A tale of three carboxylates: cooperative asymmetric crystallization of a three-dimensional microporous framework from achiral precursors

Chiroptical Activity of An Achiral Emissive Eu Metal-Organic Framework

Chiroptical activity of achiral crystals is theoretically allowed but very unusual. There is a particularly scarcity of empirical studies on optically active achiral metal-organic frameworks (MOFs). Herein we report an achiral emissive Eu MOF and its chiroptical properties both in the ground and excited states. The framework crystallizes in an achiral space group (Pna2₁ ) belonging to the polar point group (mm2), where the asymmetric arrangement of racemic trinuclear Eu-oxo clusters is responsible for the optical activity. A pair of circular dichroisms (CD) and circularly polarized luminescence (CPL) peaks with opposite signs were observed for single crystals. Importantly, the luminescence dissymmetry factor can reach up to 1.1×10^-3 , which is comparable in magnitude to the value of most of the chiral-linker-bridged MOFs. This work gives the first example of achiral MOFs with CPL response and should be instructive for the discovery of more CPL emitters from racemic MOF family.

Induction of Chiral Hybrid Metal Halides from Achiral Building Blocks

Chiral hybrid organic-inorganic metal halides (HOMHs) with intrinsic noncentrosymmetry have shown great promise for broad applications in chiroptoelectronics, spintronics, and ferroelectronics. However, the construction strategies for chiral HOMHs often involve chiral building blocks in their frameworks, which greatly limit their chemical diversity. Here, we take advantage of a chiral induction approach and have successfully constructed a series of chiral HOMHs, DMA₄MX₇ (DMA = dimethylammonium, M = Sb or Bi, X = Cl or Br), based on achiral precursors. The resulting chiral products demonstrate a clear enantioenrichment, as confirmed by single-crystal X-ray diffraction analysis and solid-state circular dichroism (CD) spectroscopy. The induction of chiral HOMHs enables superior nonlinear optical performances with very high thermal stability and laser resistance. The successful employment of such a chiral induction approach might facilitate the construction of libraries of chiral HOMH crystals from diverse achiral precursors, in particular those into which it is not easy to introduce intrinsic chiral centers, and would thus pave a new way for rational preparation and application of chiral HOMH materials.

Chiral Metal-Organic Frameworks

In the past two decades, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) assembled from metal ions or clusters and organic linkers via metal-ligand coordination bonds have captivated significant scientific interest on account of their high crystallinity, exceptional porosity, and tunable pore size, high modularity, and diverse functionality. The opportunity to achieve functional porous materials by design with promising properties, unattainable for solid-state materials in general, distinguishes MOFs from other classes of materials, in particular, traditional porous materials such as activated carbon, silica, and zeolites, thereby leading to complementary properties. Scientists have conducted intense research in the production of chiral MOF (CMOF) materials for specific applications including but not limited to chiral recognition, separation, and catalysis since the discovery of the first functional CMOF (i.e., d- or l-POST-1). At present, CMOFs have become interdisciplinary between chirality chemistry, coordination chemistry, and material chemistry, which involve in many subjects including chemistry, physics, optics, medicine, pharmacology, biology, crystal engineering, environmental science, etc. In this review, we will systematically summarize the recent progress of CMOFs regarding design strategies, synthetic approaches, and cutting-edge applications. In particular, we will highlight the successful implementation of CMOFs in asymmetric catalysis, enantioselective separation, enantioselective recognition, and sensing. We envision that this review will provide readers a good understanding of CMOF chemistry and, more importantly, facilitate research endeavors for the rational design of multifunctional CMOFs and their industrial implementation.

Role of additives and solvents in the synthesis of chiral isoreticular MOF-74 topologies

Chiral induction is a simple and inexpensive approach to synthesise chiral metal–organic frameworks, even when using achiral building-blocks. The challenge lies in selecting the proper chiral inductor. This can only be achieved upon understanding the mechanism behind the chirality transfer from the chiral guest to the achiral MOF. In this work, the role of two types of chiral additives and different solvents was investigated in the crystallization of isoreticular MOF-74. We show that pyrrolidone-based solvents can interact with the framework walls and influence the thermal stability of the MOF. The role of the different chiral additives is related to the strength of their interaction with the MOF. Unlike cinchona alkaloids that have weak interactions with the framework, l- or d-trans-4-hydroxyproline (l- or d-Hyp) can strongly bind to the Zn²⁺ metal centres and cause the twisting of the organic linker. Moreover, l- and d-Hyp additives can affect the IRMOF-74 nucleation process depending on their concentration and handedness.

Strongly interacting chiral additives play a complex role during the crystallisation of chiral isoreticular MOF-74. They can coordinate to the open sites of the metal ions and induce strain on the framework, leading to a local twisting of the organic linker.

