Jian-Rong Li

Zirconium-Metalloporphyrin PCN-222: Mesoporous Metal–Organic Frameworks with Ultrahigh Stability as Biomimetic Catalysts†

In nature, metalloporphyrins are well known for performing many biological functions in aqueous media, such as light harvesting, oxygen transportation, and catalysis. Heme, the iron–porphyrin derivative, is the cofactor for many enzyme/ protein families, including peroxidases, cytochromes, hemoglobins, and myoglobins. Using synthetic systems to mimic natural enzymes with high catalytic activity and substrate selectivity has been a sought-after goal in the last decade. Direct application of a heme as an oxidation catalyst in aqueous solution is usually challenging due to the formation of catalytically inactive dimers and catalyst self-destruction in the oxidizing reaction media. One promising approach is to load heme on supports, such as zeolites, clays, nanoparticles, hydrogels, or carbon materials, a practice which inevitably dilutes the density of active sites. An alternative approach is to protect the heme center by modifying the porphyrin to produce dendrimers or molecular crystals, which is a synthetically demanding method. Herein, we propose a unique strategy employing heme-like active centers as structural motifs for the assembly of highly stable porous materials, which should possess well-defined mesochannels and ultrahigh stability in aqueous solution. Metal-organic frameworks (MOFs) are a new class of crystalline porous materials with fascinating structures and intriguing properties, such as permanent porosity, high surface area, and uniform open cavities. The availability of various building blocks consisting of metals and organic linkers makes it possible to construct MOFs with unique properties for diverse applications. However, these desirable features of MOFs have rarely been applied to an enzymatic mimic, especially for catalysis in an aqueous medium, despite the fact that the assembly of ligands bearing high-density active sites into 3D frameworks may provide an ideal system to both enhance the catalytic activity and protect the cofactors. One of the main reasons is the lack of water-stable MOFs containing redox-active metal centers. Furthermore, most MOFs are microporous (pore size< 2 nm). Although they are suitable for gas storage, the small pore size slows down diffusion and limits the access of large substrate molecules to the active sites inside a MOF. Therefore, MOFs with mesopores, accessible redox sites, and ultrahigh stability, especially in aqueous media, are indispensible for any successful biomimetic attempt. Herein we have employed Fe-TCPP (TCPP= tetrakis(4carboxyphenyl)porphyrin) as a heme-like ligand and chosen highly stable Zr6 clusters as nodes for the assembly of stable Zr-MOFs. With carefully selected starting materials, we have successfully constructed a 3D heme-like MOF, designated as PCN-222(Fe) (Figure 1; PCN= porous coordination net-

Photocatalytic organic pollutants degradation in metal–organic frameworks

Efficient removal of organic pollutants from wastewater has become a hot research topic due to its ecological and environmental importance. Traditional water treatment methods such as adsorption, coagulation, and membrane separation suffer from high operating costs, and even generate secondary pollutants. Photocatalysis on semiconductor catalysts (TiO2, ZnO, Fe2O3, CdS, GaP, and ZnS) has demonstrated efficiency in degrading a wide range of organic pollutants into biodegradable or less toxic organic compounds, as well as inorganic CO2, H2O, NO3−, PO43−, and halide ions. However, the difficult post-separation, easy agglomeration, and low solar energy conversion efficiency of these inorganic catalysts limit their large scale applications. Exploitation of new catalysts has been attracting great attention in the related research communities. In the past two decades, a class of newly-developed inorganic–organic hybrid porous materials, namely metal–organic frameworks (MOFs) has generated rapid development due to their versatile applications such as in catalysis and separation. Recent research has showed that these materials, acting as catalysts, are quite effective in the photocatalytic degradation of organic pollutants. This review highlights research progress in the application of MOFs in this area. The reported examples are collected and analyzed; and the reaction mechanism, the influence of various factors on the catalytic performance, the involved challenges, and the prospect are discussed and estimated. It is clear that MOFs have a bright future in photocatalysis for pollutant degradation.

Bridging-ligand-substitution strategy for the preparation of metal–organic polyhedra

Metal-organic polyhedra-discrete molecular architectures constructed through the coordination of metal ions and organic linkers-have recently attracted considerable attention due to their intriguing structures, their potential for a variety of applications and their relevance to biological self-assembly. Several synthetic routes have been investigated to prepare these complexes. However, to date, these preparative methods have typically been based on the direct assembly of metal ions and organic linkers. Although these routes are convenient, it remains difficult to find suitable reaction conditions or to control the outcome of the assembly process. Here, we demonstrate a synthetic strategy based on the substitution of bridging ligands in soluble metal-organic polyhedra. The introduction of linkers with different properties from those of the initial metal-organic polyhedra can thus lead to new metal-organic polyhedra with distinct properties (including size and shape). Furthermore, partial substitution can also occur and form mixed-ligand species that may be difficult to access by means of other approaches.

