Development of Isostructural Porphyrin-Salen Chiral Metal-Organic Frameworks through Postsynthetic Metalation Based on Single-Crystal to Single-Crystal Transformation

Transmetalation in Surface-Confined Single-Layer Organometallic Networks with Alkynyl-Metal-Alkynyl Linkages

Transmetalation represents an appealing strategy toward fabricating and tuning functional metal-organic polymers and frameworks for diverse applications. In particular, building two-dimensional metal-organic and organometallic networks affords versatile nanoarchitectures of potential interest for nanodevices and quantum technology. The controlled replacement of embedded metal centers holds promise for exploring versatile material varieties by serial modification and different functionalization. Herein, we introduce a protocol for the modification of a single-layer carbon-metal-based organometallic network via transmetalation. By integrating external Cu atoms into the alkynyl-Ag organometallic network constructed with 1,3,5-triethynylbenzene precursors, we successfully realized in situ its highly regular alkynyl-Cu counterpart on the Ag(111) surface. While maintaining a similar lattice periodicity and pore morphology to the original alkynyl-Ag sheet, the Cu-based network exhibits increased thermal stability, guaranteeing improved robustness for practical implementation.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

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.

A review on chiral metal-organic frameworks: synthesis and asymmetric applications

Chiral metal-organic frameworks (CMOFs) have the characteristics of framework structure diversity and functional tunability, and have important applications in the fields of chiral identification, separation of enantiomers and asymmetric catalysis. In recent years, the application of CMOFs has also been extended to other research fields, such as circularly polarized fluorescence and chiral ferroelectrics. Compared with achiral MOFs, the design of CMOFs only considers the modes of introduction of chirality, and also takes into account the crystallization and purification. Therefore, the synthesis and characterization of CMOFs face many difficult challenges. This review discusses three effective strategies for constructing CMOFs, including direct synthesis of chiral ligands, spontaneous resolution of achiral ligands or chiral template-induced synthesis, and post-synthetic chiralization of achiral MOFs. In addition, this review also discusses the recent application progress of CMOFs in chiral molecular recognition, enantiomer separation, asymmetric catalysis, circularly polarized fluorescence, and chiral ferroelectrics.

A review of synthesis, fabrication, and emerging biomedical applications of metal-organic frameworks

The overwhelming potential of porous coordination polymers (PCP), also known as Metal-Organic Frameworks (MOFs), especially their nanostructures for various biomedical applications, have made these materials worth investigating for more applications and uses. MOFs unique structure has enabled them for most applications, particularly in biomedical and healthcare. A number of very informative review papers are available on the biomedical applications of MOFs for the reader's convenience. However, many of those reviews focus mainly on drug delivery applications, and no significant work has been reported on other MOFs for biomedical applications. This review aims to present a compact and highly informative global assessment of the recent developments in biomedical applications (excluding drug-delivery) of MOFs along with critical analysis. Researchers have recently adopted both synthetic and post-synthetic routes for the fabrication and modification of MOFs that have been discussed and analyzed. A critical review of the latest reports on the significant and exotic area of bio-sensing capabilities and applications of MOFs has been given in this study. In addition, other essential applications of MOFs, including photothermal therapy, photodynamic therapy, and antimicrobial activities, are also included. These recently grown emergent techniques and cancer treatment options have gained attention and require further investigations to achieve fruitful outcomes. MOF's role in these applications has been thoroughly discussed, along with future challenges and valuable suggestions for the research community that will help meet future demands.

Making Chiral Salen Complexes Work with Organocatalysts

Bisimine derivatives of salicylaldehyde with chiral diamines (salens) are privileged ligands in asymmetric organometallic catalysis, which can be used in cooperation with organocatalysts as additives. The latter can be a modifier of the metal reactivity by liganding or a true co-catalyst working in tandem or in a dual system. All scenarios encountered in the literature are reviewed and classified according to the organocatalyst. In each case, mechanistic and physical-organic chemistry considerations are discussed to better understand the gears of these complex catalytic settings.

