Construction of Hydration Layer for Proton Transport by Implanting the Hydrophilic Center Ag0 in Nickel Metal-Organic Frameworks.

The directional arrangement of H2O molecules can effectively regulate the ordered protons transfer to improve transport efficiency, which can be controlled by the interaction between materials and H2O. Herein, a strategy to build a stable hydration layer in metal-organic framework (MOF) platforms, in which hydrophilic centers that can manipulate H2O molecules are implanted into MOF cavities is presented. The rigid grid-Ni-MOF is selected as the supporting material due to the uniformly distributed cavities and rigid structures. The Ag0 possesses potential combination ability with the hydrophilic substances, so it is introduced into the MOF as hydration layer centers. Relying on the strong interaction between Ag0 and H2O, the H2O molecules can rearrange around Ag0 in the cavity, which is intuitively verified by DFT calculation and molecular dynamics simulation. The establishment of a hydration layer in Ag@Ni-MOF regulates the chemical properties of the material and gives the material excellent proton conduction performance, with a proton conductivity of 4.86 × 10-2 S cm-1.

Guest-Induced Multilevel Charge Transport Strategy for Developing Metal-Organic Frameworks to Boost Photocatalytic CO2 Reduction.

Encapsulating photogenerated charge-hopping nodes and space transport bridges within metal-organic frameworks (MOFs) is a promising method of boosting the photocatalytic performance. Herein, this work embeds electron transfer media (9,10-bis(4-pyridyl)anthracene (BPAN)) in MOF cavities to build multi-level electron transfer paths. The MOF cavities are accurately regulated to investigate the significance of the multi-level electron transfer paths in the process of CO2 photoreduction by evaluating the difference in the number of guest media. The prepared MOFs, {[Co(BPAN)(1,4-dicarboxybenzene)(H2 O)2 ]·BPAN·2H2 O} and {[Co(BPAN)2 (4,4'-biphenyldicarboxylic acid)2 (H2 O)2 ]·2BPAN·2H2 O} (denoted as BPAN-Co-1 and BPAN-Co-2), exhibit efficient visible-light-driven CO2 conversion properties. The CO photoreduction efficacy of BPAN-Co-2 (5598 µmol g-1  h-1 ) is superior to that of most reported MOF-based catalysts. In addition, the enhanced CO2 photoreduction ability is supported by density functional theory (DFT). This work illustrates the feasibility of realizing charge separation characteristics in MOF catalysts at the molecular level, and provides new insight for designing high-performance MOFs for artificial photosynthesis.

Electrochemical Activation of Nitromethane to Construct Isoxazoline Aldoximes.

Activation of nitromethane to endow new reactivity is an interesting and meaningful but also challenging topic. Herein, we report an electrochemical activation of nitromethane to serve as both the heterocyclic skeleton and oxime sources for the construction of isoxazoline aldoximes. The isoxazoline aldoximes that are prepared by four steps with the reported strategy are synthesized in a single step from low-cost and readily available nitromethane and olefins with moderate to excellent yields under our electrochemical conditions. The reaction also takes advantage of high atom-economy and E-selectivity. Moreover, the mechanism is studied by control experiments, a kinetic isotope effect (KIE) study, cyclic voltammogram (CV) experiments, and density functional theory (DFT) calculations. The mechanistic results reveal that nitromethane may be activated under electrochemical conditions to deliver a 1,2,5-oxadiazole 2-oxide intermediate, which undergoes [3+2] cycloaddition with olefins to yield isoxazoline aldoximes.

Metal-Ion Coupling in Metal-Organic Framework Materials Regulating the Output Performance of a Triboelectric Nanogenerator.

