Four Metalloporphyrinic Frameworks as Heterogeneous Catalysts for Selective Oxidation and Aldol Reaction

Atomic-level design of metalloenzyme-like active pockets in metal-organic frameworks for bioinspired catalysis

Natural metalloenzymes with astonishing reaction activity and specificity underpin essential life transformations. Nevertheless, enzymes only operate under mild conditions to keep sophisticated structures active, limiting their potential applications. Artificial metalloenzymes that recapitulate the catalytic activity of enzymes can not only circumvent the enzymatic fragility but also bring versatile functions into practice. Among them, metal-organic frameworks (MOFs) featuring diverse and site-isolated metal sites and supramolecular structures have emerged as promising candidates for metalloenzymes to move toward unparalleled properties and behaviour of enzymes. In this review, we systematically summarize the significant advances in MOF-based metalloenzyme mimics with a special emphasis on active pocket engineering at the atomic level, including primary catalytic sites and secondary coordination spheres. Then, the deep understanding of catalytic mechanisms and their advanced applications are discussed. Finally, a perspective on this emerging frontier research is provided to advance bioinspired catalysis.

Ligand-Mediated Regulation of the Chemical/Thermal Stability and Catalytic Performance of Isostructural Cobalt(II) Coordination Polymers

Regulating the chemical/thermal stability and catalytic activity of coordination polymers (CPs) to achieve high catalytic performance is topical and challenging. The CPs are competent in promoting oxidative cross-coupling, yet they have not received substantial attention. Here, the ligand effect of the secondary ligand of CPs for oxidative cross-coupling reactions was investigated. Specifically, four new isostructural CPs [Co(Fbtx)1.5(4-R-1,2-BDC)]n (denoted as Co-CP-R, Fbtx = 1,4-bis(1,2,4-triazole-1-ylmethyl)-2,3,5,6-tetrafluorobenzene, 4-R-1,2-BDC = 4-R-1,2-benzenedicarboxylate, R = F, Cl, Br, CF3) were prepared. It was found that in the reactions of oxidative amination of benzoxazoles with secondary amines and the oxidative coupling of styrenes with benzaldehydes, both the chemical and thermal stabilities of the four Co-CPs with the R group followed the trend of -CF3 > -Br > -Cl > -F. Density functional theory (DFT) calculations suggested that the difference in reactivity may be ascribed to the effect of substituent groups on the electron transition energy of the cobalt(II) center of these Co-CPs. These findings highlight the secondary ligand effect in regulating the stability and catalytic performance of coordination networks.

Predicting band gaps of MOFs on small data by deep transfer learning with data augmentation strategies

Porphyrin-based MOFs combine the unique photophysical and electrochemical properties of metalloporphyrins with the catalytic efficiency of MOF materials, making them an important candidate for light energy harvesting and conversion. However, accurate prediction of the band gap of porphyrin-based MOFs is hampered by their complex structure-function relationships. Although machine learning (ML) has performed well in predicting the properties of MOFs with large training datasets, such ML applications become challenging when the training data size of the materials is small. In this study, we first constructed a dataset of 202 porphyrin-based MOFs using DFT computations and increased the training data size using two data augmentation strategies. After that, four state-of-the-art neural network models were pre-trained with the recognized open-source database QMOF and fine-tuned with our augmented self-curated datasets. The GCN models predicted the band gaps of the porphyrin-based materials with the lowest RMSE of 0.2767 eV and MAE of 0.1463 eV. In addition, the data augmentation strategy rotation and mirroring effectively decreased the RMSE by 38.51% and MAE by 50.05%. This study demonstrates that, when proper transfer learning and data augmentation strategies are applied, machine learning models can predict the properties of MOFs using small training data.

