Transition-metal-catalyzed cross-coupling reactions have been well-established as indispensable tools in modern organic synthesis. One of the major research goals in cross-coupling area is expanding the scope of the coupling partners. In the past decade, diazo compounds (or their precursors N-tosylhydrazones) have emerged as nucleophilic cross-coupling partners in C–C single bond or C═C double bond formations in transition-metal-catalyzed reactions. This type of coupling reaction involves the following general steps. First, the organometallic species is generated by various processes, including oxidative addition, transmetalation, cyclization, C–C bond cleavage, and C–H bond activation. Subsequently, the organometallic species reacts with the diazo substrate to generate metal carbene intermediate, which undergoes rapid migratory insertion to form a C–C bond. The new organometallic species generated from migratory insertion may undergo various transformations. This type of carbene-based coupling has proven...

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

Water is essential in all aspects of life, being the defining characteristic of our planet and even our body. Regrettably, water pollution is increasingly becoming a challenge due to novel anthropogenic pollutants. Of particular concern are emerging organic contaminants (EOCs), the term used not only to cover newly developed compounds but also compounds newly discovered as contaminants in the environment. Aside from anthropogenic contamination, higher temperature and more extreme and less predictable weather conditions are projected to affect water availability and distribution. Therefore, wastewater treatment has to become a valuable water resource and its reuse is an important issue that must be carried out efficiently. Among the novel technologies considered in water remediation processes, metal-organic frameworks (MOFs) are regarded as promising materials for the elimination of EOCs since they present many properties that commend them in water treatment: large surface area, easy functionalizable cavities, some are stable in water, and synthesized at large scale, etc. This review highlights the advances in the use of MOFs in the elimination (adsorption and/or degradation) of EOCs from water, classifying them by the nature of the contaminant.

Metal-Organic Frameworks for the Removal of Emerging Organic Contaminants in Water.

Solid-state proton conductors (SSPCs), which are a key component for the safety and efficiency of fuel cells, have received much attention due to their broad application in electrochemical devices....

Proton Transport in Metal-Organic Frameworks.

Among the recent developments in metal-organic frameworks (MOFs), porous layered coordination polymers (CPs) have garnered attention due to their modular nature and tunable structures. These factors enable a number of properties and applications, including gas and guest sorption, storage and separation of gases and small molecules, catalysis, luminescence, sensing, magnetism, and energy storage and conversion. Among MOFs, two-dimensional (2D) compounds are also known as 2D CPs or 2D MOFs. Since the discovery of graphene in 2004, 2D materials have also been widely studied. Several 2D MOFs are suitable for exfoliation as ultrathin nanosheets similar to graphene and other 2D materials, making these layered structures useful and unique for various technological applications. Furthermore, these layered structures have fascinating topological networks and entanglements. This review provides an overview of different aspects of 2D MOF layered architectures such as topology, interpenetration, structural transformations, properties, and applications.

Two-Dimensional Metal-Organic Framework Materials: Synthesis, Structures, Properties and Applications.

As a novel class of porous crystalline materials, hydrogen-bonded organic frameworks (HOFs), self-assembled from organic or metal-organic building blocks through intermolecular hydrogen-bonding interactions, have attracted more and more attention. Over the past decade, a number of porous HOFs have been constructed through judicious selection of H-bonding motifs, which are further enforced by other weak intermolecular interactions such as π-π stacking and van der Waals forces and framework interpenetration. Since the H-bonds are weaker than coordinate and covalent bonds used for the construction of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), HOFs have some unique features such as mild synthesis condition, solution processability, easy healing, and regeneration. These features enable HOFs to be a tunable platform for the construction of functional materials. Here, we review the H-bonding motifs used for constructing porous HOFs and highlight some of their applications, including gas separation and storage, chiral separation and structure determination, fluorescent sensing, heterogeneous catalysis, biological applications, proton conduction, photoluminescent materials, and membrane-based applications.

Hydrogen-Bonded Organic Frameworks as a Tunable Platform for Functional Materials.

Three coordination polymers (CPs) have been synthesized based on a [Co(bpy)(H2 O)4 ]2+ chain (bpy=4,4'-bipyridine) by a template approach. The frameworks are neutralized by different templated polycarboxylate anions (furan di-carboxylate (fdc) in Co-fdc, benzene tri-carboxylate (btc) in Co-tri and benzene tetra-carboxylate (btec) in Co-tetra). These templates with different degrees of protonation and ionic carrier concentration played significant role on crystal packing as well as formation of well-directed H-bonded networks which made these CPs perform well in proton conduction (PC). The PC value reaches to 1.49×10-1  S cm-1 under 80 °C and 98 % relative humidity (R.H.) for Co-tri, which is the highest among CPs/MOFs/COFs and is an example of conductivity in the order of 10-1  S cm-1 . Co-tri and Co-tetra are excellent proton conductors at mild temperature (40 °C) and 98 % R.H. (conductivities up to 2.92×10-2 and 1.38×10-2  S cm-1 , respectively).

Polycarboxylate-Templated Coordination Polymers: Role of Templates for Superprotonic Conductivities of up to 10-1 S cm-1.

