Proton Conducting Metal-Organic Frameworks (MOFs) via Post Synthetic Transmetallation and Water Induced Structural Transformations

Post Synthetic Modification (PSM) of Metal-Organic Frameworks (MOFs) is a crucial strategy for developing new MOFs with enhanced functional properties compared to their parent one. PSM can be accomplished through various methods:1) modification of organic linkers; 2) exchange of metal ions or nodes; and 3) inclusion or exchange of solvent/guest molecules. Herein, PSM of bimetallic and monometallic MOFs containing biphenyl dinitro-tetra-carboxylates (NCA) are demonstrated. The tetra carboxylate NCA, produces monometallic Cd-MOF-1 and Cu-MOF-1 and bimetallic CoZn-MOF in solvothermal reactions with the corresponding metal salts. The CoZn-MOF undergoes post-synthetic transmetallation with Cd(NO3)2 and Cu(NO3)2 in aqueous solution to yield Cd-MOF-2 and Cu-MOF-2, respectively. Additionally, green crystals of Cu-MOF-1 found to undergo a single-crystal-to-single-crystal (SCSC) transformation to blue crystals of Cu-MOF-3 upon dipped into water at room temperature. These MOFs demonstrate notable proton conductivities ranging from 10-3 to 10-4 S cm-1 under variable temperatures and humidity levels. Among them, Cu-MOF-3 achieves the highest proton conductivity of 1.36×10-3 S cm-1 at 90 °C and 98 % relative humidity, attributed to its continuous and extensive hydrogen bonding network, which provides effective proton conduction pathways within the MOF. This work highlights a convenient strategy for designing proton-conducting MOFs via post-synthetic modification.

Simultaneous Control of Flexibility and Rigidity in Pore-Space-Partitioned Metal-Organic Frameworks

Flexi-MOFs are typically limited to low-connected (<9) frameworks. Here we report a platform-wide approach capable of creating a family of high-connected materials (collectively called CPM-220) that integrate exceptional framework flexibility with high rigidity. We show that the multi-module nature of the pore-space-partitioned pacs (partitioned acs net) platform allows us to introduce flexibility as well as to simultaneously impose high rigidity in a tunable module-specific fashion. The inter-modular synergy has remarkable macro-morphological and sub-nanometer structural impacts. A prominent manifestation at both length scales is the retention of X-ray-quality single crystallinity despite huge hexagonal c-axial contraction (≈ 30%) and harsh sample treatment such as degassing and sorption cycles. CPM-220 sets multiple precedents and benchmarks on the pacs platform in both structural and sorption properties. They possess exceptionally high benzene/cyclohexane selectivity, unusual C₃H₆ and C₃H₈ isotherms, and promising separation performance for small gas molecules such as C₂H₂/CO₂.

Identifying the Gate-Opening Mechanism in the Flexible Metal-Organic Framework UTSA-300

Atomic-level understanding of the gate-opening phenomenon in flexible porous materials is an important step toward learning how to control, design, and engineer them for applications such as the separation of gases from complex mixtures. Here, we report such mechanistic insight through an in-depth study of the pressure-induced gate-opening phenomenon in our earlier reported metal-organic framework (MOF) Zn(dps)₂(SiF₆) (dps = 4,4'-dipyridylsulfide), also called UTSA-300, using isotherm and calorimetry measurements, in situ infrared spectroscopy, and ab initio simulations. UTSA-300 is shown to selectively adsorb acetylene (C₂H₂) over ethylene (C₂H₄) and ethane (C₂H₆) and undergoes an abrupt gate-opening phenomenon, making this framework a highly selective gas separator of this complex mixture. The selective adsorption is confirmed by pressure-dependent in situ infrared spectroscopy, which, for the first time, shows the presence of multiple C₂H₂ species with varying strengths of bonding. A rare energetic feature at the gate-opening condition of the flexible MOF is observed in our differential heat energies, directly measured by calorimetry, showcasing the importance of this tool in adsorption property exploration of flexible frameworks and offering an energetic benchmark for further energy-based fundamental studies. Based on the agreement of this feature with ab initio-based adsorption energies of C₂H₂ in the closed-pore structure UTSA-300a ("a" refers to the activated form), this feature is assigned to the weakening of the H-bond C-H···F formed between C₂H₂ and fluorine of the MOF. Our analysis identifies the weakening of this H-bond, the expansion of the closed-pore MOF upon successive C₂H₂ coadsorption until its volume is close to that of the open-pore MOF, and the spontaneous gate opening to energetically favor C₂H₂ adsorption in the open-pore structure as crucial steps in the gate-opening mechanism in this system.