Three-dimensional Cd(ii) porphyrin metal–organic frameworks for the colorimetric sensing of Electron donors

Three MOFs with metalloporphyrin lined, large square 1D channels were used as colorimetric sensors for electron donors. Exposure to amine vapours caused a redshift of the Soret absorption bands of the metalloporphyrin.

Advances in Chiral Metal-Organic and Covalent Organic Frameworks for Asymmetric Catalysis

Asymmetric catalysis is of crucial importance owing to the huge and rising demand for optically pure substances. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), as two emerging crystalline porous materials, have presented great promising applications for heterogeneous asymmetric catalysis. The unique properties, such as, highly regular porous structures, prominent structural tunability, and well-ordered catalytic sites, render chiral MOFs (CMOFs) and chiral COFs (CCOFs) highly active and enantioselective for a large number of asymmetric catalytic organic transformations. Furthermore, they provide a useful platform for facile mechanistic understanding and catalyst design. This review provides an overview of the advancements in CMOFs and CCOFs for asymmetric catalysis. The designs, syntheses and structures of these crystalline porous materials, and their asymmetric catalytic performance are described. And the perspectives on challenges and opportunities in development of CMOFs and CCOFs are discussed. It is anticipated that this review will shed light on the heterogeneous asymmetric catalysis with CMOFs and CCOFs and motivate further research in this promising field.

Conductive Metallophthalocyanine Framework Films with High Carrier Mobility as Efficient Chemiresistors

The poor electrical conductivity of two-dimensional (2D) crystalline frameworks greatly limits their utilization in optoelectronics and sensor technology. Herein, we describe a conductive metallophthalocyanine-based NiPc-CoTAA framework with cobalt(II) tetraaza[14]annulene linkages. The high conjugation across the whole network combined with densely stacked metallophthalocyanine units endows this material with high electrical conductivity, which can be greatly enhanced by doping with iodine. The NiPc-CoTAA framework was also fabricated as thin films with different thicknesses from 100 to 1000 nm by the steam-assisted conversion method. These films enabled the detection of low-concentration gases and exhibited remarkable sensitivity and stability. This study indicates the enormous potential of metallophthalocyanine-based conductive frameworks in advanced stand-off chemical sensors and provides a general strategy through tailor-make molecular design to develop sensitive and stable chemical sensors for the detection of low-concentration gases.

Homochiral metal-organic frameworks for enantioseparation

Obtaining homochiral compounds is of high importance to human health and environmental sustainability. Currently, enantioseparation is one of the most effective approaches to obtain homochiral compounds. Thanks to their controlled synthesis and high efficiency, homochiral metal-organic frameworks (HMOFs) are one of the most widely studied porous materials to enable enantioseparation. In this review, we discuss the chiral pocket model in depth as the key to unlock enantioselective separation mechanisms in HMOFs. In particular, we classify our discussion of these chiral pockets (also regarded as "molecular traps") into: (a) achiral/chiral linker based helical channels as a result of packing modality; and (b) chiral pores inherited from chiral ligands. Driven by a number of mechanisms of enantioseparation, conceptual advances have been recently made in the design of HMOFs for achieving high enantioseparation performances. Herein, these are systematically categorised and discussed. Further we elucidate various applications of HMOFs as regards enantioseparation, systematically classifying them into their use for purification and related analytical utility according to the reported examples. Last but not the least, we discuss the challenges and perspectives concerning the rational design of HMOFs and their corresponding enantioseparations.