Synthesis of Chiral MOF-74 Frameworks by Post-Synthetic Modification by Using an Amino Acid

The synthesis of chiral metal-organic frameworks (MOFs) is highly relevant for asymmetric heterogenous catalysis, yet very challenging. Chiral MOFs with MOF-74 topology were synthesised by using post-synthetic modification with proline. Vibrational circular dichroism studies demonstrate that proline is the source of chirality. The solvents used in the synthesis play a key role in tuning the loading of proline and its interaction with the MOF-74 framework. In N,N'-dimethylformamide, proline coordinates monodentate to the Zn2+ ions within the MOF-74 framework, whereas it is only weakly bound to the framework when using methanol as solvent. Introducing chirality within the MOF-74 framework also leads to the formation of defects, with both the organic linker and metal ions missing from the framework. The formation of defects combined with the coordination of DMF and proline within the framework leads to a pore blocking effect. This is confirmed by adsorption studies and testing of the chiral MOFs in the asymmetric aldol reaction between acetone and para-nitrobenzaldehyde.

When Stereochemistry Raised Its Ugly Head in Coordination Chemistry—An Appreciation of Howard Flack

Chiral compounds have played an important role in the development of coordination chemistry. Unlike organic chemistry, where mechanistic rules allowed the establishment of absolute configurations for numerous compounds once a single absolute determination had been made, coordination compounds are more complex. This article discusses the development of crystallographic methods and the interplay with coordination chemistry. Most importantly, the development of the Flack parameter is identified as providing a routine method for determining the absolute configuration of coordination compounds.

Polymorphism of 1,3-X-adamantanes (X = Br, OH, CH3) and the crystal plastic phase formation ability

The polymorphism of 1,3-dimethyladamantane (13DMA), 1,3-adamantanediol (13DOHA) and 1,3-dibromoadamantane (13DBrA) has been studied by X-ray powder diffraction, density measurements and differential scanning calorimetry at normal and high-pressure.

Liquid-Phase Epitaxial Growth of Azapyrene-Based Chiral Metal-Organic Framework Thin Films for Circularly Polarized Luminescence

Development of chiral metal-organic frameworks (MOFs) for circularly polarized luminescence (CPL) is a challenging but important task. In this work, we report a first example of azapyrene-based chiral MOF thin films [Zn₂Cam₂DAP]_n grown on functionalized substrates (named SURchirMOF-4) for CPL property. By using a liquid-phase epitaxial layer-by-layer method, the resulted SURchirMOF-4 was constructed from chiral camphoric acid and 2,7-diazapyrene ligand, which has high orientation and homogeneity. The circular dichroism, CPL, and enantioselective adsorption results show that SURchirMOF-4 has strong chirality and CPL property as well as good enantioselective adsorption toward enantiomers of methyl-lactate. The synthesis of azapyrene-based chiral MOF thin films not only represents an ideal model for studying the enantioselective adsorption, but also will be a valuable approach for development of the chiral thin film exhibiting CPL property.

Synthesis of Chiral Labtb and Visualization of Its Enantiomeric Excess by Induced Circular Dichroism Imaging

Crystalline particles of a microporous, robust, and chiral metal-organic framework (MOF) were synthesized and their enantiomer excess (ee) was visualized for each microparticle by CD imaging. Labtb, a thermally and chemically robust MOF, was employed in this study because it shows a chiral space group. Although Labtb has been obtained as a racemic conglomerate, enantioselective synthesis of Labtb was achieved via a chiral precursor complex consisting of lanthanum and homochiral phenylalanine. Methyl orange (MO) was introduced into the micropores of chiral Labtb, which showed a strong induced CD signal for the absorption band of MO chromophores. High ee of the chiral Labtb was revealed by microscopic CD observation at the particle-level. This result provides a facile way to obtain a robust MOF that has chiral nanospace.

Metal-Organic Frameworks for Separation

Separation is an important industrial step with critical roles in the chemical, petrochemical, pharmaceutical, and nuclear industries, as well as in many other fields. Although much progress has been made, the development of better separation technologies, especially through the discovery of high-performance separation materials, continues to attract increasing interest due to concerns over factors such as efficiency, health and environmental impacts, and the cost of existing methods. Metal-organic frameworks (MOFs), a rapidly expanding family of crystalline porous materials, have shown great promise to address various separation challenges due to their well-defined pore size and unprecedented tunability in both composition and pore geometry. In the past decade, extensive research is performed on applications of MOF materials, including separation and capture of many gases and vapors, and liquid-phase separation involving both liquid mixtures and solutions. MOFs also bring new opportunities in enantioselective separation and are amenable to morphological control such as fabrication of membranes for enhanced separation outcomes. Here, some of the latest progress in the applications of MOFs for several key separation issues, with emphasis on newly synthesized MOF materials and the impact of their compositional and structural features on separation properties, are reviewed and highlighted.

