Self-Similar Ligand for 2D Zr(IV)-Based Metal-Organic Frameworks: Fluorescent Sensing and Catalysis

Two-dimensional (2D) metal-organic framework sheets, in comparison to the 3D analogues, offer potential advantages for intercalation of guest components between the layers, exfoliation/dispersion into solutions, and processing into thin films. As a versatile platform for leveraging organic functions, the 2D Zr(IV)-carboxylate net here features a dendritic Sierpinski tritopic linker with conjugated alkyne branches and a photoactive triphenylamine core. The 2D solid can be easily dispersed in water and many other solvents, resulting in stable and fluorescent suspension for sensing nitro aromatic compounds and Fe3+ ions with high quenching efficiencies and ultralow limits of detection. Also, the neighboring alkyne units of the coordination solid undergo thermal cyclization (e.g., at 320 °C) to form cross-linked nanographene-like components to afford robust porosity, which substantially takes up PdCl2 (atomic ratio of Zr/Pd, 2.4:1) to afford a heterogeneous catalyst for Suzuki-Miyaura coupling reactions─direct in air and without the need for phosphine ligands.

Minimalist Design for Solar Energy Conversion: Revamping the π-Grid of an Organic Framework into Open-Shell Superabsorbers

We apply a versatile reaction to a versatile solid: the former involves the electron-deficient alkene tetracyanoethylene (TCNE) as the guest reactant; the latter consists of stacked 2D honeycomb covalent networks based on the electron-rich β-ketoenamine hinges that also activate the conjugated, connecting alkyne units. The TCNE/alkyne reaction is a [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) that forms strong push-pull units directly into the backbone of the framework-i.e., using only the minimalist "bare-bones" scaffold, without the need for additional side groups of alkynes or other functions. The ability of the stacked alkyne units (i.e., as part of the honeycomb mass) to undergo such extensive rearrangement highlights the structural flexibility of these covalent organic framework (COF) hosts. The COF solids remain porous, crystalline, and air-/water-stable after the CA-RE modification, while the resulting push-pull units feature distinct open-shell/free-radical character, are strongly light-absorbing, and shift the absorption ends from 590 nm to around 1900 nm (band gaps from 2.17-2.23 to 0.87-0.95 eV), so as to better capture sunlight (especially the infrared region which takes up 52% of the solar energy). As a result, the modified COF materials achieve the highest photothermal conversion performances, holding promise in thermoelectric power generation and solar steam generation (e.g., with solar-vapor conversion efficiencies >96%).

Carbazole-Equipped Metal-Organic Framework for Stability, Photocatalysis, and Fluorescence Detection

The useful yet underutilized backfolded design is invoked here for functionalizing porous solids with the versatile carbazole function. Specifically, we attach carbazole groups as backfolded side arms onto the backbone of a linear dicarboxyl linker molecule. The bulky carbazole side arms point away from the carboxyl links and do not disrupt the Zr-carboxyl framework formation; namely, the resultant MOF solid ZrL1 features the same net as that of the unfunctionalized dicarboxyl linker, also known as the PCN-111 net or UiO-66 net. The ZrL1 structure features only half linker occupancy (about 6 out of the 12 linkers around the Zr₆O₈ cluster being missing) and partially collapses upon activation (acetone exchange and evacuation). Notably, the stability improves after heating in diphenyl oxide at 260 °C (POP-260 treatment; to form ZrL1-260), as indicated by the higher crystallinity and surface area of the activated ZrL1-260 sample. The ZrL1-260 samples achieve 72% yield in photocatalyzing reductive dehalogenation of phenacyl bromide; ZrL1 can detect nitro-aromatic compounds via fluorescence quenching, with selectivity and sensitivity toward 4-nitroaniline, featuring a limit of detection of 96 ppb.

A Ferrocene Metal-Organic Framework Solid for Fe-Loaded Carbon Matrices and Nanotubes: High-Yield Synthesis and Oxygen Reduction Electrocatalysis

Using a carbon-rich designer metal-organic framework (MOF), we open a high-yield synthetic strategy for iron-nitrogen-doped carbon (Fe-N-C) nanotube materials that emulate the electrocatalysis performance of commercial Pt/C. The Zr(IV)-based MOF solid boasts multiple key functions: (1) a dense array of alkyne units over the backbone and the side arms, which are primed for extensive graphitization; (2) the open, branched structure helps maintain porosity for absorbing nitrogen dopants; and (3) ferrocene units on the side arms as atomically dispersed precursor catalyst for targeting micropores and for effective iron encapsulation in the carbonized product. As a result, upon pyrolysis, over 89% of the carbon component in the MOF scaffold is successfully converted into carbonized products, thereby contrasting the easily volatilized carbon of most MOFs. Moreover, over 97% of the iron ends up being encased as acid-resistant Fe/Fe₃C nanoparticles in carbon nanotubes/carbon matrices.

