Bimetallic Organic Frameworks via In Situ Solvothermal Sol-Gel-Crystal and Sol-Crystal Transformation as Durable Electrocatalysts for Oxygen Reduction Reaction.

The in situ solvothermal conversion of metal-organic gels (MOGs) to crystalline metal-organic frameworks (MOFs) represents a versatile and ingenious strategy that has been employed for the synthesis of MOF materials with specific morphologies, high yield, and improved functional properties. Herein, we have adopted an in situ solvothermal conversion of bimetallic MOGs to crystalline bimetallic MOFs with the aim of introducing a redox-active metal heterogeneity into the monometallic counterpart. The formation of bimetallic NiZn-MOF and CoZn-MOF via in situ solvothermal sol-gel-crystal and sol-crystal transformation is found to depend on the solvent systems used. The sol-to-gel-to-crystal transformation of NiZn-MOF via the formation of NiZn-MOG is found to occur through the gradual disruption of gel fibers leading to subsequent formation of microcrystals and single crystals of NiZn-MOF. These bimetallic MOFs and MOGs serve as promising electrocatalysts for oxygen reduction reaction (ORR) with an excellent methanol tolerance property, which can be attributed to the enhanced mass and charge transfer, higher oxygen vacancies, and bimetallic synergistic interactions among the heterometals. This work demonstrates a convenient strategy for producing bimetallic MOGs to MOFs through the introduction of a redox-active metal heterogeneity in the inorganic hybrid functional materials for fundamental and applied research. Our results connect MOGs and MOFs which have been regarded as having opposite physical states, that is, soft vs hard, and provide promising structural correlation between MOGs and MOFs at the molecular level.

Diffusion and Flux Improvement of Drugs through Complexation.

Improving the solubility and permeability of drugs via cocrystallization is an important theme in crystal engineering with practical applications for the discovery and development of high bioavailability medicines. The past decade has witnessed a surge of publications on pharmaceutical cocrystals/salts to improve the permeability of Biopharmaceutics Classification System (BCS) class IV drugs. In this review article, the reader is introduced to the fundamentals of drug permeability mechanisms and then examples of pharmaceutical cocrystals and salts designed to enhance drug diffusion and permeability are presented, in order to understand the different structural factors that modulate drug flux and transport across a semipermeable membrane. Broadly, two main phenomena can be summarized from the 50 or so examples: (1) The heterosynthons in hydrogen-bonded drug-coformer aggregates survive long enough in the experimental media such that the drug, which is present in high concentration due to supersaturation, exhibits higher flux across the semipermeable membrane. (2) The coformer or cocrystal is able to reduce the transepithelial electrical resistance (TEER) values of lipid monolayers, which impairs their tight junctions, and facilitates drug passage to improve its diffusion/permeability. The medicinal chemistry literature on high permeability drugs is recapitulated with the idea that these principles may be utilized in the de novo design of high permeability coformers for the synthesis of improved-performance pharmaceutical cocrystals. Enhancing drug solubility and permeability without changing its molecular structure in supramolecular complexes of pharmaceutical cocrystals and salts will address the poor bioavailability challenge for a majority of BCS class II and IV drugs.

Photoinduced Bending of Single Crystals of a Linear Bis-Olefin via Water-Templated Solid-State [2+2] Photopolymerization Reaction.

The single crystals of two structural isomers of bis-olefinic molecules were shown to have contrasting properties in terms of their photoreactivity: one exhibits an excellent ability to form polymers, accompanied with bending of crystals upon irradiation, while the other is photostable. The photoreactive crystal is a first example in which [2+2] polymerization leads to bending of the crystals, with implications for the design of photoactuators. The hydrate formation ability of one of these molecular isomers promotes the solid-state reactivity in its crystal, as the H2 O molecules act as a template to bring the olefin molecules into the required arrangement for [2+2] polymerization. Further, the crystals of the polymer exhibited better flexibility and smoothed surfaces compared to those of the monomers. In addition, under UV-light the diene emits bluish violet light while the polymer emits green light, indicating that the luminescence property can be tuned through photoirradiation.

