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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

Advances in flexible and functional metal–organic frameworks (MOFs), also called soft porous crystals, are reviewed by covering the literature of the five years period 2009–2013 with reference to the early pertinent work since the late 1990s. Flexible MOFs combine the crystalline order of the underlying coordination network with cooperative structural transformability. These materials can respond to physical and chemical stimuli of various kinds in a tunable fashion by molecular design, which does not exist for other known solid-state materials. Among the fascinating properties are so-called breathing and swelling phenomena as a function of host–guest interactions. Phase transitions are triggered by guest adsorption/desorption, photochemical, thermal, and mechanical stimuli. Other important flexible properties of MOFs, such as linker rotation and sub-net sliding, which are not necessarily accompanied by crystallographic phase transitions, are briefly mentioned as well. Emphasis is given on reviewing the recent progress in application of in situ characterization techniques and the results of theoretical approaches to characterize and understand the breathing mechanisms and phase transitions. The flexible MOF systems, which are discussed, are categorized by the type of metal-nodes involved and how their coordination chemistry with the linker molecules controls the framework dynamics. Aspects of tailoring the flexible and responsive properties by the mixed component solid-solution concept are included, and as well examples of possible applications of flexible metal–organic frameworks for separation, catalysis, sensing, and biomedicine.

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Flexible metal–organic frameworks

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-

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

Metal-organic frameworks (MOFs) are typically highlighted for their potential application in gas storage, separations and catalysis. In contrast, the unique prospects these porous and crystalline materials offer for application in electronic devices, although actively developed, are often underexposed. This review highlights the research aimed at the implementation of MOFs as an integral part of solid-state microelectronics. Manufacturing these devices will critically depend on the compatibility of MOFs with existing fabrication protocols and predominant standards. Therefore, it is important to focus in parallel on a fundamental understanding of the distinguishing properties of MOFs and eliminating fabrication-related obstacles for integration. The latter implies a shift from the microcrystalline powder synthesis in chemistry labs, towards film deposition and processing in a cleanroom environment. Both the fundamental and applied aspects of this two-pronged approach are discussed. Critical directions for future research are proposed in an updated high-level roadmap to stimulate the next steps towards MOF-based microelectronics within the community.

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An updated roadmap for the integration of metal–organic frameworks with electronic devices and chemical sensors

We demonstrate how a single-crystal to single-crystal transformation resulting from bridging-linker replacement is possible in extended 2D and 3D metal–organic frameworks (MOFs) by introducing pillared paddlewheel MOF structures into a solution containing dipyridyl linkers. No lateral movement of the layers was observed during this transformation, creating a templating effect from the “parent” structure to the “daughter” structure. A previously unattainable structure was obtained by a two-step synthetic method utilizing the bridging-linker replacement transformation method. Additionally, a bridging-linker insertion was observed when excess linker was used with the 2D MOF structure, inducing an overall 2D to 3D transformation.

Stepwise synthesis of metal-organic frameworks: replacement of structural organic linkers.

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.

This highlight presents a review of porphyrin paddlewheel frameworks (PPFs) created using porphyrin metalloligands and paddlewheel secondary building units. The combination of these two components, along with dipyridyl pillaring linkers, results in the assembly of 25 PPFs. We describe the topology control exhibited in this series by considering the preferred coordination geometry of the porphyrin building blocks, the length and steric effects of the pillars, and the use of sequential self-assembly. These PPFs and related porphyrinic metal–organic frameworks are emerging as important materials for applications such as heterogeneous catalysis, gas storage, and light harvesting.

Recent advances in porphyrinic metal–organic frameworks: materials design, synthetic strategies, and emerging applications

A thermoresponsive, 3D hinged metal–organic framework (HMOF-1) assembled from meso-tetra(4-pyridyl)porphine and CdI2 exhibits a 3D “lattice fence” topology with extraordinary thermal expansion and shrinkage. A simple structural model is established to elucidate such a drastic thermal response. The hinged structure model presented here can also be applied to other “lattice fence” topologies with little or no modification, depending on the symmetry of the molecular building blocks.

“Nanoscale Lattice Fence” in a Metal–Organic Framework: Interplay between Hinged Topology and Highly Anisotropic Thermal Response

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.

Paul M. Barron

Papers

Stepwise synthesis of metal-organic frameworks: replacement of structural organic linkers.

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

Recent advances in porphyrinic metal–organic frameworks: materials design, synthetic strategies, and emerging applications

“Nanoscale Lattice Fence” in a Metal–Organic Framework: Interplay between Hinged Topology and Highly Anisotropic Thermal Response

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