Solvent-triggered single-crystal-to-single-crystal transformation from a monomeric to polymeric copper(II) complex based on an aza macrocyclic ligand

Reversible solvent-triggered single-crystal-to-single-crystal (SCSC) transformations are observed between two copper(II) azamacrocyclic complexes: [Cu(C₁₆H₃₈N₆)(H₂O)₂](C₁₂H₆O₄) (1) and [Cu(C₁₆H₃₈N₆)(C₁₂H₆O₄)] (2). Complex (1) was prepared via self-assembly of a copper(II) azamacrocyclic complex containing butyl pendant groups, [Cu(C₁₆H₃₈N₆)(ClO₄)₂], with 2,7-naphthalenedicarboxylic acid. When monomeric compound (1) was immersed in CH₃OH, coordination polymer (2) was obtained, indicating a solvent-triggered SCSC transformation. Furthermore, when (2) was immersed in water, an reverse SCSC transformation from (2) to (1) occurred. Complex (1) presents a 3D supramolecular structure formed via intermolecular hydrogen-bonding interactions, whereas complex (2) features a 1D zigzag coordination polymer. The reversible SCSC transformation of (1) and (2) was characterized using single-crystal X-ray diffraction and in situ powder X-ray diffraction techniques. Despite its poor porosity, complex (2) displayed interesting CO₂ adsorption behaviour under CO₂ gas.

Metalloporphyrinic metal-organic frameworks: Controlled synthesis for catalytic applications in environmental and biological media

Recently, as a new sub-family of porous coordination polymers (PCPs), porphyrinic-MOFs (Porph-MOFs) with biomimetic features have been developed using porphyrin macrocycles as ligands and/or pillared linkers. The control over the coordination of the porphyrin ligand and its derivatives however remains a challenge for engineering new tunable Porph-MOF frameworks by self-assembly methods. The key challenges exist in the following respects: (i) collapse of the large open pores of Porph-MOFs during synthesis, (ii) deactivation of unsaturated metal-sites (UMCs) by axial coordination, and (iii) the tendency of both coordinated moieties (at peripheral meso- and beta-carbon sites) and the N₄-pyridine core to coordinate with metal cations. In this respect, this review covers the advances in the design of Porph-MOFs relative to their counterpart covalent organic frameworks (Porph-COFs). The potential utility of custom-designed porphyrin/metalloporphyrins ligands is highlighted. Synthesis strategies of Porph-MOFs are also illustrated with modular design of hybrid guest@host composites (either Porph@MOFs or guest@Porph-MOFs) with exceptional topologies and stability. This review summarizes the synergistic benefits of coordinated porphyrin ligands and functional guest molecules in Porph-MOF composites for enhanced catalytic performance in various redox applications. This review shed lights on the engineering of new tunable hetero-metals open active sites within (metallo)porphyrin-MOFs as out-of-the-box platforms for enhanced catalytic processes in chemical and biological media.

Recent highlights and future prospects on mixed-metal MOFs as emerging supercapacitor candidates

Mixed-metal metal-organic frameworks (M-MOFs) consist of at least two different metal ions as nodes in the same framework. The incorporation of a second or more metal ions provides structural/compositional diversity, multi-functionality and stability to the framework. Moreover, the periodical array of different metal ions in the framework may alter the physical/chemical properties of M-MOFs and result in fascinating applications. M-MOFs with exciting structural features offer superior supercapacitor performances compared to single metal MOFs due to the synergic effect of different metal ions. In this review, we summarize several synthetic methods to construct M-MOFs by employing various organic ligands or metalloligands. Further, we discuss the electrochemical performance of several M-MOFs and their derived composite materials for supercapacitor applications.

Computational Exploration of Chiral Iron Porphyrin-Catalyzed Asymmetric Hydroxylation of Ethylbenzene Where Stereoselectivity Arises from π-π Stacking Interaction

The mechanism and origins of stereoselectivity of chiral iron porphyrin-catalyzed asymmetric hydroxylation of ethylbenzene were explored with density functional theory. The hydrogen atom abstraction is the rate- and stereoselectivity-determining step. In good agreement with experimental results, the formation of the (R)-1-phenylethanol product is found to be the most favorable pathway. The transition state of hydrogen atom abstraction which leads to the (S)-1-phenylethanol product is unfavorable by 1.7 kcal/mol compared to the corresponding transition state which leads to the (R)-1-phenylethanol product. Enantioselectivity arises from an attractive π-π stacking interaction between the phenyl group of ethylbenzene substrate and the naphthyl group of the porphyrin ligand.

