Hydrophobic Shielding of Outer Surface: Enhancing the Chemical Stability of Metal–Organic Polyhedra

Metal–organic polyhedra (MOP) are a promising class of crystalline porous materials with multifarious potential applications. Although MOPs and metal–organic frameworks (MOFs) have similar potential in terms of their intrinsic porosities and physicochemical properties, the exploitation of carboxylate MOPs is still rudimentary because of the lack of systematic development addressing their chemical stability. Herein we describe the fabrication of chemically robust carboxylate MOPs via outer‐surface functionalization as an a priori methodology, to stabilize those MOPs system where metal–ligand bond is not so strong. Fine‐tuning of hydrophobic shielding is key to attaining chemical inertness with retention of the framework integrity over a wide range of pH values, in strong acidic conditions, and in oxidizing and reducing media. These results are further corroborated by molecular modelling studies. Owing to the unprecedented transition from instability to a chemically ultra‐stable regime using a rapid ambient‐temperature gram‐scale synthesis (within seconds), a prototype strategy towards chemically stable MOPs is reported.     

A Coordinative Solubilizer Method to Fabricate Soft Porous Materials from Insoluble Metal–Organic Polyhedra

Porous molecular cages have a characteristic processability arising from their solubility, which allows their incorporation into porous materials. Attaining solubility often requires covalently bound functional groups that are unnecessary for porosity and which ultimately occupy free volume in the materials, decreasing their surface areas. Here, a method is described that takes advantage of the coordination bonds in metal–organic polyhedra (MOPs) to render insoluble MOPs soluble by reversibly attaching an alkyl‐functionalized ligand. We then use the newly soluble MOPs as monomers for supramolecular polymerization reactions, obtaining permanently porous, amorphous polymers with the shape of colloids and gels, which display increased gas uptake in comparison with materials made with covalently functionalized MOPs.     

Inorganic–Organic Hybrid Polyoxoniobates: Polyoxoniobate Metal Complex Cage and Cage Framework

The combination of polyoxoniobates (PONbs) with 3d metal ions, azoles, and organoamines is a general synthetic procedure for making unprecedented PONb metal complex cage materials, including discrete molecular cages and extended cage frameworks. By this method, the first two PONb metal complex cages K₄@{[Cu₂₉(OH)₇(H₂O)₂(en)₈(trz)₂₁][Nb₂₄O₆₇(OH)₂(H₂O)₃]₄} and [Cu(en)₂]@{[Cu₂(en)₂(trz)₂]₆(Nb₆₈O₁₈₈)} have been made. The former exhibits a huge tetrahedral cage with more than 120 metal centers, which is the largest inorganic–organic hybrid PONb known to date. The later shows a large cubic cage, which can act as building blocks for cage‐based extended assembly to form a 3D cage framework {[Cu(en)₂]@{[Cu₂(trz)₂(en)₂]₆[H₁₀Nb₆₈O₁₈₈]}}. These materials exhibit visible‐light‐driven photocatalytic H₂ evolution activity and high vapor adsorption capacity. The results hold promise for developing both novel cage materials and largely unexplored inorganic–organic hybrid PONb chemistry.     

A Smart Metal–Organic Framework Nanomaterial for Lung Targeting

Despite high morbidity and mortality associated with lung diseases, addressing drugs towards lung tissue remains a pending task. Particle lung filtration has been proposed for passive lung targeting and drug delivery. However, toxicity issues derived from the long‐term presence of the particles must be overcome. By exploiting some of the ignored properties of nanosized metal–organic frameworks it is possible to achieve impressive antitumoral effects on experimental lung tumors, even without the need to engineer the surface of the material. In fact, it was discovered that, based on unique pH‐responsiveness and reversible aggregation behaviors, nanoMOF was capable of targeting lung tissue. At the neutral pH of the blood, the nanoMOFs form aggregates with the adequate size to be retained in lung capillaries. Within 24 h they then disaggregate and release their drug payload. This phenomenon was compatible with lung tissue physiology.     

From Octahedral to Icosahedral Metal–Organic Polyhedra Assembled from Two Types of Polyoxovanadate Clusters

Design and synthesis of metal–organic polyhedra (MOPs) with targeted geometries from predetermined secondary building units (SBUs) is a long‐standing challenge in chemistry and material science. Theoretical prediction shows that there are 6 possible polyhedra from the 3‐coordinated, 4‐coordinated octahedron ((3,4)‐c octahedron) to (3,5)‐c icosahedron with minimal transitivity (simplest possible). Except for one missing polyhedron ( mtr ) due to the unfavorable angles, we report five MOPs based on these structures, including an octahedral (3,4)‐c VMOP‐ 21 ( rdo ), an icosahedral (3,5)‐c VMOP‐ 25 ( trc ), and three intermediate derived trinodal (3,4,5)‐c VMOP‐ 22 – 24 ( ghm , hmg , xum ). Remarkably, all these MOPs obey the minimal transitivity principle and are consistent with geometrical predictions.     

