Charge Separation in Metal-Organic Framework Enables Heterogeneous Thiol Catalysis.

Photoinduced metal-organic framework (MOF) enabled heterogeneous thiol catalysis has been achieved for the first time. MOF Zr-TPDCS-1, consisting of Zr6 -clusters and TPDCS linkers (TPDCS=3,3'',5,5''-tetramercapto[1,1':4',1''-terphenyl]-4,4''-dicarboxylate), effectively catalyzed the borylation, silylation, phosphorylation, and thiolation of organic molecules. Upon irradiation, the fast electron transfer from TPDCS to Zr6 -cluster is believed to facilitate the formation of the thiyl radical, a hydrogen atom transfer catalyst, which competently abstracts the hydrogen from borane, silane, phosphine, or thiol for generating the corresponding element radical to engender the chemical transformations. The elaborate control experiments evidenced the generation of thiyl radicals in MOF and illustrated a radical reaction pathway. The gram-scale reaction worked well, and the product was conveniently separated via centrifugation and vacuum with a turnover number (TON) of ≈3880, highlighting the practical application potential of heterogeneous thiyl-radical catalysis.

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.

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%.

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.

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.

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

Conjugated porous polymers (CPPs) are a class of amorphous polymer networks that are, in their design, fully cross-linked and fully π-conjugated. The cross-linked nature of CPPs means that they have permanent intrinsic porosity (on the nanometer scale), which, in combination with the fully π-conjugated framework, makes these materials unique among the wider class of porous polymeric materials. In recent years, the need for new and efficient functional materials has driven the development of CPPs as versatile platforms for applications including (photo)catalysis, light harvesting, gas separation and storage, chemosensing, environmental remediation and energy storage. The efficiency of these materials towards these described applications is heavily influenced by the choice of molecular building blocks and synthetic conditions, allowing for facile tailoring and optimisation of the structure and properties. The aim of this review is to highlight select works on CPPs, including basic structural design principles, various synthetic protocols and topical applications of these versatile materials.

Halogen-C2 H2 Binding in Ultramicroporous Metal-Organic Frameworks (MOFs) for Benchmark C2 H2 /CO2 Separation Selectivity.

Acetylene (C2 H2 ) capture is a step in a number of industrial processes, but it comes with a high-energy footprint. Although physisorbents have the potential to reduce this energy footprint, they are handicapped by generally poor selectivity versus other relevant gases, such as CO2 and C2 H4 . In the case of CO2 , the respective physicochemical properties are so similar that traditional physisorbents, such as zeolites, silica, and activated carbons cannot differentiate well between CO2 and C2 H2 . Herein, we report that a family of three isostructural, ultramicroporous (<7 Å) diamondoid metal-organic frameworks, [Cu(TMBP)X] (TMBP=3,3',5,5'-tetramethyl-4,4'-bipyrazole), TCuX (X=Cl, Br, I), offer new benchmark C2 H2 /CO2 separation selectivity at ambient temperature and pressure. We attribute this performance to a new type of strong binding site for C2 H2 . Specifically, halogen⋅⋅⋅HC interactions coupled with other noncovalent in a tight binding site is C2 H2 specific versus CO2 . The binding site is distinct from those found in previous benchmark sorbents, which are based on open metal sites or electrostatic interactions enabled by inorganic fluoro or oxo anions.

Porphyrin Grafting on a Mercapto-Equipped Zr(IV)-Carboxylate Framework Enhances Photocatalytic Hydrogen Production.