Postsynthetic Metal Exchange in a Metal-Organic Framework Assembled from Co(III) Diphosphine Pincer Complexes

A Zr metal-organic framework (MOF) 1-CoCl₃ has been synthesized by solvothermal reaction of ZrCl₄ with a carboxylic acid-functionalized Co^III-P^NN^NP pincer complex H₄(L-CoCl₃) ([L-CoCl₃]^4- = [(2,6-(NHPAr₂)₂C₆H₃)CoCl₃]^4-, Ar = p-C₆H₄CO₂^-). The structure of 1-CoCl₃ has been determined by X-ray powder diffraction and exhibits a csq topology that differs from previously reported ftw-net Zr MOFs assembled from related Pd^II- and Pt^II-P^NN^NP pincer complexes. The Co-P^NN^NP pincer species readily demetallate upon reduction of Co^III to Co^II, allowing for transmetalation with late second and third row transition metals in both the homogeneous complex and 1-CoCl₃. Reaction of 1-CoCl₃ with [Rh(nbd)Cl]₂ (nbd = 2,5-nobornadiene) results in complete Rh/Co metal exchange at the supported diphosphine pincer complexes to generate 1-RhCl, which has been inaccessible by direct solvothermal synthesis. Treating 1-CoCl₃ with PtCl₂(SMe₂)₂ in the presence of the mild reductant NEt₃ resulted in nearly complete Co substitution by Pt. In addition, a mixed metal pincer MOF, 1-PtRh, was generated by sequential substitution of Co with Pt followed by Rh.

Metal-salen molecular cages as efficient and recyclable heterogeneous catalysts for cycloaddition of CO2 with epoxides under ambient conditions

A salen based molecular cage, salen@cage, was synthesized and complexed with Co and Al to yield metal-salen molecular cages, Co(ii)@cage, Co(iii)@cage and Al(iii)@cage. These cages were demonstrated to be efficient heterogeneous catalysts for the cycloaddition of CO2 with styrene oxide, achieving full conversion at 25 °C and 1 atm CO2. Good to excellent yields of various cyclic carbonates were also achieved under mild conditions. Al(iii)@cage can be reused up to five times without any significant loss of its high catalytic activity. The capability to access a variety of heterogeneous organometallic catalysts with salen@cage offers new prospects for practical CO2 utilization and chemical manufacturing.

Compartmentalized host spaces accommodating guest aromatic molecules in a chiral way in a helix-peptide-aromatic framework

A novel host molecular assembly of a free-standing flat nanosheet with compartmentalized spaces was prepared using a bolaamphiphilic peptide composed of two amphiphilic helical peptides and an oligo(naphthaleneethynylene) (ONE) unit at the center of the molecule. The nanosheet possesses void host spaces that can accommodate two mol-equivalent ONE groups to form columns of ONE groups in a right-handed helical way and ONE channels over a long distance. The present molecular system therefore can provide a chiral pore channel for relatively large molecules.

Highly Stable Chiral Zirconium-Metallosalen Frameworks for CO2 Conversion and Asymmetric C-H Azidation

The engineering of highly stable metal-organic frameworks (MOFs) will unveil the intrinsic potential of these materials for practical applications, especially for heterogeneous catalyzes. However, it is fairly challenging to rationally design robust MOFs serving as highly effective and reusable heterogeneous catalysts. Here, for the first time, we report the construction of four robust UiO-type chiral zirconium-metallosalen frameworks, denoted ZSF-1-4. Single-crystal X-ray-diffraction reveals that the frameworks consist of twelve-connected Zr₆O₈ clusters with privileged chiral metallosalen ligands anchored at ideal positions, generating confined chiral cages that enable synergistic activation. Unlike UiO-68 that is highly sensitive to aqueous solutions, ZSF-1-4 exhibit excellent chemical stability in aqueous solutions with a wide range of pH owing to the abundant hydrophobic groups within metallosalen ligands. These features render ZSF-1 and ZSF-2 to be excellent recycled heterogeneous catalysts for the conversion of imitated industrial CO₂ with epoxides into cyclic carbonates with the highest reported turnover numbers in Zr-MOFs. With regard to asymmetric catalysis, ZSF-3 and ZSF-4 can effectively catalyze C-H azidation reaction in water medium with ee value up to 94%. Moreover, these robust ZSFs can be further extended to other analogues with various metal centers through demetallization-remetallization strategy, which renders them to be an excellent platform for broader fields.