Self-Sorting of Metal-Organic Polymeric Assemblies in Gels: Selective Templation and Catalysis of Homodimers.

The development of generic strategy is essential for the construction of higher order supramolecular assemblies from the mixture of molecular components. Such higher order aggregations are possible through a self-sorting phenomenon, which is not well-explored in gel materials. Here, first examples of self-sorting in the coordination polymer (CP) based gels have been explored using three and four component systems. The self-sorting phenomenon has been monitored through a [2+2] photochemical reaction in the gel state and characterized by 1 H NMR, diffuse reflectance spectroscopy (DRS) and single crystal XRD analyses. Furthermore, AgI was shown to act as a supramolecular catalyst for the [2+2] photochemical reaction in gels.

Superprotonic Conductivity by Synergistic Blending of Coordination Polymers with Organic Polymers: Fabrication of Durable and Flexible Proton Exchange Membranes.

Creation of an efficient and cost-effective proton exchange membrane (PEM) has emerged as a propitious solution to address the challenges of renewable energy development. Coordination polymers (CPs) have garnered significant interest due to their multifunctional applications and moldability, along with long-range order. To leverage the potential of CPs in fuel cells, it is essential to integrate microcrystalline CPs into organic polymers to prepare membranes and avoid grain boundary issues. In this study, we designed and synthesized CPs containing imidazole and sulfonate moieties via gel-to-crystal transformation. The integration of CPs into the PVDF-PVP matrix resulted in superprotonic conductivity in the order of 10-2 S cm-1 at room temperature (30 ºC) and 98% RH. The proton conductivity achieved with CP-integrated composite membrane was 4.69 × 10-2 S cm-1 at 80 °C and 98% RH, the highest among all CP/MOF-integrated PVDF-PVP membranes under hydrous conditions. The excellent compatibility of CPs with PVDF-PVP produced highly flexible membranes with superior mechanical, chemical, and thermal stability. About 25 times higher proton conductivity value was achieved with membrane, compared to intrinsic CPs, at RT and 98% RH. Thus, we present a cost-effective CP-integrated mixed-matrix membrane with superprotonic conductivity and long-term durability for cutting-edge fuel cell development.

In situ conversion of a MOG to a crystalline MOF: a case study on solvent-dependent gelation and crystallization.

Herein, we report in situ transformation of a metal-organic gel (MOG) to a crystalline metal-organic framework (MOF) and solvent-dependent gelation/crystallization via solvothermal reactions of a tetracarboxylic acid, namely 4,4'-dinitro-2,2',6,6'-tetracarboxybiphenyl, and ZnSO4. The results provide structural insights into MOGs at the molecular level and also help in the synthesis of crystalline MOFs that are otherwise difficult to obtain.

Solid or gel? Which one works better for [2 + 2] photochemical polymerization in pyridine appended flexible phenylene 1, 4-bis-olefins by Ag(i) templation?

Two diene molecules were shown to undergo photopolymerization reactions in their metal-organic gels and xerogels, while their respective crystalline CPs are photostable. These reactions reveal the advantages of the gels and xerogels compared to their crystalline counterparts and also the utility of AgAg interactions in the gels to promote topochemical polymerizations.

Proton-Conducting Hydrogen-Bonded 3D Frameworks of Imidazo-Pyridine-Based Coordination Complexes Containing Naphthalene Disulfonates in Rhomboid Channels.

Coordination complexes of an olefinic molecule (PIP) containing pyridine and imidazopyridine moieties with ZnII /NiII metal salts were shown to exhibit appreciable proton conductivity. These complexes form 3D-hydrogen bonded frameworks containing rhomboidal channels that are occupied by uncoordinated 1,5-naphthalenedisulfonate (NDS). The extensive hydrogen bonding between the frameworks and NDS resulted in thermally stable and water-insoluble materials. Irrespective of the metal atom present, both complexes exhibited moderate to high proton conduction in the range of 10-5 to 0.5×10-3  S cm-1 depending on the temperature and humidity levels.