Isolating Fe-O2 Intermediates in Dioxygen Activation by Iron Porphyrin Complexes

Dioxygen (O₂) is an environmentally benign and abundant oxidant whose utilization is of great interest in the design of bioinspired synthetic catalytic oxidation systems to reduce energy consumption. However, it is unfortunate that utilization of O₂ is a significant challenge because of the thermodynamic stability of O₂ in its triplet ground state. Nevertheless, nature is able to overcome the spin state barrier using enzymes, which contain transition metals with unpaired d-electrons facilitating the activation of O₂ by metal coordination. This inspires bioinorganic chemists to synthesize biomimetic small-molecule iron porphyrin complexes to carry out the O₂ activation, wherein Fe-O₂ species have been implicated as the key reactive intermediates. In recent years, a number of Fe-O₂ intermediates have been synthesized by activating O₂ at iron centers supported on porphyrin ligands. In this review, we focus on a few examples of these advances with emphasis in each case on the particular design of iron porphyrin complexes and particular reaction environments to stabilize and isolate metal-O₂ intermediates in dioxygen activation, which will provide clues to elucidate structures of reactive intermediates and mechanistic insights in biological processes.

Tailored Inorganic-Organic Architectures via Metalloligands

This article discusses the design principles and strategies and the structural outcome of various supramolecular architectures constructed by utilizing well-defined coordination complexes as the metalloligands. We have included selected examples of metalloligands, offering either pyridyl or arylcarboxylic acid groups as the appended functional groups, for illustrating the construction of their supramolecular architectures. Both geometrical position and the number of the appended functional groups emerging from a metalloligand were found to critically regulate the structural aspects and dimensionality of the resultant material. The article concludes by delineating the structure-directing lessions as well as the potential applications of the metalloligand-based supramolecular architectures for the generation of next-level materials.

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.

Immobilizing a homogeneous manganese catalyst into MOF pores for α-alkylation of methylene ketones with alcohols

An encapsulation strategy via nano-confinement of a homogeneous manganese–phenanthroline complex into MOF pores selectively produced functionalized branched ketones.

Catalysis in Single Crystalline Materials: From Discrete Molecules to Metal-Organic Frameworks

Catalysis is one of the key techniques for people's modern life. It has created numerous essential chemicals such as biomedicines, agricultural chemicals and unique materials. Heterogeneous catalysis is the new emerging method with reusable catalysts. Among heterogenous catalysis patterns developed so far, single crystalline catalysis has become the promising one owing to its high catalytic density and selectivity resulted by the inherent porosity, orderliness of the lattices and permeability. These crystalline catalysts could be used in various reactions such as photo-dimerization, Diels-Alder reaction, CO₂ transformation and so on. In this review, we highlighted the reported works about the single crystalline catalysts. Both discrete small molecules and metal-organic frameworks (MOFs) have been used to prepare single crystals for catalysis. For discrete molecules based crystalline catalysts, coordinated and covalent molecules have been used. There were more catalytic modes in crystalline MOF catalysts. Three patterns were identified in this review: single crystalline MOFs i) without catalytic sites, ii) with inherent catalytic features and iii) with introducing catalytic units by post synthetic modification. Based on these examples, this review committed to provide the inspirations for the further design and application of single crystalline materials.

A historical perspective on porphyrin-based metal-organic frameworks and their applications

Porphyrins are important molecules widely found in nature in the form of enzyme active sites and visible light absorption units. Recent interest in using these functional molecules as building blocks for the construction of metal-organic frameworks (MOFs) have rapidly increased due to the ease in which the locations of, and the distances between, the porphyrin units can be controlled in these porous crystalline materials. Porphyrin-based MOFs with atomically precise structures provide an ideal platform for the investigation of their structure-function relationships in the solid state without compromising accessibility to the inherent properties of the porphyrin building blocks. This review will provide a historical overview of the development and applications of porphyrin-based MOFs from early studies focused on design and structures, to recent efforts on their utilization in biomimetic catalysis, photocatalysis, electrocatalysis, sensing, and biomedical applications.

