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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Surfactant-assisted assembly of nanoscale zinc coordination compounds to enhance tandem conversion reactions in water.

Precise control over the morphology and size of coordination polymers (CPs) is crucial for extending these inorganic-organic materials to many advanced applications, in particular for heterogeneous catalysis. In this work, two Zn-based CPs, {[Zn3(idbt)2(4,4'-dmbpy)2]·H2O}n (1) and {[Zn3(idbt)2(H2O)3]·H2O}n (2) (H3idbt = 5,5'-(1H-imidazole-4,5-diyl)-bis-(2H-tetrazole), 4,4'-dmbpy = 4,4''-dimethyl-2,2'-bipyridine), were synthesized through solvothermal reactions. The morphologies and particle sizes of 1 and 2 could be controlled from large scale to nanoscale by regulating the amount of poly(vinyl alcohol) (PVA). Furthermore, for the conversion reactions of nitromethylbenzenes into benzoic acids, the catalytic properties of nanoscale 1 and 2 were much more efficient than those of large size of 1 and 2, because of the benefit of readily accessible active sites in the nanoscale sized particles, which provide a tunable and functionalizable platform for the conversion reaction by minimizing the diffusion distance but do little for the selectivity.

Porous functionalized MOF self-evolution promoting molecule encapsulation and Hg2+ removal.

Single-crystal-to-single-crystal (SC-SC) structural self-evolution has been successfully performed on an amino-functionalized MOF material, which has greatly improved the Hg2+ removal performance of the material and implemented dye molecule encapsulation through a dissolution-encapsulation-recrystallization process for the first time.

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.

Proton coupled electron transfer: novel photochromic performance in a host-guest collaborative MOF.

Proton coupled electron transfer has been successfully introduced to a host-guest collaborative metal-organic framework material, which exhibits new photochromic properties with reversible, controllable and efficient characteristics.

Ion exchange collaborating coordination substitution: More efficient Cr(VI) removal performance of a water-stable CuII-MOF material.

An unusual water-stable cationic metal-organic framework {[Cu(L)0.5(bpe)(H2O)](NO3)•(H2O)0.5}n (1) (H4L = bis(3,5-dicarboxypyridinium)-p-xylylene) was synthesized, which was developed into an effective capture material for removal chromate from water. The results show that this material efficiently traps HCrO4- pollutant ions via single-crystal to single-crystal (SCSC) coordination substitution process. The HCrO4- uptake capacity of 1 is high to 190 mg/g. Meaningfully, the structure of 1-HCrO4 ({[Cu(L)0.5(bpe)(HCrO4)]}n) can be accurately obtained by single-crystal X-ray diffraction, where the chromate enter the framework to form stable coordination with central metal ions Cu2+. This is the first example of a stable coordination between chromate and the framework during the capture process. The captured HCrO4- are not dissociated easily into the solution due to the coordination bond. This interaction makes the enrichment of HCrO4- more stable and the capture capacity excellent. Furthermore, the HCrO4- releasing process displays good regeneration in a single crystal state, which further elaborates the reversible SCSC transformation. The mechanism of Cr(VI) removal was also confirmed by DFT calculation studies. This work provides a new way to design and develop efficient MOF capture materials.