/pdf/metal-organic-frameworks-and-self-assembled-supramolecular-2xs948xxr9.pdf

Metal–Organic Frameworks and Self-Assembled Supramolecular Coordination Complexes: Comparing and Contrasting the Design, Synthesis, and Functionality of Metal–Organic Materials

Metal–organic frameworks (MOFs) are constructed from metal ions/clusters coordinated by organic linkers (or bridging-ligands). The hallmark of MOFs is their permanent porosity, which is frequently found in MOFs constructed from metal-clusters. These clusters are often formed in situ, whereas the linkers are generally pre-formed. The geometry and connectivity of a linker dictate the structure of the resulting MOF. Adjustments of linker geometry, length, ratio, and functional-group can tune the size, shape, and internal surface property of a MOF for a targeted application. In this critical review, we highlight advances in MOF synthesis focusing on linker design. Examples of building MOFs to reach unique properties, such as unprecedented surface area, pore aperture, molecular recognition, stability, and catalysis, through linker design are described. Further search for application-oriented MOFs through judicious selection of metal clusters and organic linkers is desirable. In this review, linkers are categorized as ditopic (Section 1), tritopic (Section 2), tetratopic (Section 3), hexatopic (Section 4), octatopic (Section 5), mixed (Section 6), desymmetrized (Section 7), metallo (Section 8), and N-heterocyclic linkers (Section 9).

Tuning the structure and function of metal–organic frameworks via linker design

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-

/pdf/zirconium-metalloporphyrin-pcn-222-mesoporous-metal-organic-5fxw4zlsqt.pdf

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

Two-dimensional (2D) metal–organic framework (MOF) nanosheets are attracting increasing research attention due to their unique properties originating from their ultrathin thickness, large surface area and high surface-to-volume atom ratios. Many great advances have been made in the synthesis and application of 2D MOF nanosheets over the past few years. In this review, we summarize the recent advances in the synthesis of 2D MOF nanosheets by using top-down methods, e.g. sonication exfoliation, mechanical exfoliation, Li-intercalation exfoliation and chemical exfoliation, and bottom-up methods, i.e. interfacial synthesis, three-layer synthesis, surfactant-assisted synthesis, modulated synthesis, and sonication synthesis. In addition, the recent progress in 2D MOF nanosheet-based nanocomposites is also briefly introduced. The potential applications of 2D MOF nanosheets in gas separation, energy conversion and storage, catalysis, sensors and biomedicine are discussed. Finally, we give our personal insights into the challenges and opportunities for the future research of 2D MOF nanosheets and their composites.

Two-dimensional metal-organic framework nanosheets: synthesis and applications.

Pyridine-functionalized graphene (reduced graphene oxide) can be used as a building block in the assembly of metal organic framework (MOF). By reacting the pyridine-functionalized graphene with iron–porphyrin, a graphene–metalloporphyrin MOF with enhanced catalytic activity for oxygen reduction reactions (ORR) is synthesized. The structure and electrochemical property of the hybrid MOF are investigated as a function of the weight percentage of the functionalized graphene added to the iron–porphyrin framework. The results show that the addition of pyridine-functionalized graphene changes the crystallization process of iron–porphyrin in the MOF, increases its porosity, and enhances the electrochemical charge transfer rate of iron–porphyrin. The graphene–metalloporphyrin hybrid shows facile 4-electron ORR and can be used as a promising Pt-free cathode in alkaline Direct Methanol Fuel Cell.

Electrocatalytically active graphene-porphyrin MOF composite for oxygen reduction reaction.

We report three new porphyrin-based, pillared paddle-wheel homologous series: porphyrin paddle-wheel frameworks PPF-3, -4, and -5. These compounds are assembled from free base or palladium tetrakis(4-carboxyphenyl)porphine, M(NO3)2 (M = Co and Zn), and 4,4′-bipyridine via solvothermal reactions. The resulting solids exhibit 3D metal−organic frameworks, where 2D layers are pillared by bipyridine with three different packing arrangements.

Pillared porphyrin homologous series: intergrowth in metal-organic frameworks.

Seven new heterometallic frameworks have been synthesized with specific control over the metallic composition of each structure. Using Mn, Fe, Ni, V, and Pt metallated tetra-(4-carboxyphenyl)porphine (MTCPP) metalloligands and Zn and Co paddle-wheel building units, two series of heterometallic frameworks are assembled that maintain topology with changing metal composition.

Highly Tunable Heterometallic Frameworks Constructed from Paddle-Wheel Units and Metalloporphyrins

A new series of porphyrin paddlewheel frameworks (PPFs) has been assembled from 5,10,15,20-tetrakis(4-carboxyl)-21H,23H-porphine (TCPP), Zn(NO3)2·6H2O, and organic pillars such as N,N′-di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNI) and 3,6-di-4-pyridyl-1,2,4,5-tetrazine (DPT). Here we report the synthesis and structural characterization of five new PPFs based on 2D porphyrin paddlewheel grid. In this homologous series, the stacking patterns of such 2D porphyrin sheets are varied and exhibit two new structures, namely, bilayer and interpenetrated AA stacking pattern, in addition to the ABBA pattern previously observed.

Structural Variation in Porphyrin Pillared Homologous Series: Influence of Distinct Coordination Centers for Pillars on Framework Topology

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Pillared porphyrin homologous series: intergrowth in metal-organic frameworks.

Highly Tunable Heterometallic Frameworks Constructed from Paddle-Wheel Units and Metalloporphyrins

Structural Variation in Porphyrin Pillared Homologous Series: Influence of Distinct Coordination Centers for Pillars on Framework Topology