Metal-organic frameworks (MOFs) as friction nanopower generation materials have attracted more and more research and attention because of the inherent three-dimensional framework structure and large aperture. In this work, the ZUT-75(Mn) with a one-dimensional pore structure was synthesized by using electron-rich benzimidazole carboxylic acid ligands, and isomorphic offspring MOF materials were obtained by single crystal-single crystal solvent-assisted metal-ion exchange. The exchange process was monitored by liquid UV-vis spectroscopy, atomic absorption spectrometry, and energy-dispersive X-ray spectroscopy. The metal-oxygen coordination energy, X-ray photoelectron spectroscopy binding energy, and hard-soft acid-base principle verified the spontaneity of the central-metal-exchange reaction. The four materials were applied to a triboelectric nanogenerator (TENG), and the output performance law of ZUT-75 was Co-MT > Zn-MT > Cu-MT > Mn-MT. Among them, the charge and power densities of Co-MT were up to 127.05 µC m-2 and 3280.50 mW m-2. When the density functional theory calculation and variable-temperature magnetic susceptibility test results were combined, it was concluded that low metal-ion-coupling degree promoted the formation and transfer of contact electrifications, which greatly improved the output performance of the TENG. This work provided a new idea for improving the output performance of the TENG.

A Double-Helix Metal-Chain Metal-Organic Framework as a High-Output Triboelectric Nanogenerator Material for Self-Powered Anticorrosion.

The development of novel metal organic framework (MOF) friction power generation materials with high stability is important. This paper reports the first example of a double-helix metal chain organic framework with a network structure (ZUT-8). ZUT-8 shows high chemical stability, functional adjustability, and excellent output performance of friction power generation, which is superior to traditional coordination polymer materials. The cathodic protection system with ZUT-8 can prevent metal corrosion significantly. The output performance can be improved effectively by enhancing the conjugate effect of the linker. The theoretical calculation results showed that an increase in the degree of conjugation could significantly reduce the band gap, thereby affecting the friction power output signal. This study opens the door to constructing MOF materials with a double-helix metal chain and will promote their potential applications in self-powered electrochemical cathodic protection.

Enhancement of Proton Conductivity in Fe-Metal-Organic Frameworks by Postsynthetic Oxidation and High-Performance Hybrid Membranes with Low Acidity.

The postsynthetic oxidation (PSO) of metal nodes in metal-organic frameworks (MOFs) has received widespread attention because PSO can significantly improve the performance of materials without changing the framework. This study investigates the influence of PSO on the proton conductivity of MOFs. The PSO product {[FeIII3L2(H2O)6]•3(OH)}n (2) is obtained by oxidizing {[FeII3L2(H2O)6]•3H2O}n (1) with Cu(NO3)2. At 98% RH and 70 °C, the proton conductivity of 2 is 66 times higher than that of 1, indicating that PSO can promote proton conduction. In the PSO process, metal ions shuttle in the MOF framework to functionalize the pores, and the change in the guest molecule forms more host-guest collaborative hydrogen bonds. All of these have made a significant contribution to proton conduction. Because 2 exhibits high proton conductivity (2.66 × 10-4 S·cm-1) at 98% RH and 80 °C, we doped 2 into a highly economical poly(vinylidene fluoride) (PVDF)/polyvinylpyrrolidone (PVP) substrate to make a hybrid membrane. The resulting hybrid membrane exhibits a high proton conductivity of 1.77 × 10-3 S·cm-1 at 98% RH and 80 °C, which is 4 times higher than the proton conductivity of the PVDF/PVP membrane and 6.6 times higher than that of 2.

High Proton Conductivity in Nafion/Ni-MOF Composite Membranes Promoted by Ligand Exchange under Ambient Conditions.

Metal-organic frameworks (MOFs) have appeared to be promising competitive candidates as crystalline porous materials for proton conduction. Explorations of the method of preparation of proton conductive MOFs and the proton transfer mechanism have enabled them to attract widespread attention, and tremendous efforts have been made to improve the proton conductivity of MOFs. On the basis of our previous work, we explicitly propose that ligand exchange can upgrade the proton conduction performance of MOFs. Using MOF-azo as the precursor, the proton conductivities of exchange products MOF-bpy and MOF-bpe increase by 3.5- and 2.8-fold, respectively. After the MOFs had been doped into the Nafion matrix to prepare composite membranes, the proton conduction performance of composite membranes filled with subproducts (2.6 × 10-2 and 1.95 × 10-2 S cm-1) is significantly better than that of a composite membrane filled with a parent product (1.12 × 10-2 S cm-1) under ambient conditions (without heating or humidifying). The ligand exchange strategy presented herein demonstrates great promise for the development of high-proton conductivity MOFs and MOF composites with expanded future applications.