A stable and highly selective metalloporphyrin based framework for the catalytic oxidation of cyclohexene

A new metalloporphyrin framework of molybdenum Mo2O4(C48H28N4O8)·(CH3)2NH·5H2O·2DMF (Mo2TCPP) was synthesized from tetrakis(4-carboxyphenyl)porphyrin (H4TCPP) and sodium molybdate dihydrate by a hydrothermal method. Mo2TCPP is a 3D network with two sub-units, in which both TCPP ligands and each Mo2 dimer act as four connection nodes. The crystal structure was determined by single crystal analysis and further characterized by FTIR, SEM, EDX, PXRD, XPS and TGA. Here cumene hydrogen peroxide and hydrogen peroxide were used as oxidants to study the catalytic activity of new metalloporphyrins in the oxidation of cyclohexene at different temperatures. The conversion rate of cyclohexene and the selectivity of epoxycyclohexane were both higher than 99%, which was better than the previously published research results. The stability of the catalyst before and after the reaction was further tested for 10 runs without obvious degradation. The catalyst was stable in different solutions such as acidic, water and alkaline. These results shed light on the future development of new catalytic materials based on metalloporphyrin.

Enhancing catalytic alkane hydroxylation by tuning the outer coordination sphere in a heme-containing metal-organic framework

Catalytic heme active sites of enzymes are sequestered by the protein superstructure and are regulated by precisely defined outer coordination spheres. Here, we emulate these protective functions in the porphyrinic metal-organic framework PCN-224 by post-synthetic acetylation and subsequent hydroxylation of the Zr6 nodes. A suite of physical methods demonstrates that both transformations preserve framework structure, crystallinity, and porosity without modifying the inner coordination spheres of the iron sites. Single-crystal X-ray analyses establish that acetylation replaces the mixture of formate, benzoate, aqua, and terminal hydroxo ligands at the Zr6 nodes with acetate ligands, and hydroxylation affords nodes with seven-coordinate, hydroxo-terminated Zr4+ ions. The chemical influence of these reactions is probed with heme-catalyzed cyclohexane hydroxylation as a model reaction. By virtue of passivated reactive sites at the Zr6 nodes, the acetylated framework oxidizes cyclohexane with a yield of 68(8)%, 2.6-fold higher than in the hydroxylated framework, and an alcohol/ketone ratio of 5.6(3).

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.

Charge, adsorption, water stability and bandgap tuning of an anionic Cd(ii) porphyrinic metal–organic framework

By changing the occupancies of the metal ions and counterions, the tuning of the framework charge, band-edge position and bandgap of a novel Cd(ii) porphyrinic MOF 1 was achieved.

Collagenase-Encapsulated pH-Responsive Nanoscale Coordination Polymers for Tumor Microenvironment Modulation and Enhanced Photodynamic Nanomedicine

The abundant tumor extracellular matrix (ECM) could result in insufficient tumor retention and ineffective intratumor penetration of therapeutic agents as well as an acidic and hypoxic tumor microenvironment (TME), leading to unsatisfactory therapeutic outcomes for many types of therapies. Therefore, developing strategies to modulate the TME by selectively degrading the condensed ECM may be helpful to improve existing cancer therapies. Herein, collagenase (CLG)-encapsulated nanoscale coordination polymers (NCPs) are synthesized based on Mn²⁺ and an acid-sensitive benzoic-imine organic linker and then modified by polyethylene glycol (PEG). Upon intravenous (iv) injection, these CLG@NCP-PEG nanoparticles show efficient accumulation within the tumor, in which CLG would be released because of the collapse of NCP structures within the acidic TME. The released CLG enzyme could then specifically degrade collagens, the major component of ECM, leading to a loosened ECM structure, enhanced tumor perfusion, and relieved hypoxia. As a result, the second wave of nanoparticles, chlorin e6 (Ce6)-loaded liposomes (liposome@Ce6), would exhibit enhanced retention and penetration within the tumor. Such phenomena together with relieved tumor hypoxia could then lead to greatly enhanced photodynamic therapeutic effect of liposome@Ce6 for mice pretreated with CLG@NCP-PEG. Our work thus presents a unique strategy for TME modulation using pH-responsive NCPs as smart enzyme carriers.