Bottom-Up Assembly of a Highly Efficient Metal-Organic Framework for Cooperative Catalysis

In this study, we demonstrate a bottom-up assembly of a monomeric copper complex and a two-dimensional (2-D) heterometallic metal-organic framework (MOF) from a carboxylate-functionalized tridentate Schiff base ligand and metal ions. The obtained 2-D MOF features a unique bimetallic copper center which is different from its monometallic precursor and acts as an efficient heterogeneous catalyst for the Friedel-Crafts reaction and Henry reaction. The MOF catalyst shows a remarkably superior activity compared to its homogeneous counterparts in a wide range of substrates. It is presumably ascribed to the dual activation of the substrates by the active bimetallic copper center confined in the MOF network, which is supported by the significant changes in catalytic activity at low catalyst/substrates ratios when using the 2-D MOF and its precursor as catalysts, respectively. Moreover, the MOF catalyst also shows an excellent stability and recyclability. Our work, therefore, provides a stepwise strategy to design a heterogeneous cooperative catalyst, by taking advantage of the modulated structure of MOF and tunable functionality of the tridentate Schiff base, with high performance in a variety of organic synthesis.

Enhanced Activity and Enantioselectivity of Henry Reaction by the Postsynthetic Reduction Modification for a Chiral Cu(salen)-Based Metal-Organic Framework

Metal-organic frameworks (MOFs) imbedded privileged molecular catalysts are of particular interest due to their higher catalytic activities derived from the MOFs pore/channel confinement effect, improved lifetime through eliminating intermolecular deactivation pathway, and the recyclability based on their heterogeneity. In this work, a 3D chiral metallosalen-based MOF [Cd₂(Cu(salen))(DMF)₃]·DMF·3H₂O (1) with a 1D open channel was synthesized and characterized by single-crystal X-ray diffraction and other physicochemical methods. Upon postsynthetic reduction modification with NaBH₄, the conversion from imino to amino group on salen cores of 1 generates the reduction product 2 with a more flexible chiral group and more alkaline backbone, meanwhile still maintaining the original porous framework. 2 can be used as an efficient heterogeneous catalyst for the asymmetric Henry reaction with broad substrate applicability and exhibits higher activity and enantioselectivity (ee up to 98%) compared with the unreduced 1. Note that 2 can accelerate the Henry reaction of pyridine-2-carboxaldehyde possessing a potential coordination atom with excellent ee value; however, the homogeneous counterpart does not. In addition, the bulky aldehydes show a decrease in activity but almost the same enantioselectivity with an increase in the molecular size of substrates as a result of the chiral confinement effect of 2, indicating the size-dependent selectivity. To the best of our knowledge, this is the highest enantioselectivity for asymmetric Henry reaction catalyzed by MOF-based catalysts.

Iron(ii) and cobalt(ii) complexes based on anionic phenanthroline-imidazolate ligands: reversible single-crystal-to-single-crystal transformations

Among the series of Fe^IIand Co^IIcomplexes based on phenanthroline-imidazolate ligands, the Fe^IIcomplex (H₉O₄)[Fe(PIMP)₃]·(C₄H₁₀O)₂(H₂O) (1a) exhibits reversible single-crystal-to-single-crystal transformations by solvent uptake/release.

Three novel Co(ii)/Ni(ii)-based coordination polymers as efficient heterogeneous catalysts for dye degradation

Three novel Co(ii)/Ni(ii)-based coordination polymers have been synthesized and characterized. Compound 2 shows a rare 2D + 2D heterogeneous framework. Compound 3 is a chiral 0-D molecular complex driven by the solvent-assisted homochiral helix. The photocatalytic oxidation activities and mechanism have been studied.

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.