The 50-Fold Enhanced Proton Conductivity Brought by Aqueous-Phase Single-Crystal-to-Single-Crystal Central Metal Exchange.

Coordination polymer {[Co3L2(H2O)6]·2H2O}n goes through aqueous-phase single-crystal-to-single-crystal (SC-SC) central metal exchange to produce {[Cu3L2(H2O)6]·2H2O}n. The daughter product presents a higher proton conductivity of 0.004 S cm-1 at 95 °C and 100% RH, increasing by 50-fold relative to the parent product. The water vapor adsorption reveals that the uptake capacity of 2 reaches 145.08 mg/g, which is 7.5 times that of 1 (19.36 mg/g). High water affinity is confirmed by the smaller water contact angle of 2. Replacing water vapor with vapors of dilute hydrochloric acid and ammonia, the improvement of proton conductivity is also realized. Exchanged products all give enhanced conductivities in different vapor atmospheres, which shows that the aqueous-phase central metal exchange is a judicious choice for the preparation of excellent proton conducting coordination polymers.

Investigation of Regulating Third-Order Nonlinear Optical Property by Coordination Interaction.

In this paper, a series of organic compounds (L1-L6) with a D-π-A conjugation system were prepared. The investigations of third-order nonlinear optical (NLO) properties indicate that L1-L6 show different degrees of third-order NLO responses. It is surprising that introducing metal ions can effectively regulate their third-order NLO properties and even change the type of nonlinear absorption signal from reverse saturable absorption to saturable absorption, which can be attributed to the formation of coordination bonds between metal ions and L1-L6. It has aroused our tremendous interests in regulating third-order NLO performance. The regulation mechanisms were also discussed through the pump-probe measurements and the density functional theory. The enhancement of electron transfer efficiency is considered to be the key to improving NLO performance. Furthermore, we also obtained two coordination complexes [Cu(L1)2(NO3)2] (1) and [Cd(L1)2I2] (2) based on L1, which further proved the coordination between metal ions and L1-L6. Ligand-to-metal or metal-to-ligand charge transfer makes more electronic delocalization, leading to better third-order NLO properties. This work provides new ideas and explorations for the excogitation of third-order NLO materials.

Alkenone-enol-alkenone [2+2+2] Cyclotrimerization Producing Functional Coordination Polymers with Excellent Adsorption Performance.

[2+2+2] cycloaddition reactions are one of the most elegant routes for the construction of six-membered rings. Here, we report initially the alkenone-enol-alkenone [2+2+2] cycloaddition reaction and introduce the cycloaddition into the system of in situ building complexes, where three novel coordination polymers (CPs) with functional groups, namely {[CdCl2(L1)]} n (1), {[CdCl2(L2)]} n (2), and {[CdCl2(L3)]} n (3), have been obtained. Particularly, the new 1,2,3,4,5-pentasubtituted cyclohexanols ligands L1, L2, and L3 are created from the starting chalcone derivatives via the [2+2+2] cyclotrimerization. Cadmium chloride not only plays the role of constructing CPs but also acts as a catalyst to promote the cycloaddition reaction. In addition, benefiting from numerous exposed carbonyl and hydroxyl function groups, CPs 1-3 are applied to the adsorptive removal of dyes (congo red (CR) and methyl orange (MO)) from aqueous solutions. As a result, 1-3 show excellent dye adsorption capacity. 1 exhibits maximum CR adsorption capacity of 485.4 mg g-1, and 3 has ultrahigh MO uptake capacity of 492.6 mg g-1. Experimental results suggest that the dye removal effect derives from the interactions between dye molecules and the exposed carbonyl and hydroxyl groups.

Directed Structural Transformations of Coordination Polymers Supported Single-Site Cu(II) Catalysts To Control the Site Selectivity of C-H Halogenation.