Long‐Term Photostability in Terephthalate Metal–Organic Frameworks

Prolonged (weeks) UV/Vis irradiation under Ar of UiO‐66(Zr), UiO66 Zr‐NO₂, MIL101 Fe, MIL125 Ti‐NH₂, MIL101 Cr and MIL101 Cr(Pt) shows that these MOFs undergo photodecarboxylation of benzenedicarboxylate (BDC) linker in a significant percentage depending on the structure and composition of the material. Routine characterization techniques such as XRD, UV/Vis spectroscopy and TGA fail to detect changes in the material, although porosity and surface area change upon irradiation of powders. In contrast to BCD‐containing MOFs, zeolitic imidazolate ZIF‐8 does not evolve CO₂ or any other gas upon irradiation.     

On‐Chip Tailorability of Capacitive Gas Sensors Integrated with Metal–Organic Framework Films

Gas sensing technologies for smart cities require miniaturization, cost‐effectiveness, low power consumption, and outstanding sensitivity and selectivity. On‐chip, tailorable capacitive sensors integrated with metal–organic framework (MOF) films are presented, in which abundant coordinatively unsaturated metal sites are available for gas detection. The in situ growth of homogeneous Mg‐MOF‐74 films is realized with an appropriate metal‐to‐ligand ratio. The resultant sensors exhibit selective detection for benzene vapor and carbon dioxide (CO₂) at room temperature. Postsynthetic modification of Mg‐MOF‐74 films with ethylenediamine decreases sensitivity toward benzene but increases selectivity to CO₂. The reduced porosity and blocked open metal sites caused by amine coordination account for a deterioration in the sensing performance for benzene (by ca. 60 %). The enhanced sensitivity for CO₂ (by ca. 25 %) stems from a tailored amine–CO₂ interaction. This study demonstrates the feasibility of tuning gas sensing properties by adjusting MOF–analyte interactions, thereby offering new perspectives for the development of MOF‐based sensors.     

Control of Metal–Organic Framework Crystallization by Metastable Intermediate Pre‐equilibrium Species

There is an increasing amount of interest in metal–organic frameworks (MOFs) for a variety of applications, from gas sensing and separations to electronics and catalysis. However, the mechanisms by which they crystallize remain poorly understood. Herein, an important new insight into MOF formation is reported. It is shown that, prior to network assembly, crystallization intermediates in the canonical ZIF‐8 system exist in a dynamic pre‐equilibrium, which depends on the reactant concentrations and the progress of reaction. Concentration can, therefore, be used as a synthetic handle to directly control particle size, with potential implications for industrial scale‐up and gas sorption applications. These findings enable the rationalization of apparent contradictions between previous studies of ZIF‐8 and opens up new opportunities for the control of crystallization in network solids more generally.     

Nanocrystal/Metal–Organic Framework Hybrids as Electrocatalytic Platforms for CO2 Conversion

The tunable chemistry linked to the organic/inorganic components in colloidal nanocrystals (NCs) and metal–organic frameworks (MOFs) offers a rich playground to advance the fundamental understanding of materials design for various applications. Herein, we combine these two classes of materials by synthesizing NC/MOF hybrids comprising Ag NCs that are in intimate contact with Al‐PMOF ([Al₂(OH)₂(TCPP)]) (tetrakis(4‐carboxyphenyl)porphyrin (TCPP)), to form Ag@Al‐PMOF. In our hybrids, the NCs are embedded in the MOF while still preserving electrical contact with a conductive substrate. This key feature allows the investigation of the Ag@Al‐PMOFs as electrocatalysts for the CO₂ reduction reaction (CO₂RR). We show that the pristine interface between the NCs and the MOFs accounts for electronic changes in the Ag, which suppress the hydrogen evolution reaction (HER) and promote the CO₂RR. We also demonstrate a minor contribution of mass‐transfer effects imposed by the porous MOF layer under the chosen testing conditions. Furthermore, we find an increased morphological stability of the Ag NCs when combined with the Al‐PMOF. The synthesis method is general and applicable to other metal NCs, thus revealing a new way to think about rationally tailored electrocatalytic materials to steer selectivity and improve stability.     