Double role of metalloporphyrins in catalytic bioinspired supramolecular metal-organic frameworks (SMOFs)

Heterogeneous catalysts are of great interest in many industrial processes for environmental reasons and, during recent years, a great effort has been devoted to obtain metal-organic frameworks (MOFs) with improved catalytic behaviour. Few supramolecular metal-organic frameworks (SMOFs) are stable under ambient conditions and those with anchored catalysts exhibit favourable properties. However, this paper presents an innovative approach that consists of using metal nodes as both structural synthons and catalysts. Regarding the latter, metalloporphyrins are suitable candidates to play both roles simultaneously. In fact, there are a number of papers that report coordination compounds based on metalloporphyrins exhibiting these features. Thus, the aim of this bioinspired work was to obtain stable SMOFs (at room temperature) based on metallo-porphyrins and explore their catalytic activity. This work reports the environmentally friendly microwave-assisted synthesis and characterization of the compound [H(bipy)]2[(MnTPPS)(H2O)2]·2bipy·14H2O (TPPS = meso-tetra-phenyl-porphine-4,4',4'',4'''-tetra-sulfonic acid and bipy = 4,4'-bi-pyridine). This compound is the first example of an MnTPPS-based SMOF, as far as we are aware, and has been structurally and thermally characterized through single-crystal X-ray diffraction, IR spectroscopy, thermogravimetry and transmission electron microscopy. Additionally, this work explores not only the catalytic activity of this compound but also of the compounds μ-O-[FeTCPP]2·16DMF and [CoTPPS0.5(bipy)(H2O)2]·6H2O. The structural features of these supra-molecular materials, with accessible networks and high thermal stability, are responsible for their excellent behaviour as heterogeneous catalysts for different oxidation, condensation (aldol and Knoevenagel) and one-pot cascade reactions.

Metal-Organic Frameworks as Platforms for Catalytic Applications

Metal-organic frameworks (MOFs), also called porous coordination polymers, represent a class of crystalline porous materials built from organic linkers and metal ions/clusters. The unique features of MOFs, including structural diversity and tailorability as well as high surface area, etc., enable them to be a highly versatile platform for potential applications in many fields. Herein, an overview of recent developments achieved in MOF catalysis, including heterogeneous catalysis, photocatalysis, and eletrocatalysis over MOFs and MOF-based materials, is provided. The active sites involved in the catalysts are particularly emphasized. The challenges, future trends, and prospects associated with MOFs and their related materials for catalysis are also discussed.

High-Throughput Computational Screening of the Metal Organic Framework Database for CH4/H2 Separations

Metal organic frameworks (MOFs) have been considered as one of the most exciting porous materials discovered in the last decade. Large surface areas, high pore volumes, and tailorable pore sizes make MOFs highly promising in a variety of applications, mainly in gas separations. The number of MOFs has been increasing very rapidly, and experimental identification of materials exhibiting high gas separation potential is simply impractical. High-throughput computational screening studies in which thousands of MOFs are evaluated to identify the best candidates for target gas separation is crucial in directing experimental efforts to the most useful materials. In this work, we used molecular simulations to screen the most complete and recent collection of MOFs from the Cambridge Structural Database to unlock their CH₄/H₂ separation performances. This is the first study in the literature, which examines the potential of all existing MOFs for adsorption-based CH₄/H₂ separation. MOFs (4350) were ranked based on several adsorbent evaluation metrics including selectivity, working capacity, adsorbent performance score, sorbent selection parameter, and regenerability. A large number of MOFs were identified to have extraordinarily large CH₄/H₂ selectivities compared to traditional adsorbents such as zeolites and activated carbons. We examined the relations between structural properties of MOFs such as pore sizes, porosities, and surface areas and their selectivities. Correlations between the heat of adsorption, adsorbility, metal type of MOFs, and selectivities were also studied. On the basis of these relations, a simple mathematical model that can predict the CH₄/H₂ selectivity of MOFs was suggested, which will be very useful in guiding the design and development of new MOFs with extraordinarily high CH₄/H₂ separation performances.