Enantiopure Chiral Coordination Polymers Based on Polynuclear Paddlewheel Helices and Arsenyl Tartrate

Herein, we report the preparation of chiral, one-dimensional coordination polymers based on trinuclear paddlewheel helices [M₃(dpa)₄]2+ (M = Co(II) and Ni(II); dpa = the anion of 2,2'-dipyridylamine). Enantiomeric resolution of a racemic mixture of [M₃(dpa)₄]2+ complexes was achieved by chiral recognition of the respective enantiomer by [Δ-As₂(tartrate)₂]2- or [Λ-As₂(tartrate)₂]2- in N,N-dimethylformamide (DMF), affording crystalline coordination polymers formed from [(Δ-Co₃(dpa)₄)(Λ-As₂(tartrate)₂)]·3DMF (Δ-1), [(Λ-Co₃(dpa)₄)(Δ-As₂(tartrate)₂)]·3DMF (Λ-1), [(Δ-Ni₃(dpa)₄)(Λ-As₂(tartrate)₂)]·(4 - n)DMF∙nEt₂O (Δ-2) or [(Λ-Ni₃(dpa)₄)(Δ-As₂(tartrate)₂)]·(4 - n)DMF∙nEt₂O (Λ-2) repeating units. UV-visible circular dichroism spectra of the complexes in DMF solutions demonstrate the efficient isolation of optically active species. The helicoidal [M₃(dpa)₄]2+ units that were obtained display high stability towards racemization as shown by the absence of an evolution of the dichroic signals after several days at room temperature and only a small decrease of the signal after 3 h at 80 °C.

Novel MOF-Derived Co@N-C Bifunctional Catalysts for Highly Efficient Zn-Air Batteries and Water Splitting

Metal-organic frameworks (MOFs) and MOF-derived materials have recently attracted considerable interest as alternatives to noble-metal electrocatalysts. Herein, the rational design and synthesis of a new class of Co@N-C materials (C-MOF-C2-T) from a pair of enantiotopic chiral 3D MOFs by pyrolysis at temperature T is reported. The newly developed C-MOF-C2-900 with a unique 3D hierarchical rodlike structure, consisting of homogeneously distributed cobalt nanoparticles encapsulated by partially graphitized N-doped carbon rings along the rod length, exhibits higher electrocatalytic activities for oxygen reduction and oxygen evolution reactions (ORR and OER) than that of commercial Pt/C and RuO₂ , respectively. Primary Zn-air batteries based on C-MOF-900 for the oxygen reduction reaction (ORR) operated at a discharge potential of 1.30 V with a specific capacity of 741 mA h g_Zn^-1 under 10 mA cm^-2 . Rechargeable Zn-air batteries based on C-MOF-C2-900 as an ORR and OER bifunctional catalyst exhibit initial charge and discharge potentials at 1.81 and 1.28 V (2 mA cm^-2 ), along with an excellent cycling stability with no increase in polarization even after 120 h - outperform their counterparts based on noble-metal-based air electrodes. The resultant rechargeable Zn-air batteries are used to efficiently power electrochemical water-splitting systems, demonstrating promising potential as integrated green energy systems for practical applications.

Design and synthesis of multifunctional metal-organic zeolites

Metal-organic zeolites (MOZs) are an important branch of metal-organic frameworks (MOFs) and combine the advantages of zeolites and MOFs, such as high surface area and porosity as well as the exceptional stability of zeolites, which would have a significant impact on catalysis chemistry, inorganic chemistry, coordination chemistry, materials science and other areas. In this review, we focus on the recent advances in MOZs with a brief outline of the most prominent examples. In particular, we highlight the basic principles of the design and synthesis approaches toward the construction of MOZs. Obeying the principle of charge matching, tuning tetrahedral metal centers, using enlarged tetrahedral building units as clusters, introducing functional groups into ligands, and combining traditional inorganic TO₄ sites in MOZs enable the final materials with diverse topological structures to exhibit superior performance for various applications, such as gas sorption/separation, catalysis, enantio-selectivity, luminescence, etc.

Hydrothermal synthesis and crystal structure of poly[aqua-(μ2-1,3-bis(4-pyridyl)propane-κ2 N:N′)-(μ2-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene-2,3-dicarboxylato-κ2 O:O′)manganese(II) hydrate, C22H20Cl6N2O6Mn

C22H20Cl6N2O6Mn, monoclinic, P21/c (no. 14), a = 12.0080(7) Å, b = 14.9289(8) Å, c = 15.2997(8) Å, β = 103.590(2)°, V = 2665.9(3) Å3, Z = 4, R gt(F) = 0.0428, wR ref(F 2) = 0.1147, T = 296 K.

Homochiral metal–organic frameworks as heterogeneous catalysts

Homochiral metal–organic frameworks (HMOFs) are attractive materials for asymmetric catalysis because they possess high surface area and uniform active sites.