We employ facile aromatic nucleophilic substitution between the mercapto (-SH) and arylfluoro (Ar-F) groups to achieve extensive and robust cross-linking of a coordination host by porphyrin guests that also serve the purpose of versatile postsynthetic functionalization. For this, a tritopic linker with three trident-like thiol-flanked carboxyl units are reacted with ZrOCl2·8H2O to afford a two-dimensional (3,6-connected) net. The wide aperture of the porous framework solid, together with its stability in both air and boiling water, facilitates the entry of bulky metalloporphyrin guests and the subsequent property studies. On the porphyrin side, four pentafluorophenyl (C6F5-) groups offer multiple fluoro groups to facilitate their replacement by the thiol groups from the host net. The inserted metalloporphyrin bridges impart to the metal-organic framework (MOF) host stable and recyclable activities for photocatalytic hydrogen production. We also disclose an improvement in synthetic methodology, in which BBr3 is used to simultaneously cleave the ester and benzyl thioether groups to more efficiently access thiol-equipped carboxylic acid building block.

Dense Alkyne Arrays of a Zr(IV) Metal-Organic Framework Absorb Co2(CO)8 for Functionalization.

Finely dispersed Co(0) and CoO species were efficiently loaded into a stable metal-organic framework to impart catalytic activities to the porous solid. The metalation of the MOF host is facilitated by the dense arrays of accessible alkyne units that boost the alkyne-Co2(CO)8 interaction. The tetrakis(4-carboxylphenylethynyl)pyrene linker, with eight symmetrically backfolded alkyne side arms, features strong fluorescence and a dendritic Sierpinski shape. The resultant Zr(IV)-MOF features NU-901 topology (scu net, with rhombus channels) and breathing properties (e.g., the contracted (porous) phase reverts to the as-made phase upon contact with DMF (dimethylformamide)). The inserted Co2(CO)8 guests quickly react with air to form atomically dispersed CoO species (nondiffracting), and subsequent thermal treatment at 600 °C of the CoO-loaded solid generates an electrocatalyst for the oxygen evolution reaction (OER).

A Bumper Crop of Boiling-Water-Stable Metal-Organic Frameworks from Controlled Linker Sulfuration.

The series of highly stable porous solids here feature systematic, regiospecific sulfur substitutions on the organic linkers for versatile functions. One major surprise lies in the controllable sequential reactions between sodium thiomethoxide (NaSMe) and octafluorobiphenyl-4,4'-dicarboxylic acid (H2bpdc-8F; this was readily made without precious metal catalysts). Namely, 3, 4, 6, and 8 methylthio-substitutions can be respectively achieved with regiospecificity (i.e., to produce the four molecules H2bpdc-3S5F, H2bpdc-4S4F, H2bpdc-6S2F, H2bpdc-8MS). A second surprise lies in their persistent formation of the UiO-67-type net with Zr(IV) ions, e.g., even in the case of the fully sulfurated H2bpdc-8MS. In addition to the remarkable breadth of functional control, all the Zr(IV)-based crystalline solids here are stable in boiling water (e.g., for 24 h) and in air as solventless, activated porous solids. Moreover, the thioether groups allow for convenient H2O2 oxidation to fine-tune the hydrophilicity and luminescence properties and improve proton conductivity.

Sulfur Chemistry for Stable and Electroactive Metal-Organic Frameworks: The Crosslinking Story.

The versatile sulfur chemistry, as exemplified by the reactive yet tractable thiol group (-SH), offers unique opportunities for bypassing the obstacles in accessing stable and electroactive metal-organic frameworks. Of particular interest are recent advances in assembling MOF materials equipped with free-standing thiol functions: metal guests can be conveniently inserted to install electroactive metal-sulfur bonds, which, as crosslinks, also stabilize the host coordination net. Here the historical development of the bifunctional, two-step design embodied by the thiol-tagged MOF solids is traced, in order to highlight the underlying spirit that has driven research efforts in the past two decades. Going forward, broad new horizons are foreseen in solid-state materials synthesis, for example, arising from various sulfur-based hard-and-soft combinations, in synergy with the crystalline and modular MOF structural platform.

A Thiol-Functionalized UiO-67-Type Porous Single Crystal: Filling in the Synthetic Gap.