Catalysis and photocatalysis by metal organic frameworks

This review aims to provide different strategies employed to use MOFs as solid catalysts and photocatalysts in organic transformations.

A structurally-characterized peroxomanganese(iv) porphyrin from reversible O2 binding within a metal-organic framework

Within a MOF, a side-on peroxomanganese(iv) porphyrin has been isolated and comprehensively examined.

The role of peroxometal species as reactive intermediates in myriad biological processes has motivated the synthesis and study of analogous molecular model complexes. Peroxomanganese(iv) porphyrin complexes are of particular interest, owing to their potential ability to form from reversible O₂ binding, yet have been exceedingly difficult to isolate and characterize in molecular form. Alternatively, immobilization of metalloporphyrin sites within a metal–organic framework (MOF) can enable the study of interactions between low-coordinate metal centers and gaseous substrates, without interference from bimolecular reactions and axial ligation by solvent molecules. Here, we employ this approach to isolate the first rigorously four-coordinate manganese(ii) porphyrin complex and examine its reactivity with O₂ using infrared spectroscopy, single-crystal X-ray diffraction, EPR spectroscopy, and O₂ adsorption analysis. X-ray diffraction experiments reveal for the first time a peroxomanganese(iv) porphyrin species, which exhibits a side-on, η² binding mode. Infrared and EPR spectroscopic data confirm the formulation of a peroxomanganese(iv) electronic structure, and show that O₂ binding is reversible at ambient temperature, in contrast to what has been observed in molecular form. Finally, O₂ gas adsorption measurements are employed to quantify the enthalpy of O₂ binding as h_ads = –49.6(8) kJ mol^–1. This enthalpy is considerably higher than in the corresponding Fe- and Co-based MOFs, and is found to increase with increasing reductive capacity of the M^II/III redox couple.

Incorporation of Molecular Catalysts in Metal-Organic Frameworks for Highly Efficient Heterogeneous Catalysis

Porous metal-organic frameworks (MOFs) are built from periodically alternate organic moieties and metal ions/clusters. The unique features of the open framework structures, the high surface areas, the permanent porosity, and the appropriate hydrophilic and hydrophobic pore nature mean that MOF materials are a class of ideal host matrices for immobilization of molecular catalysts. The emerging porous materials can not only retain but are also able to enhance the catalytic functions of the single individuals. MOF catalysts have the following super characters: i) uniformly dispersed catalytic sites on the pore surfaces to improve the utility, ii) appropriate hydrophilic and hydrophobic pore nature to facilitate the recognition and transportation of reactant and product molecules, iii) a collaborative microenvironment to realize synergistic catalysis, and iv) simple separation and recovery for long-term usage. Accompanying the development of the synthetic strategies and the technologies for the characterization of MOF materials, MOF catalysis has undergone an upsurge, which has transcended the stage of opportunism. Here, the rational design and synthesis of MOF catalysts are discussed, along with the key factors of active sites, microenvironments, and transmission channels that lead to the distinct catalytic properties of MOF catalysts.

cis-Decalin oxidation as a stereochemical probe of in-MOF versus on-MOF catalysis

Development of catalyst-controlled C–H hydroxylation could provide direct access to valuable synthetic targets, such as primary metabolites.

Three metalloporphyrin frameworks containing imidazole groups: synthesis, characterization and properties

Three metalloporphyrin frameworks containing imidazole groups were structurally characterized, revealing the moderate CO₂ adsorption capacity and high catalytic activity for ethylbenzene oxidation.