Selective Gas Adsorption and Unique Structural Topology of a Highly Stable Guest-Free Zeolite-Type MOF Material with N-rich Chiral Open Channels

A new multifunctional di-topic tetrazolate-based ligand, 2,3-di-1H-tetrazol-5-ylpyrazine (H(2)dtp) has been designed and synthesized. The solvothermal reaction of this ligand with ZnCl(2) gave a robust guest-free three-dimensional zeolite-like chiral metal-organic framework (MOF) complex, [Zn(dtp)], which crystallized in chiral space group P6(1) and possessed chiral open channels with nitrogen-rich walls and the diameter of approximately 4.1 A. This framework presents a unique uniform etd (8,3) topology, is the first example of its type in MOFs, and exhibits high thermal stability with the decomposition temperature above 380 degrees C and permanent porosity. It is interesting that this material is able to selectively adsorb O(2) and CO(2) over N(2) gas, being a rare example in MOFs. In addition, C(2)H(2) and MeOH adsorption results show that although the framework channel holds nitrogen-rich walls that may provide H-bonding sites, no NH H-bond effect between the guest molecules and microporous surface was observed.

Reversible Alteration of CO2 Adsorption upon Photochemical or Thermal Treatment in a Metal–Organic Framework

A metal-organic framework (MOF) for reversible alteration of guest molecule adsorption, here carbon dioxide, upon photochemical or thermal treatment has been discovered. An azobenzene functional group, which can switch its conformation upon light irradiation or heat treatment, has been introduced to the organic linker of a MOF. The resulting MOF adsorbs different amount of CO(2) after UV or heat treatment. This remarkable stimuli-responsive adsorption effect has been demonstrated through experiments.

Interconversion between Molecular Polyhedra and Metal−Organic Frameworks

The construction of a metal-organic framework (MOF) with a pcu-a topology using a preassembled soluble molecular octahedron has been realized experimentally. The resulting MOF can also be reversibly converted to the molecular octahedron. All such conversions are based on axial-ligand substitution reactions on the molecular octahedron.

Pore Surface Engineering with Controlled Loadings of Functional Groups via Click Chemistry in Highly Stable Metal–Organic Frameworks

Reactions of ZrCl(4) and single or mixed linear dicarboxylic acids bearing methyl or azide groups lead to highly stable isoreticular metal-organic frameworks (MOFs) with content-tunable, accessible, reactive azide groups inside the large pores. These Zr-based MOFs offer an ideal platform for pore surface engineering by anchoring various functional groups with controlled loadings onto the pore walls via the click reaction, endowing the MOFs with tailor-made interfaces. Significantly, the framework and crystallinity of the functionalized MOFs are well-retained, and the engineered pore surfaces have been demonstrated to be readily accessible, thus providing more opportunities for powerful and broad applications of MOFs.

Cooperative template-directed assembly of mesoporous metal-organic frameworks.

Despite great efforts, the development of a reliable way to assemble mesoporous metal-organic frameworks (mesoMOFs) remains a challenge. In this work, we have designed a cooperative template system, comprising a surfactant (cetyltrimethylammonium bromide) and a chelating agent (citric acid), for the generation of a mesoMOF containing a hierarchical system of mesopores interconnected with microspores. The surfactant molecules form micelles and the chelating agent bridges the MOF and the micelles, making self-assembly and crystal growth proceed under the direction of the cooperative template. However, when the surfactant or the chelating agent was applied individually, no mesoMOF was obtained.

Multipoint interactions enhanced CO2 uptake: a zeolite-like zinc-tetrazole framework with 24-nuclear zinc cages.

A zeolite-like microporous tetrazole-based metal-organic framework (MOF) with 24 nuclear zinc cages was synthesized and characterized. It exhibits high CO(2) adsorption capacity up to 35.6 wt % (8.09 mmol/g) and excellent CO(2)/CH(4) selectivity at 273 K/1 bar, being among the highest values known to date. Theoretical calculations based on simulated annealing techniques and periodic DFT revealed that CO(2) is predominantly located around the inner surface of the cages through multipoint interactions, in particular, around the aromatic tetrazole rings. Importantly, it is the first time that multipoint interactions between CO(2) molecules and frameworks resulting in high CO(2) uptake are observed.

A pcu-type metal–organic framework with spindle [Zn7(OH)8]6+ cluster as secondary building units

The in situ solvothermal reaction of 9,10-dicyanoanthracene and ZnCl(2)/NaN(3) gave the complex, {[Zn(7)(OH)(8)(DTA)(3)].H(2)O}(n) () (DTA(2-) = 9,10-ditetrazolateanthracene), which presents a pcu-type topological framework formed by DTA(2-) bridging unprecedented heptanuclear spindle [Zn(7)(OH)(8)](6+) clusters as SBUs, and exhibits strong luminescent emission with long lifetime.