Proton-conducting materials in the solid state have received immense attention for their role as electrolytes in proton-exchange membrane fuel cells. Recently, crystalline materials-metal-organic frameworks (MOFs), hydrogen-bonded organic frameworks (HOFs), covalent organic frameworks (COFs), polyoxometalates (POMs), and porous organic crystals-have become an exciting research topic in the field of proton-conducting materials. For a better electrolyte, a high proton conductivity on the order of 10-2  S cm-1 or higher is preferred as efficient proton transport between the electrodes is ultimately necessary. With an emphasis on design principles, this Concept will focus on MOFs and other crystalline solid-based proton-conducting platforms that exhibit "ultrahigh superprotonic" conductivities with values in excess of 10-2  S cm-1 . While only a handful of MOFs exhibit such an ultrahigh conductivity, this quality in other systems is even rarer. In addition to interpreting the structural-functional correlation by taking advantage of their crystalline nature, we address the challenges and promising directions for future research.

Metal–Organic Frameworks and Other Crystalline Materials for Ultrahigh Superprotonic Conductivities of 10−2 S cm−1 or Higher

Self-assembly of bent dicarboxylate linker 4,4′-sulfonyldibenzoic acid (H2SDB) and flexible N,N-donor spacer 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (L) with Co(NO3)2·6H2O forms a twofold interpenetrated {[Co2(SDB)2(L)]·(H2O)4·(DMF)}n, (IITKGP-6) network via solvothermal synthesis with sql(2,6L1) topology, which is characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, elemental analysis, powder X-ray diffraction (XRD), and single-crystal XRD. The framework is microporous with a solvent-accessible volume of 25.5% and forms a one-dimensional channel along [1–1 0] direction with the dimensions of ∼3.4 × 5.0 A2. As the stability of metal–organic frameworks (MOFs) in the presence of water is a topic of significant importance while considering them for practical applications, this framework reveals its high stability toward water. The desolvated framework shows modest uptake of CO2 (50.6 and 37.4 cm3 g–1 at 273 and 295 K under 1 bar pressure, respectively), with high selectivi...

A Water-Stable Twofold Interpenetrating Microporous MOF for Selective CO2 Adsorption and Separation.

Recently, proton conduction has been a thread of high potential owing to its wide applications in fuel-cell technology. In the search for a new class of crystalline materials for protonic conductors, three metalo hydrogen-bonded organic frameworks (MHOFs) based on [Ni(Imdz)6 ]2+ and arene disulfonates (MHOF1 and MHOF2) or dicarboxylate (MHOF3) have been reported (Imdz=imidazole). The presence of an ionic backbone with charge-assisted H-bonds, coupled with amphiprotic imidazoles made these MHOFs protonic conductors, exhibiting conduction values of 0.75×10-3 , 3.5×10-4 and 0.97×10-3  S cm-1 , respectively, at 80 °C and 98 % relative humidity, which are comparable to other crystalline metal-organic framework, coordination polymer, polyoxometalate, covalent organic framework, and hydrogen-bonded organic framework materials. This report initiates the usage of MHOF materials as a new class of solid-state proton conductors.

Metalo Hydrogen-Bonded Organic Frameworks (MHOFs) as New Class of Crystalline Materials for Protonic Conduction.

Two cadmium-based 3D luminescent MOFs {[Cd2(SA)2(L)2]·H2O}n (Cd-MOF-1) and [Cd(CDC)(L)]n (Cd-MOF-2) (H2SA = succinic acid, H2CDC = 1,4-cyclohexanedicarboxylic acid, L = [3,3′-azobis(pyridine)]) have been assembled by employing organic dicarboxylic acid linkers with an unexploited azo-functionalized N,N′ spacer via a room temperature slow evaporation process, and they are characterized by single crystal X-ray analysis, TGA, FT-IR, PXRD and elemental analysis. The topological analysis reveals that Cd-MOF-1 features a 6c-uninodal rare ‘rob’ topology with the point symbol {48·66·8}, whereas Cd-MOF-2 shows a ‘pcu’ alpha-Po primitive cubic topology with the point symbol {412·63}. These MOFs are highly emissive at 382 nm and 398 nm when excited at 305 nm and 312 nm, respectively. The exposed azo groups are presumed to act as functional sites for the recognition of metal ions through quenching of fluorescence intensity. The fluorescence measurements show that these MOFs can selectively and sensitively detect Fe3+ as well as Al3+ and thus, they demonstrate potential as dual-responsive luminescent probes for metal-ion sensing. EDS elemental mapping, PXRD of loaded MOF materials, and a plausible quenching mechanism have been discussed. More importantly, both the MOFs exhibit rapid response times toward sensing of Fe3+ and Al3+.

Arun Pal

Papers

Polycarboxylate-Templated Coordination Polymers: Role of Templates for Superprotonic Conductivities of up to 10-1 S cm-1.

Metal–Organic Frameworks and Other Crystalline Materials for Ultrahigh Superprotonic Conductivities of 10−2 S cm−1 or Higher

A Water-Stable Twofold Interpenetrating Microporous MOF for Selective CO2 Adsorption and Separation.

Metalo Hydrogen-Bonded Organic Frameworks (MHOFs) as New Class of Crystalline Materials for Protonic Conduction.

Two azo-functionalized luminescent 3D Cd(II) MOFs for highly selective detection of Fe3+ and Al3+