Lanthanide Metal–Organic Framework Microrods: Colored Optical Waveguides and Chiral Polarized Emission

Lanthanide metal–organic frameworks (Ln‐MOFs) have received much attention owing to their structural tunability and widely photofunctional applications. However, successful examples of Ln‐MOFs with well‐defined photonic performances at micro‐/nanometer size are still quite limited. Herein, self‐assemblies of 1,3,5‐benzenetricarboxylic acid (BTC) and lanthanide ions afford isostructural crystalline Ln‐MOFs. Tb‐BTC, Eu@Tb‐BTC, and Eu‐BTC have 1D microrod morphologies, high photoluminescence (PL) quantum yields, and different emission colors (green, orange, and red). Spatially PL resolved spectra confirm that Ln‐MOF microrods exhibit an optical waveguide effect with low waveguide loss coefficient (0.012≈0.033 dB μm⁻¹) during propagation. Furthermore, these microrods feature both linear and chiral polarized photoemission with high anisotropy.     

Chiral Phosphoric Acids in Metal–Organic Frameworks with Enhanced Acidity and Tunable Catalytic Selectivity

Chiral phosphoric acids are incorporated into indium‐based metal–organic frameworks (In‐MOFs) by sterically preventing them from coordination. This concept leads to the synthesis of three chiral porous 3D In‐MOFs with different network topologies constructed from three enantiopure 1,1′‐biphenol‐phosphoric acid derived tetracarboxylate linkers. More importantly, all the uncoordinated phosphoric acid groups are periodically aligned within the channels and display significantly enhanced acidity compared to the non‐immobilized acids. This facilitates the Brønsted acid catalysis of asymmetric condensation/amine addition and imine reduction. The enantioselectivities can be tuned (up to >99 % ee) by varying the substituents to achieve a nearly linear correlation with the concentrations of steric bulky groups in the MOFs. DFT calculations suggest that the framework provides a chiral confined microenvironment that dictates both selectivity and reactivity of chiral MOFs.     

Two‐Dimensional Metal–Organic Framework Nanosheets for Membrane‐Based Gas Separation

Metal–organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes owing to their structural diversity and minimized mass‐transfer barrier. To date, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet‐based membranes remain as great challenges. A modified soft‐physical exfoliation method was used to disintegrate a lamellar amphiprotic MOF into nanosheets with a high aspect ratio. Consequently sub‐10 nm‐thick ultrathin membranes were successfully prepared, and these demonstrated a remarkable H₂/CO₂ separation performance, with a separation factor of up to 166 and H₂ permeance of up to 8×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at elevated testing temperatures owing to a well‐defined size‐exclusion effect. This nanosheet‐based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.     

Evaporation‐Directed Crack‐Patterning of Metal–Organic Framework Colloidal Films and Their Application as Photonic Sensors

A straightforward crack‐patterning method is reported allowing the direct formation of periodic cracks in metal–organic framework (MOF) nanoparticle films during dip‐coating deposition. The crack propagation and periodicity can be easily tailored by controlling the evaporation front and the withdrawal speed. Several MOF‐patterned films can be fabricated on large surfaces and on several substrates (flat, curved or flexible) including the inner surface of a tube, not achievable by other lithographic techniques. We demonstrate that the periodic cracked arrays diffract light and, due to the MOF sorption properties, photonic vapor sensors are fabricated. A new concept of “in‐tube”, MOF‐based diffraction grating sensors is proposed with outstanding sensitivity that can be easily tuned “on‐demand” as function of the desired detection range.     

Cooperative Multifunctional Catalysts for Nitrone Synthesis: Platinum Nanoclusters in Amine‐Functionalized Metal–Organic Frameworks

Nitrones are key intermediates in organic synthesis and the pharmaceutical industry. The heterogeneous synthesis of nitrones with multifunctional catalysts is extremely attractive but rarely explored. Herein, we report ultrasmall platinum nanoclusters (PtNCs) encapsulated in amine‐functionalized Zr metal–organic framework (MOF), UiO‐66‐NH₂ (Pt@UiO‐66‐NH₂) as a multifunctional catalyst in the one‐pot tandem synthesis of nitrones. By virtue of the cooperative interplay among the selective hydrogenation activity provided by the ultrasmall PtNCs and Lewis acidity/basicity/nanoconfinement endowed by UiO‐66‐NH₂, Pt@UiO‐66‐NH₂ exhibits remarkable activity and selectivity, in comparison to Pt/carbon, Pt@UiO‐66, and Pd@UiO‐66‐NH₂. Pt@UiO‐66‐NH₂ also outperforms Pt nanoparticles supported on the external surface of the same MOF (Pt/UiO‐66‐NH₂). To our knowledge, this work demonstrates the first examples of one‐pot synthesis of nitrones using recyclable multifunctional heterogeneous catalysts.