A main difficulty in C-H bond functionalization is to undertake the catalyst control accurately where the reaction takes place. In this work, to achieve highly effective and regioselective single-site catalysts, a three-dimensional (3D) rhombus-like framework of {[Mn(Hidbt)DMF]·H2O}n (1) [H3idbt = 5,5'-(1H-imidazole-4,5-diyl)-bis(2H-tetrazole)] containing coordinated DMF molecules was constructed. For the dissolution-recrystallization structural transformation process, attractive structural transformations proceeded from 1 to a new crystalline species formulated as {[Mn3(idbt)2(H2O)2]·3H2O}n (2) with a 3D windowlike architecture, and then the Mn ions in 2 could be exchanged with Cu ions through cation exchange in a single-crystal to single-crystal fashion to produce the Cu-exchanged product {[Mn2Cu(idbt)2(H2O)2]·3H2O}n (2a), which had a windowlike framework like that of 2. Furthermore, 2 and 2a were used as heterogeneous catalysts for the regioselective C-H halogenation of phenols with N-halosuccinimides (NCS and NBS) to produce the site selective single monohalogenated products. It was found that the catalytic activity and site selectivity of 2a were much higher than those of 2, because the unique structural features of 2a with the uniformly dispersed CuII active centers served as a single-site catalyst with a site-isolated and well-defined platform to promote the C-H halogenation reaction in regiocontrol and guide an orientation that favored the para selectivity during the reaction process.

Efficient Catalytic Performance for Acylation-Nazarov Cyclization Based on an Unusual Postsynthetic Oxidization Strategy in a Fe(II)-MOF.

A rare example of SC-SC triggered by Cu2+ heterogeneous oxidation was demonstrated in a Fe(II)-based MOF {[FeII3(L)2(H2O)6]·3H2O} n (1), which occurred a slow conversion into an oxidized MOF 2 ({[FeIII3(L)2(H2O)6]·3(OH)} n) with retention of single crystallinity. The FeII → FeIII progress of the reaction oxidation was proved by single crystal XRD, PXRD, XPS, 57Fe Mössbauer spectroscopy, and UV-vis. We used 1 and 2 as catalysts to catalyze the tandem Nazarov cyclization, and the results show that acylation products were only harvested when 1 was a catalyst, while the oxidized transformer 2 lead mainly to the formation of cyclization products under identical conditions. Through the test of different substrates, 2 can be a good heterogeneous catalyst candidate for the formation of cyclopentenone[ b] benzenes. This work provides a new way to design efficient and hard-synthesized heterogeneous catalyst materials.

Solvent-Induced Assembly of Sliver Coordination Polymers (CPs) as Cooperative Catalysts for Synthesizing of Cyclopentenone[b]pyrroles Frameworks.

A series of solvent-induced Ag-based coordination polymers (CPs) (1-3), which were synthesized by regulating the size, shape, and polarity of solvent molecules (EtOH, iPrOH, 1,4-dioxane) as structure-directing agents, has been used as heterogeneous catalysts to explore how the metal-metal bonds could serve as cooperative catalysis for tandem acylation-Nazarov cyclization reaction to prepare cyclopentenone[b]pyrroles frameworks. Structural conversions from a three-dimensional (3D) framework of 1 to the structure of 2 with Ag-Ag-Ag bonds and to the two-dimensional (2D) framework of 3 with Ag-Ag bonds, arising from the different sizes, shapes, and polarities of the solvent molecules, were observed. Futhermore, the effective cooperative catalysis for tandem acylation-Nazarov cyclization reaction has been evidenced by the significantly transformed connection modes of Ag-Ag interactions in 1-3. As a result, the unique structural characteristics of 2, especially containing the Ag-Ag-Ag bonds, endow 2 with intact multinuclear reaction pathways and well-defined multinuclear platforms that can execute tandem acylation-Nazarov cyclization reaction through the cooperative effect of Ag-Ag-Ag interactions during the catalytic process.

Sustainable Electrochemical Benzylic C-H Oxidation Using MeOH as an Oxygen Source.