Invisible Silver Guests Boost Order in a Framework That Cyclizes and Deposits Ag3Sb Nanodots

The infusion of metal guests into (i.e., metalating) the porous medium of metal-organic frameworks (MOFs) is a topical approach to wide-ranging functionalization purposes. We report the notable interactions of AgSbF₆ guests with the designer MOF host ZrL1 [Zr₆O₄(OH)₇(L1)_4.5(H₂O)₄]. (1) The heavy-atom guests of AgSbF₆ induce order in the MOF host to allow the movable alkyne side arm to be fully located by X-ray diffraction, but they themselves curiously remain highly disordered and absent in the strucutral model. The enhanced order of the framework can be generally ascribed to interaction of the silver guests with the host alkyne and thioether functions, while the invisible heavy-atom guest represents a new phenomenon in the metalation of open framework materials. (2) The AgSbF₆ guests also participate in the thermocyclization of the vicinal alkyne units of the L1 linker (at 450 °C) and form the rare nanoparticle of Ag₃Sb supported on the concomitantly formed nanographene network. The resulted composite exhibits high electrical conductivity (1.0 S/cm) as well as useful, mitigated catalytic activity for selectively converting nitroarenes into the industrially important azo compounds, i.e., without overshooting to form the amine side products. The heterogeneous/cyclable catalysis entails only the cheap reducing reagents of NaBH₄, ethanol, and water, with yields being generally close to 90%.

Conjugated crosslinks boost the conductivity and stability of a single crystalline metal-organic framework

A linker molecule with four pendant thiophene functions was crystallized with Zr(iv) ions to form a semiconductive porous coordination solid (1.1 × 10^-5 S cm^-1). Oxidative treatment with FeCl₃ guests then coupled the thiophene units to form conjugated bridges as covalent crosslinks. The resulting hybrid of a metal-organic framework and conjugated polymer featured robust crystalline order that withstood long-term air exposure and broad pH (from 0 to 12) conditions. Moreover, the homocoupled thiophene units, conjugated through sulfide links (-S-) with the linker backbone, afforded higher electronic conductivity (e.g., >2.2 × 10^-3 S cm^-1), which is characteristic of conductive polymer prototypes of polythiophene and polyphenylene sulfide. The crosslinked solid also exhibited proton conductivity that could be increased broadly upon H₂SO₄ treatment (e.g., from 5.0 × 10^-7 to 1.6 × 10^-3 S cm^-1).

Linker Deficiency, Aromatic Ring Fusion, and Electrocatalysis in a Porous Ni8-Pyrazolate Network

The cruciform linker molecule here features two designer functions: the pyrazole donors for framework construction, and the vicinal alkynyl units for benzannulation to form nanographene units into the Ni₈-pyrazolate scaffold. Unlike the full 12 connections of the Ni₈(OH)₄(H₂O)₂ clusters in other Ni₈-pyrazolate networks, significant linker deficiency was observed here, leaving about half of the Ni(II) sites capped by acetate ligands, which can be potentially removed to open the metal sites for reactivity. The crystalline Ni₈-pyrazolate scaffold also retains the crystalline order even after thermal treatments (up to 300 °C) that served to partially graphitize the neighboring alkyne units. The resultant nanographene components enhance the electroactive properties of the porous hosts, achieving hydrogen evolution reaction (HER) activity that rivals that of topical nickel/palladium-enabled materials.

Metal-Organic Frameworks Based on Group 3 and 4 Metals

Metal-organic frameworks (MOFs) based on group 3 and 4 metals are considered as the most promising MOFs for varying practical applications including water adsorption, carbon conversion, and biomedical applications. The relatively strong coordination bonds and versatile coordination modes within these MOFs endow the framework with high chemical stability, diverse structures and topologies, and interesting properties and functions. Herein, the significant progress made on this series of MOFs since 2018 is summarized and an update on the current status and future trends on the structural design of robust MOFs with high connectivity is provided. Cluster chemistry involving Y, lanthanides (Ln, from La to Lu), actinides (An, from Ac to Lr), Ti, and Zr is initially introduced. This is followed by a review of recently developed MOFs based on group 3 and 4 metals with their structures discussed based on the types of inorganic or organic building blocks. The novel properties and arising applications of these MOFs in catalysis, adsorption and separation, delivery, and sensing are highlighted. Overall, this review is expected to provide a timely summary on MOFs based on group 3 and 4 metals, which shall guide the future discovery and development of stable and functional MOFs for practical applications.