Thiol groups (-SH) offer versatile reactivity for functionalizing metal-organic frameworks, and yet thiol-equipped MOF solids remain underexplored due to synthetic challenges. Building on the recent breakthrough using benzyl mercaptan as the sulfur source and AlCl3 for uncovering the thiol function, we report on the thiol-equipped linker 3,3'-dimercaptobiphenyl-4,4'-dicarboxylic acid and its reaction with Zr(IV) ions to form a UiO-67-type MOF solid with distinct functionalities. The thiol-equipped UiO-67 scaffold shows substantial stability toward oxidation, e.g., it can be treated with 30% H2O2 to afford oxidation of the thiol to the strongly acidic sulfonic function while maintaining the ordered porous MOF structure. The thiol groups also effectively take up palladium(II) ions from solutions to allow for comparative studies on catalytic activities and to help elucidate how the spatial configuration of the thiol groups can be engineered to impact the performance of heterogeneous catalysis in the solid state. Comparative studies on the stability in the solventless (activated) state also help to highlight the steric factor in stabilizing UiO-67-type frameworks.

Symmetrically backfolded molecules emulating the self-similar features of a Sierpinski triangle.

We synthesized self-similar molecules (G3 and G2; based on phenylalkynyl backbones) with symmetrically backfolded shapes inspired by the famous fractal of a Sierpinski triangle. Unlike the more traditional, starburst dendrimers, the centripetal-shaped Sierpinski molecules feature side branches symmetrically bent away from the growth direction of the main branch, thus contrasting the natural-tree shape. Molecule G3 exhibits three distinct levels of the structural hierarchy comprising the primary, secondary and tertiary branches, while the smaller G2 contains only features of the 1st and 2nd orders. In spite of the much larger conjugated backbone of G3, its solution UV-vis absorption and fluorescence exhibit no red shift relative to G2. In a test of nitrobenzene sensing, a thin film of G3 deposited from THF was more sensitively quenched in fluorescence than the smaller G2.

Single-Crystalline UiO-67-Type Porous Network Stable to Boiling Water, Solvent Loss, and Oxidation.

With methylthio groups flanking the carboxyl groups, the 3,3',5,5'-tetrakis(methylthio)biphenyl dicarboxylate (TMBPD) linker forms a zirconium(IV) carboxylate porous framework featuring the topology of the UiO-67 prototype, i.e., with a face-centered-cubic array of the Zr6O4(OH)4 clusters. Thioether functionalization proves valuable because the ZrTMBPD crystal is found to be exceptionally stable not only upon long-term exposure to air but also in boiling water and a broad range of pH conditions. The hydrophobicity of the metal-organic framework can also be tuned by simple H2O2 oxidation, as illustrated in the water contact-angle measurement of the pristine and H2O2-treated ZrTMBPD solid.

Made in Water: A Stable Microporous Cu(I)-carboxylate Framework (CityU-7) for CO2, Water, and Iodine Uptake.

Using water as the sole solvent, the bifunctional molecule tetrakis(methylthio)-1,4-benzenedicarboxylic acid (TMBD) was reacted with Cu(CH3CN)4BF4 to form a robust microporous metal-organic framework (MOF, CityU-7) featuring Cu(I) ions being simultaneously bonded to the carboxyl and thioether donors. The MOF solid is stable in air and can be easily activated by heating, without the need for treatment with organic solvents. The subnanoscopic pores (ca. 0.6 nm) of the host net allow for uptake of CO2 and H2O but exhibit lesser sorption for N2 at 77 K. The microporous net can also be penetrated by I2 molecules.

Multiphase-Assembly of Siloxane Oligomers with Improved Mechanical Strength and Water-Enhanced Healing.

Healable silicone materials have great technical impact in coatings, smart actuators, and flexible electronics, however, current healable silicone materials lack mechanical tunability. Herein, we designed and synthesized a new type of healable silicone through hydrogen-bond assisted multiphase assembly of siloxane oligomers. Besides the enhanced mechanical strength, unique water-enhanced healing was observed in the polymer network which is due to the reversible dissociation/association of multivalent hydrogen bonds in the presence of water.