New methods and strategies for the direct oxidation of benzylic C-H bonds are highly desirable, owing to the importance of ketone motifs in significant organic transformations and the synthesis of valuable molecules, including pharmaceuticals, pesticides, and fine chemicals. Herein, we describe an electrochemical benzylic C-H oxidation strategy for the synthesis of ketones using MeOH as an oxygen source. Inexpensive and safe KBr serves as both an electrolyte and a bromide radical precursor in the reaction. This transformation also offers several advantages such as mild conditions, broad functional group tolerance, and operational simplicity. Mechanistic investigations by control experiments, radical scavenging experiments, electron paramagnetic resonance (EPR), kinetic studies, cyclic voltammetry (CV), and in-situ Fourier transform infrared (FTIR) spectroscopy support a pathway involving the formation and transformation of benzyl methyl ether via hydrogen atom transfer (HAT) and single-electron transfer (SET). The practical application of our strategy is highlighted by the successful synthesis of five pharmaceuticals, namely lenperone, melperone, diphenhydramine, cinnarizine, and flunarizine.

3D nanocrystalline metal-organic framework materials for the improved output performance of triboelectric nanogenerators.

Triboelectric nanogenerators (TENGs) based on contact electrification and electrostatic induction can effectively convert low-frequency mechanical energy into electrical energy and has attracted considerable attention. However, the low current output performance seriously hinders the wide application of TENGs. Herein, a 3D nanocrystalline metal-organic framework (Cd-MOF) with a specific structure and morphology was reasonably designed as a high-performance triboelectric positive electrode material. The triboelectric test results showed that the maximum instantaneous short-circuit current of Cd-MT was 55.32 µA and the stable output performance maintained a long-term continuous operation for 10 000 s. The peak values of the charge density and electric power density were 102.39 µC m-2 and 2451.04 mW m-2, respectively. In addition, the Cd-MT could quickly fully charge commercial capacitors and light a large number of LED lamps. This work provides a new idea for the development and design of functional MOF triboelectric materials.

Cationic metal-organic framework with charge separation effect as a high output triboelectric nanogenerator material for self-powered anticorrosion.

New stable frictional materials based on metal-organic frameworks (MOFs) are greatly desired for applications in self-powered systems. This work reports an ionic MOF material with efficient charge separation mediated by charge induction. ZUT-iMOF-1(Cu) is chemically stable and its triboelectric output performance surpasses those of traditional MOF materials. The short-circuit current of the iMOF triboelectric nanogenerator is 73.79 µA at 5 Hz. The output performance remains stable over 50 000 cycles of continuous operation. The charge and power densities peak at 123.20 µC m-2 and 3133.23 mW m-2. Owing to its high output performance, ZUT-iMOF-1(Cu) effectively prevents metal corrosion in cathodic-protection systems. Theoretical calculations show that increasing the charge-separation effect promotes the frictional electricity generation behaviour. This study provides research suggestions for ionic MOF frictional materials and will promote their application in self-powered electrochemical cathodic-protection systems.

Sequentially Regulating the Structural Transformation of Copper Metal-Organic Frameworks (Cu-MOFs) for Controlling Site-Selective Reaction.

Regulating atomically precise sites in catalysts to achieve site-selective reactions is remarkable but challenging. In this work, a convenient and facile solid-gas/liquid reaction strategy was used to construct controllable active sites in metal-organic frameworks (MOFs) to guide an orientation site-selective reaction. A flexible CuI-MOF-1 with dynamics originating from an anionic and tailorable framework could undergo a reversible structural transformation to engineer a topologically equivalent mixed-valent CuICuII-MOF-2 via a solid-gas/liquid oxidation/reduction process. More importantly, CuI-MOF-1 and CuICuII-MOF-2 could further execute the solid-gas/liquid reaction under ammonia vapor/solution to generate CuII-MOF-3. Furthermore, the transformation from CuI-MOF-1 to CuICuII-MOF-2 and CuII-MOF-3 served as controllable catalysts to facilitate site-selective reactions to realize direct C-N bond arylations. The results demonstrated that CuI-MOF-1 and CuII-MOF-3 possessed well-defined platforms with uniformly and accurately active sites to attain a "turn-on/off" process via different reaction routes, providing the desired site-selective ring-opening products.