Crystallinity after decarboxylation of a metal-carboxylate framework: indestructible porosity for catalysis

We report a curious case study of a Zr(iv)-carboxylate framework, which retains significant crystalline order after cascade thermocyclization of its linker components, and - more notably - after the crucial carboxylate links were severed by heat. Vigorous heat treatment (e.g., 450 °C and above) benzannulates the multiple alkyne groups on the linker to generate linked nanographene blocks and to afford real stability. The resultant Zr oxide/nanographene hybrid solid is stable in saturated NaOH and concentrated H₃PO₄, allowing a convenient anchoring of H₃PO₄ into its porous matrix to enable size-selective heterogeneous acid catalysis. The Zr oxide components can also be removed by strong hydrofluoric acid to further enhance the surface area (up to 650 m² g^-1), without collapsing the nanographene scaffold. The crystallinity order and the extensive thermal transformations were characterized by X-ray diffraction, scanning transmission electron microscopy (STEM), IR, solid state NMR and other instrumental methods.

Building Conjugated Donor-Acceptor Cross-Links into Metal-Organic Frameworks for Photo- and Electroactivity

We convert a coordination network into a covalent solid, while maintaining the crystallinity and greatly enhancing the framework rigidity and redox-active and photochemical properties. Specifically, intensely light-absorbing push-pull functions are postsynthetically installed by reacting the electrophilic TCNE (tetracyanoethylene) guests and the electron-rich alkyne side arms on a microporous Zr-organic framework, generating black microporous crystallites with a band gap smaller than 1.0 eV. The reaction proceeds in the known [2 + 2] cycloaddition-retroelectrocyclization mechanism and extensively establishes conjugated (polyene) bridges across the linker molecules. The donor (4-methoxyphenyl) and acceptor (dicyanovinyl) couples of the polyene bridges also act as an efficient fluorescent quencher and can be selectively installed in a thin outer layer of the host crystallite to form a core-shell assembly for turn-on fluorescent sensing of small amine molecules in water solutions.

High-Performance Metal-Organic Framework-Templated Sorbent for Selective Eu(III) Capture

A stable porous sorbent M1 was achieved through the specific transformation of flexible thioalkyl groups and metal cluster sites in a zirconium MOF (metal-organic framework; Zr-L) template. The target polymer combines sulfoxide/sulfone and phosphoric acid in a single framework, which was fully characterized by ¹H-NMR, PXRD, IR, and elemental analysis. When employed as the heavy metal adsorbent, M1 exhibit a remarkable Eu(III) sorption behavior, achieving both high chemical affinity (K _d = 10⁵) and sorption capacity (the maximum Eu(III) sorption capacity reached 220 mg g^-1 at pH = 4.0 and T = 298 K calculated from the Langmuir model). Recyclability and selectivity test of M1 further prove that the sorbent is highly stable and effective for europium enrichment in the aqueous solution. This work takes focus on the introduction of multifunctional groups into a single polymeric framework in a feasible and environmentally friendly way and highlights the sorption efficiency for europium removal from the aqueous solution.

Conjugated porous polymers: incredibly versatile materials with far-reaching applications

This review discusses conjugated porous polymers and focuses on relating design principles and synthetic methods to key properties and applications such as (photo)catalysis, gas storage, chemical sensing, energy storage and environmental remediation.

Effective loading of cisplatin into a nanoscale UiO-66 metal-organic framework with preformed defects

Defects within the nanoscale UiO-66 metal-organic framework (MOF) are created to lock a hybrid phosphonoacetate ligand through Zr-O-P linkages, leaving the carboxyl group free to anchor cisplatin prodrug cis, cis, trans-[Pt(NH3)2Cl2(OH)2]. A drug loading of 256.5 mg g-1 (25.7 wt% based on cisplatin) was achieved with a Zr6 : Pt : P ratio of 1.5 : 1 : 1, which surpasses defect-free UiO-66 and several other MOF carriers. This framework exhibited a burst release of its payload in PBS solution in the first 2 h, releasing 71% of the drug, including a 50% payload release in less than 1 h. This work demonstrates that MOF defects can be intentionally engineered to achieve a high drug loading, and serves as an alternative to drug encapsulation using the pore void and through the association of the functionalized ligand.