Design and synthesis of coordination polymers with chelated units and their application in nanomaterials science

The advances and problems associated with the preparation, properties and structure of coordination polymers with chelated units are presented and assessed.

Porphyrin-Based Metal-Organic Frameworks as Heterogeneous Catalysts in Oxidation Reactions

Porphyrin-based Metal-Organic Frameworks (Por-MOFs) constitute a special branch of the wide MOF family that has proven its own value and high potential in different applications. In this mini-review the application of these materials as catalysts in oxidation reactions is highlighted.

Metal-Organic Frameworks as Catalysts for Oxidation Reactions

This Concept is aimed at describing the current state of the art in metal-organic frameworks (MOFs) as heterogeneous catalysts for liquid-phase oxidations, focusing on three important substrates, namely, alkenes, alkanes and alcohols. Emphases are on the nature of active sites that have been incorporated within MOFs and on future targets to be set in this area. Thus, selective alkene epoxidation with peroxides or oxygen catalyzed by constitutional metal nodes of MOFs as active sites are still to be developed. Moreover, no noble metal-free MOF has been reported to date that can act as a general catalyst for the aerobic oxidation of primary and secondary aliphatic alcohols. In contrast, in the case of alkanes, a target should be to tune the polarity of MOF internal pores to control the outcome of the autooxidation process, resulting in the selective formation of alcohol/ketone mixtures at high conversion.

Highly Stable Mesoporous Zirconium Porphyrinic Frameworks with Distinct Flexibility

The construction of highly stable metal-porphyrinic frameworks (MPFs) is appealing as these materials offer great opportunities for applications in artificial light-harvesting systems, gas storage, heterogeneous catalysis, etc. Herein, we report the synthesis of a novel mesoporous metal-porphyrinic framework (denoted as NUPF-1) and its catalytic properties. NUPF-1 is constructed from a new porphyrin linker and a Zr6 O8 structural building unit, possessing an unprecedented doubly interpenetrating scu net. The structure exhibits not only remarkable chemical and thermal stabilities, but also a distinct structural flexibility, which is seldom seen in metal-organic framework (MOF) materials. By the merit of high chemical stability, NUPF-1 could be easily post-metallized with [Ru3 (CO)12 ], and the resulting {NUPF-1-RuCO} is catalytically active as a heterogeneous catalyst for intermolecular C(sp(3) )-H amination. Excellent yields and good recyclability for amination of small substrates with various organic azides have been achieved.

Metal organic frameworks mimicking natural enzymes: a structural and functional analogy

In this review, we have portrayed the structure, synthesis and applications of a variety of biomimetic MOFs from an unprecedented angle.

Porphyrinic metal–organic frameworks from custom-designed porphyrins

This paper highlights porphyrinic metal–organic frameworks (porph-MOFs) assembled from metal ions and custom-designed porphyrins: pyridyl-based, carboxyphenyl-based porphyrins and other custom-designed porphyrins.

Two heterotrimetallic organic frameworks constructed using a functionalized Schiff base ligand: syntheses, structures and visible photocatalytic activities for the degradation of chlorophenols

Two 3D heterotrimetallics have been synthesized and their photocatalytic degradation performances for chlorophenols have been investigated.

Recent developments in metal-metalloporphyrin frameworks

Metal-organic frameworks (MOFs) based on porphyrin or metalloporphyrin components are of particular interest due to their potential applications in molecular sorption, light-harvesting, and heterogeneous catalysis. This perspective is focused on recent advances in the syntheses and functional properties of metal-metalloporphyrin frameworks, which are classified according to coordination moieties on the porphyrin ligands.

Bottom-up approach to engineer two covalent porphyrinic frameworks as effective catalysts for selective oxidation

Two kinds of novel functional covalent organic frameworks were assembled with the porphyrin building block and terephthalaldehyde or squaric acid via bottom-up approach.