Enhancement of Output Performance of Triboelectric Nanogenerator by Switchable Stimuli in Metal-Organic Frameworks for Photocatalysis.

Precise control of the structure of crystalline materials is an efficient strategy to manipulate the fundamental performance of solids. In metal-organic framework (MOF) materials, this control can be realized by reversible cation-exchange through chemically driven changes in the crystalline state. Herein, we reported that the reversible structural transformations between an anionic Zn-MOF (1) and a topologically equivalent bimetallic Zn/Co-MOF (2) were accomplished. Both MOFs powders and their hybrid composites were used as positive electrode materials to assemble triboelectric nanogenerators (TENGs). The results demonstrated that the output performance of the Zn/Co-MOF-TENG was effectively improved because the introduction of Co ions makes electron transfer easier. Moreover, the output performance of the TENGs based on MOF@PVDF (PVDF = polyvinylidene fluoride) composite films showed that the Zn/Co-MOF@PVDF-TENG possessed much higher output than these corresponding film-based and MOF-based TENGs. As a practical application, the superior output of Zn/Co-MOF@PVDF-TENG was used to light an ultraviolet lamp plate for the [2 + 2] photochemical cycloaddition of organometallic macrocycles.

Proton Conduction of Nafion Hybrid Membranes Promoted by NH3-Modified Zn-MOF with Host-Guest Collaborative Hydrogen Bonds for H2/O2 Fuel Cell Applications.

It is of great significance to develop creative proton exchange membrane materials for proton exchange membrane fuel cells (PEMFCs). The strategy of doping metal-organic frameworks (MOFs) with guest molecules into the Nafion matrix is adopted to improve the electrochemical performance of Nafion hybrid membranes. Various and abundant hydrogen bonds can make a tremendous contribution to the proton conduction of hybrid membranes. In this work, we used high proton-conducting Zn-MOFs with the characteristics of host-guest collaborative hydrogen bonds as the filler to prepare Zn-MOF/Nafion hybrid membranes. Alternating current (AC) impedance tests show that when the doping amount of Zn-MOF is 5%, the proton conductivity reaches 7.29 × 10-3 S·cm-1, being 1.87 times that of the pure Nafion membrane at 58% relative humidity (RH) and 80 °C. In an attempt to prove the promotion effect of guest NH3 on proton conductivity of Nafion hybrid membranes, Zn-MOF-NH3 was filled into the Nafion matrix. Under the same conditions, its proton conductivity reaches the maximum value of 2.13 × 10-2 S·cm-1, which is 5.47 times that of the pure Nafion membrane. Zn-MOF-NH3/Nafion-5 was used to fabricate a proton exchange membrane for application in H2/O2 fuel cells. The maximum power density of 212 mW cm-2 and a current density of 630 mA cm-2 reveal a respectable single cell performance. This study provides a promising method for optimizing the structure of MOF proton conductors and inspires the preparation of high-performance Nafion hybrid membranes.

Efficient Identification for Alcohol Homologues and Hyperthermy Based on Coordination Polymer Multiple Structural Transformations.

Recently, coordination polymer materials are of high interest due to the potential applications for chemical sensing and luminescent materials. In this work, we designed a photofluorescence coordination polymer material based on a donor-metal-acceptor structure. The donor-metal-acceptor architecture showed unusual multiple environmental responsiveness accompanied by a great change of fluorescence behaviors. Generally, organic homologue molecules are not easily distinguished by coordination polymer sensors because they have similar molecular structures and interaction sites. However, using the feature of multiple structural transformations, the accurate identification for organic homologue molecules can be realized, especially in a short time to quickly identify MeOH in other alcohol homologues (even in mixed atmospheres with only 10% MeOH). The visualization transformation of fluorescence can also be realized by single crystal to single crystal thermal-induced coordination bond ON/OFF behavior. The reversible structure conversion strategy provides new ideas for the accurate identification of organic homolog molecules or external environmental stimuli.