Stability and Phase Transitions of Nontoxic γ-Cyclodextrin-K+ Metal-Organic Framework in Various Solvents

Cyclodextrin (CD) has been used to prepare biocompatible and nontoxic metal-organic frameworks (MOFs) suitable for biomedical applications as drug carriers. In this study, γ-CD/K-based MOF (γ-CD-MOF-1-α) was synthesized and its stability in various solvents was explored by single-crystal X-ray diffractometry (SCXRD) and powder X-ray diffractometry (PXRD). As a result of solvent-induced phase transformations, two novel crystalline phases of γ-CD-MOF-1 were discovered. The newly formed ε- and δ-phases crystallize in orthorhombic and tetragonal symmetry, respectively. In ε-phase, toluene was determined as a guest molecule by SCXRD. Interactions between γ-cyclodextrin and solvent molecules in ε-phase were evaluated using Hirshfeld surface analysis. The thermal stability of the new crystal forms of γ-CD-MOF-1 was analyzed by differential scanning calorimetry and thermogravimetric analysis.

Single-crystal-to-single-crystal transformations triggered by dehydration in polyoxometalate-based compounds

Single-crystal-to-single-crystal transformations are solid-state phase transitions between different crystalline states in which the crystal integrity and the long-range structural order are retained through the whole transformation process. Such a phenomenon constitutes the structural response that some compounds afford when being exposed to a given external stimulus (temperature, pressure, light, etc.) and, therefore, its study has become a relevant focus of interest within crystal engineering because it allows for monitoring how certain properties (colour, magnetism, luminescence, porosity) of the stimuli-responsive material are modified as the structure evolves into the activated form. A range of organic, inorganic and hybrid systems have been found to undergo such phase transitions, but these examples only include a small number of compounds that incorporate polyoxometalate anions, among which the removal of guest solvent molecules (dehydration) stands out as the most common external stimulus able to induce the occurrence of a single-crystal-to-single-crystal transformation. This feature article compiles the examples of dehydration-triggered single-crystal-to-single-crystal transformation studies that have been reported to date for polyoxometalate-based compounds and reviews some of their most relevant structural aspects.

Solvent-Triggered Reversible Phase Changes in Two Manganese-Based Metal-Organic Frameworks and Associated Sensing Events

A flexible Mn-based MOF, Mn-sdc-1, has been successfully synthesized by using the ligand 4,4'-stilbenedicarboxylic acid (H₂ sdc). Attributed to the flexibility of the framework, Mn-sdc-1 can transform into a new phase (Mn-sdc-2) with completely different structural geometry; this is induced by trace levels of H₂ O at room temperature. Reversibly, the transformation from Mn-sdc-2 to Mn-sdc-1 can be triggered by DMF upon heating beyond 100 °C. These results inspired a study of the influences of temperature and H₂ O volume in the solid-state transformation of two MOF phases and, for the first time, a phase diagram of MOFs has been depicted. This phase diagram reflects the gradual H₂ O-/temperature-dependent changes between Mn-sdc-1 and Mn-sdc-2, which is very meaningful in achieving the controllable synthesis of these two MOFs and lead to targeting the desired water-stable structure. As a result, the obtained water-stable Mn-sdc-2 can be developed as an excellent Pb²⁺ sensor in aqueous solution through the luminescence quenching effect with a limit of detection of 31.4 nm.

Structure and properties of dynamic metal–organic frameworks: a brief accounts of crystalline-to-crystalline and crystalline-to-amorphous transformations

External stimuli-driven structural changes and the associated properties of dynamic MOFs are discussed with examples.

Pre-design and synthesis of a five-fold interpenetrated pcu-type porous coordination polymer and its CO2/CO separation

The first five-fold interpenetrated PCP with pcu topology and high porosity was rationally designed and prepared, which showed high separation potential of CO₂/CO mixtures.