Metalation Triggers Single Crystalline Order in a Porous Solid.

We report the dramatic triggering of structural order in a Zr(IV)-based metal-organic framework (MOF) through docking of HgCl2 guests. Although as-made crystals were unsuitable for single crystal X-ray diffraction (SCXRD), with diffraction limited to low angles well below atomic resolution due to intrinsic structural disorder, permeation of HgCl2 not only leaves the crystals intact but also resulted in fully resolved backbone as well as thioether side groups. The crystal structure revealed elaborate HgCl2-thioether aggregates nested within the host octahedra to form a hierarchical, multifunctional net. The chelating thioether groups also promote Hg(II) removal from water, while the trapped Hg(II) can be easily extricated by 2-mercaptoethanol to reactivate the MOF sorbent.

A Boiling-Water-Stable, Tunable White-Emitting Metal-Organic Framework from Soft-Imprint Synthesis.

A new avenue for making porous frameworks has been developed by borrowing an idea from molecularly imprinted polymers (MIPs). In lieu of the small molecules commonly used as templates in MIPs, soft metal components, such as CuI, are used to orient the molecular linker and to leverage the formation of the network. Specifically, a linear dicarboxylate linker with thioether side groups reacted simultaneously with Ln(3+) ions and CuI, leading to a bimetallic net featuring strong, chemically hard Eu(3+) -carboxylate links, as well as soft, thioether-bound Cu2 I2 clusters. The CuI block imparts water stability to the host; with the tunable luminescence from the lanthanide ions, this creates the first white-emitting MOF that is stable in boiling water. The Cu2 I2 block also readily reacts with H2 S, and enables sensitive colorimetric detection while the host net remains intact.

Highly Polarizable Triiodide Anions (I3(-)) as Cross-Linkers for Coordination Polymers: Closing the Semiconductive Band Gap.

From a hydrothermal reaction using CuI, KI, and 3,3'5,5'-tetramethyl-4,4'-bipyrazole (TMBP), the triiodide anion I3(-) has been integrated into the water-stable 2D coordination polymer Cu(TMBP)I3 (1). In contrast with other metal triiodide complexes, 1 features remarkably small distortions in the bond distances associated with the I3(-) units (i.e., the Cu-I and I-I bonds), which effectively link up the copper(I) centers into infinite CuI3 chains. The electronic band gaps and electrical conductivity data are also found to be consistent with the I3(-) ion acting as an effective linker across the copper(I) centers.

Selective Ag(I) binding, H2S sensing, and white-light emission from an easy-to-make porous conjugated polymer.

Separating silver (Ag(+)) from lead (Pb(2+)) is one of the many merits of the porous polymer framework reported here. The selective metal binding stems from the well-defined chelating unit of N-heterocycles, which consists of a triazine (C3N3) ring bonded to three 3,5-dimethylpyrazole moieties. Such a rigid and open triad also serves as the distinct building unit in the fully conjugated 3D polymer scaffold. Because of its strong fluorescence and porosity (e.g., BET surface area: 355 m(2)/g), and because of the various types of metal species that can be readily taken up, this versatile framework is especially fit for functionalization. For example, with AgNO3 loaded, the framework solid exhibits a brown color in response to water solutions of H2S, even at the dilution of 5.0 µM (0.17 ppm); whereas cysteine and other biologically relevant thiols do not cause notable change in color. In another example, tunable white-light emission was produced when an Ir(III) complex was doped (e.g., about 0.02% of the polymer weight) onto the framework. Mechanistically, the bound Ir(III) centers become highly emissive in the orange-red region, complementing the broad, bluish emission from the polymer host to result in the overall white-light quality: the color attributes of the emission are therefore easily tunable by the Ir(III) dopant concentration. With this exemplary study, we intend to highlight metal uptake as an effective approach to modify and enrich the properties of porous polymer frameworks and to stimulate interest in further examining metal-polymer interactions in the context of sensing, separation, catalyzes, and other applications.