐dispersive X‐ray spectroscopy on microtome‐sectioned crystals. The observed cation exchange is in excellent agreement with the Irving–Williams series (Mn&lt;Fe&lt;Co&lt;Ni&lt; Cu&gt;Zn) associated with the relative stability of the resulting coordination nodes. Furthermore, we demonstrate how the metal cation controls the optical and magnetic properties. The crystals maintain their morphology, allowing a quantitative comparison of their properties at both the ensemble and single‐crystal level.

Directing the Morphology, Packing, and Properties of Chiral Metal-Organic Frameworks by Cation Exchange.

We show that metal–organic frameworks, based on tetrahedral pyridyl ligands, can be used as a morphological and structural template to form a series of isostructural crystals having different metal ions and properties. An iterative crystal‐to‐crystal conversion has been demonstrated by consecutive cation exchanges. The primary manganese‐based crystals are characterized by an uncommon space group (P622). The packing includes chiral channels that can mediate the cation exchange, as indicated by energy‐dispersive X‐ray spectroscopy on microtome‐sectioned crystals. The observed cation exchange is in excellent agreement with the Irving–Williams series (Mn<Fe<Co<Ni< Cu>Zn) associated with the relative stability of the resulting coordination nodes. Furthermore, we demonstrate how the metal cation controls the optical and magnetic properties. The crystals maintain their morphology, allowing a quantitative comparison of their properties at both the ensemble and single‐crystal level.

Bifunctionalized Metal–Organic Frameworks for Pore‐Size‐Dependent Enantioselective Sensing

AbstractPorosity is a fundamental property of metal–organic frameworks (MOFs). However, the role of the pore size has always been underestimated in MOF‐based luminescent sensors for enantioselective sensing. The construction of isoreticular MOFs (IRMOFs) with variable pore sizes and the synergy between chirality and luminescence is challenging. Herein, a general strategy was developed to introduce chirality into two well‐known IRMOF‐74 analogs with nanochannels of identical shapes but different pore sizes by functionalizing the open metal site under mild conditions. To enhance the detection accuracy, a second luminescent center was introduced into the IRMOF‐74 system to achieve ratiometric sensing. The two bifunctionalized IRMOF‐74 compounds exhibited pore‐size‐dependent sensing performance for enantiomers. This study not only provides a convenient method to construct chiral MOFs as advanced sensing materials but also reveals the fundamental of the pores in MOF‐based luminescent sensors.

Bifunctionalized Metal-Organic Frameworks for Pore-Size-Dependent Enantioselective Sensing.

Porosity is a fundamental property of metal-organic frameworks (MOFs). However, the role of the pore size has always been underestimated in MOF-based luminescent sensors for enantioselective sensing. The construction of isoreticular MOFs (IRMOFs) with variable pore sizes and the synergy between chirality and luminescence is challenging. Herein, a general strategy was developed to introduce chirality into two well-known IRMOF-74 analogs with nanochannels of identical shapes but different pore sizes by functionalizing the open metal site under mild conditions. To enhance the detection accuracy, a second luminescent center was introduced into the IRMOF-74 system to achieve ratiometric sensing. The two bifunctionalized IRMOF-74 compounds exhibited pore-size-dependent sensing performance for enantiomers. This study not only provides a convenient method to construct chiral MOFs as advanced sensing materials but also reveals the fundamental of the pores in MOF-based luminescent sensors.

A robust ethane-selective metal-organic framework with nonpolar pore surface for efficient C2H6/C2H4 separation

A robust chiral chain-based microporous MOF with nonpolar pore surface and suitable cavity space was designed and synthesized for C 2 H 6 /C 2 H 4 separation. The MOF shows simultaneously high C 2 H 6 uptake and inverse C 2 H 6 /C 2 H 4 selectivity, affording the one-step purification of C 2 H 4 from C 2 H 6 -C 2 H 4 mixture under ambient condition. • Adopting the methyl-modified ligands afforded a Zn-MOF with nonpolar channels. • The MOF shows excellent stability and uncommon preferential adsorption for C 2 H 6 . • The methyl groups in MOF are crucial for the separation of C 2 H 6 over C 2 H 4 . • The MOF achieves effective one-step purification of C 2 H 4 from C 2 H 6 -C 2 H 4 mixture. Due to similar physical and chemical properties, the separation of ethane (C 2 H 6 ) and ethylene (C 2 H 4 ) is an indispensable and extremely challenging task in petrochemical process. Although traditional C 2 H 4 -selective porous materials realize the separation of C 2 H 6 -C 2 H 4 mixture, but subsequent multiple separation operations are required to produce high-purity C 2 H 4 . Herein, we designed a hydrophobic methyl group-modified metal-organic framework (MOF) for one-step efficient purification of C 2 H 4 from C 2 H 6 -C 2 H 4 mixture. The as-synthesized MOF constructed from inexpensive carboxylic acid and pyrazole mixed linkers shows high thermal stability and chemical stability (water resistance). Cooperative study on the pure-component sorption isotherm, Ideal Adsorbed Solution Theory method, adsorption heat calculation and grand canonical Monte Carlo (GCMC) simulation supports the unique C 2 H 6 -selective feature of the MOF. Breakthrough experiments revealed the one-step C 2 H 4 purification from C 2 H 6 -C 2 H 4 (1/1, 1/10 and 1/15) mixtures through the MOF around room temperature condition with nice recyclability.

Pillar-Layer Chiral MOFs as a Crystalline Platform for Circularly Polarized Luminescence and Single-Phase White-Light Emission

The construction of circularly polarized luminescence (CPL) materials with high porosity and high rigidity is still challenging. Herein, we propose a chiral reticular chemistry strategy to prepare the homochiral porous metal-organic frameworks (MOFs) as CPL-active materials. Two pairs of enantiomeric MOFs are synthesized through the self-assembly of chiral D/L-cam (DL-camphorates) and achiral fluorescent ligand TPB (1,2,4,5-tetra(pyridin-4-yl)benzene). The glum values of Cd-CMOF-D and Cd-CMOF-L were up to 0.010 and 0.009; the high glum values could be compared to those of the partially pure multicomponent self-assembly systems obtained by the complicated process. We further trace the generation and transfer of the hierarchical chirality from chiral molecule to 3D framework, demonstrating that the CPL was dominated by the original molecular chirality rather than the global chirality of the hierarchical structure. Moreover, the single-phase white-light materials with nearly ideal CIE coordinates (0.33, 0.33) were constructed through the introduction of dye emitters into Zn-CMOF (Zn-based chiral MOF). This work provided not only an insightful view of the chirality transfer and disappearance mechanism but also an efficient method for the preparation of the highly porous CPL materials.