Chiral metal–organic frameworks constructed from four-fold helical chain SBUs for enantioselective recognition of α-hydroxy/amino acids

Three chiral 3D metal–carboxylate frameworks have been successfully synthesized, featuring four-fold helical metal chains as SBUs. Co-MOFs could recognize enantio-selectively α-hydroxy/amino acids by the change of CD signals.

Homochiral coordination polymers constructed from V-shaped oxybisbenzoyl-based amino acid derivatives: structures, magnetic and photoluminescence properties

Five novel amino acid-modified HCPs have been prepared using the V-shaped semi-rigid 4,4′-oxybis(benzoic acid) instead of usual rigid polycarboxylate units.

Coordinative Alignment of Chiral Molecules to Control over the Chirality Transfer in Spontaneous Resolution and Asymmetric Catalysis

The production and availability of enantiomerically pure compounds that spurred the development of chiral technologies and materials are very important to the fine chemicals and pharmaceutical industries. By coordinative alignment of enantiopure guests in the metal‒organic frameworks, we reported an approach to control over the chirality of homochiral crystallization and asymmetric transformation. Synthesized by achiral triphenylamine derivatives, the chirality of silver frameworks was determined by the encapsulated enantiopure azomethine ylides, from which clear interaction patterns were observed to explore the chiral induction principles. With the changing of addition sequence of substrates, the enantioselectivity of asymmetric cycloaddition was controlled to verify the determinant on the chirality of the bulky MOF materials. The economical chirality amplification that merges a series of complicated self-inductions, bulk homochiral crystallization and enantioselective catalysis opens new avenues for enantiopure chemical synthesis and provides a promising path for the directional design and development of homochiral materials.

Metal-Organic Frameworks for Heterogeneous Basic Catalysis

Great attention has been given to metal-organic frameworks (MOFs)-derived solid bases because of their attractive structure and catalytic performance in various organic reactions. The extraordinary skeleton structure of MOFs provides many possibilities for incorporation of diverse basic functionalities, which is unachievable for conventional solid bases. The past decade has witnessed remarkable advances in this vibrant research area; however, MOFs for heterogeneous basic catalysis have never been reviewed until now. Therefore, a review summarizing MOFs-derived base catalysts is highly expected. In this review, we present an overview of the recent progress in MOFs-derived solid bases covering preparation, characterization, and catalytic applications. In the preparation section, the solid bases are divided into two categories, namely, MOFs with intrinsic basicity and MOFs with modified basicity. The basicity can originate from either metal sites or organic ligands. Different approaches used for generation of basic sites are included, and each approach is described with representative examples. The fundamental principles for the design and fabrication of MOFs with basic functionalities are featured. In the characterization section, experimental techniques and theoretical calculations employed for characterization of basic MOFs are summarized. Some representive experimental techniques, such as temperature-programmed desorption of CO₂ (CO₂-TPD) and infrared (IR) spectra of different probing molecules, are covered. Following preparation and characterization, the catalytic applications of MOFs-derived solid bases are dealt with. These solid bases have potential to catalyze some well-known "base-catalyzed reactions" like Knoevenagel condensation, aldol condensation, and Michael addition. Meanwhile, in contrast to conventional solid bases, MOFs show some different catalytic properties due to their special structural and surface properties. Remarkably, characteristic features of MOFs-derived solid bases are described by comparing with conventional inorganic counterparts, keeping in mind the current opportunities and challenges in this field.

Synthesis, structures and magnetic properties of two chiral mixed-valence iron(ii,iii) coordination networks

Two rare chiral mixed-valence iron(ii,iii) coordination networks have been synthesized through spontaneous asymmetrical crystallization from achiral units in the assembly process.

A second-order nonlinear optical material with a 5-fold interpenetrating diamondoid framework based on two achiral precursors: spontaneous resolution to absolute chiral induction

A pair of enantiomeric Zn(ii) complexes have been synthesized through spontaneous resolution. Fortunately, the handedness of bulk crystals can be controlled by using a chiral induction agent. Notably, they display strong SHG response.

Achiral polyamine-induced chiral zinc gallophosphates with fused double helix-like strands exhibiting blue to green photoluminescence

Chiral zinc gallophosphate structures with achiral linear-type polyamine templates exhibiting inorganic–organic double helix-like strands and intriguing blue to green photoluminescence.

Achiral aromatic solvent-induced assembly of 3-D homochiral porous 3d–4f heterometallic-organic frameworks based on isonicotinic acid

Via induction of achiral aromatic solvents, two 3-D monochiral 3d–4f MOFs (1 and 2) from the ligand HIN without any chiral auxiliary are uncommonly constructed. And the results show homochirality of 1 and 2 are enantiopurity rather than enantiomeric excess.