There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

The Chemistry and Applications of Metal-Organic Frameworks

From molecules to crystal engineering: supramolecular isomerism and polymorphism in network solids.

Synthesis of metal-organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites.

Hydrogen Storage in Metal–Organic Frameworks

Improvements in the efficiency of combustion within a vehicle can lead to reductions in the emission of harmful pollutants and increased fuel efficiency. Gas sensors have a role to play in this process, since they can provide real time feedback to vehicular fuel and emissions management systems as well as reducing the discrepancy between emissions observed in factory tests and ‘real world’ scenarios. In this review we survey the current state-of-the-art in using porous materials for sensing the gases relevant to automotive emissions. Two broad classes of porous material – zeolites and metal–organic frameworks (MOFs) – are introduced, and their potential for gas sensing is discussed. The adsorptive, spectroscopic and electronic techniques for sensing gases using porous materials are summarised. Examples of the use of zeolites and MOFs in the sensing of water vapour, oxygen, NOx, carbon monoxide and carbon dioxide, hydrocarbons and volatile organic compounds, ammonia, hydrogen sulfide, sulfur dioxide and hydrogen are then detailed. Both types of porous material (zeolites and MOFs) reveal great promise for the fabrication of sensors for exhaust gases and vapours due to high selectivity and sensitivity. The size and shape selectivity of the zeolite and MOF materials are controlled by variation of pore dimensions, chemical composition (hydrophilicity/hydrophobicity), crystal size and orientation, thus enabling detection and differentiation between different gases and vapours.

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Gas sensing using porous materials for automotive applications

Multidimensional crystal engineering of bifunctional metal complexes containing complementary triple hydrogen bonds

Mixed-component metal–organic frameworks (MC-MOFs) are metal–organic frameworks that have different linkers or metals with the same structural role. Many of these mixed-ligand or mixed-metal MOFs are solid solutions, in which the proportions of the ligands or metals can be adjusted or even controlled. These MC-MOFs can be prepared directly, using more than one metal or ligand in the synthesis, or formed by post-synthetic modification. A second class of MC-MOFs have core–shell structures, and these can be prepared through epitaxial growth of one MOF on the surface of another or post-synthetic modification of the crystal surfaces. This review describes the syntheses, structures and properties of mixed-ligand, mixed-metal and core–shell MOFs, and highlights some of the potential benefits in functionality that these materials have.

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Mixed-component metal–organic frameworks (MC-MOFs): enhancing functionality through solid solution formation and surface modifications

Metal–organic frameworks (MOFs) are currently attracting considerable attention, largely because of their potential for porosity, and their consequent use in applications as diverse as gas storage, catalysis, separations, and drug delivery. The first generation of MOFs were formed by linking together metal centers with simple, commercially available bridging ligands, such as 1,4-benzenedicarboxylate (bdc), but there has since been an increasing shift towards more complex structures and increased functionality. For example, MOFs in which the pores contain accessible hydrogenbonding groups, unsaturated metal centers, or chirality have been reported and studied, and the preparation of dynamic porous materials, capable of undergoing guestinduced transformations or reformations, has been explored. Another approach to forming functionalized networks is to undertake reactions on preformed MOFs, converting one solid state material into another. The incorporation of an additional functional group, a “tag”, into a linking ligand offers the opportunity to form structures in which this group is preserved during the MOF synthesis, allowing it to project into the pores or channels of the network structure. We define a “tag” as a group or functionality that is stable and innocent (that is, non-structure-defining) during MOF formation, but that can be transformed by a post-synthetic modification. This approach is shown schematically in Figure 1. A similar concept of tagging has also recently been applied in medicinal chemistry. Post-synthetic modification allows the pores in a preformed MOF to be tailored for a specific purpose, which offers the possibility of fine-tuning for selective adsorption and catalysis. The strategy also facilitates the incorporation into a MOF of functional groups that would not survive the conditions of the MOF synthesis (e.g., temperature and pH) and of functional groups that might compete with the donor groups on the bridging ligands. Given these advantages, it is surprising that there has been very little focus on postsynthetic modification of MOFs. Kim and co-workers showed that the pendant pyridyl groups in a chiral zinc network could be methylated and, very recently, Wang and Cohen, and Gamez and co-workers have both demonstrated that the amino groups in 2-amino-1,4-benzenedicarboxylate MOFs can be converted into amides or urethanes. Rosseinsky and co-workers have converted these amines into salicylidenes, and then used these to coordinate vanadium. Fujita and co-workers have shown that guest molecules can undergo similar transformations within the pores of a MOF. Herein, we report our endeavors to prepare tagged MOFs suitable for post-synthetic modification, starting from an aldehyde-modified dicarboxylate. Following seminal work from Yaghi and co-workers, it is now well-established that the octahedral zinc secondary building unit (SBU) Zn4O(O2CR)6 forms an isoreticular series of MOFs containing the same framework topology with linear dicarboxylates, such as bdc and 4,4’-biphenyldicarboxylate (bpdc). We have prepared the aldehyde-tagged dicarboxylic acid H2L 1 (2-formylbiphenyl-4,4’-dicarboxylic acid, Scheme 1), and used it in MOF synthesis. The coordinated L ligand is suitable for Figure 1. Schematic representation of the post-synthetic modification strategy for MOFs.

Post-synthetic modification of tagged metal-organic frameworks

Hydrolysis of the DMF or DEF solvent influences the nature of the product observed in the reaction between zinc(II) nitrate and 1,4-benzenedicarboxylic acid, with dialkylammonium cations able to template the formation of anionic networks.

Andrew D. Burrows

Papers

Gas sensing using porous materials for automotive applications

Multidimensional crystal engineering of bifunctional metal complexes containing complementary triple hydrogen bonds

Mixed-component metal–organic frameworks (MC-MOFs): enhancing functionality through solid solution formation and surface modifications

Post-synthetic modification of tagged metal-organic frameworks

Solvent hydrolysis and templating effects in the synthesis of metal-organic frameworks