Nanoscale Trimetallic Metal–Organic Frameworks Enable Efficient Oxygen Evolution Electrocatalysis

Metal–organic frameworks (MOFs) are a class of promising materials for diverse heterogeneous catalysis, but they are usually not directly employed for oxygen evolution electrocatalysis. Most reports focus on using MOFs as templates to in situ create efficient electrocatalysts through annealing. Herein, we prepared a series of Fe/Ni‐based trimetallic MOFs (Fe/Ni/Co(Mn)‐MIL‐53 accordingly to the Material of Institute Lavoisier) by solvothermal synthesis, which can be directly adopted as highly efficient electrocatalysts. The Fe/Ni/Co(Mn)‐MIL‐53 shows a volcano‐type oxygen evolution reaction (OER) activity as a function of compositions. The optimized Fe/Ni_2.4/Co_0.4‐MIL‐53 can reach a current density of 20 mA cm^?2 at low overpotential of 236 mV with a small Tafel slope of 52.2 mV dec^?1. In addition, the OER performance of these MOFs can be further enhanced by directly being grown on nickel foam (NF).     

Metal‐Ion Metathesis and Properties of Triarylboron‐Functionalized Metal–Organic Frameworks

An anionic metal–organic framework, H₃[(Mn₄Cl)₃ L ₈]?30 H₂O?2.5 DMF?5 Diox ( UPC‐15 ), was successfully prepared by the reaction of MnCl₂ with tris(p‐carboxylic acid)tridurylborane (H₃ L ) under solvothermal conditions. UPC‐15 with wide‐open pores (～18.8 Å) is constructed by packing of octahedral and cuboctahedral cages, and exhibits high gas‐sorption capabilities. Notably, UPC‐15 shows selective adsorption of cationic dyes due to the anion framework. Moreover, the catalytic and magnetic properties were investigated, and UPC‐15 can highly catalyze the cyanosilylation of aromatic aldehydes. UPC‐15 exhibits the exchange of metal ions from Mn to Cu in a single‐crystal‐to‐single‐crystal manner to generate UPC‐16 , which could not be obtained by the direct solvothermal reaction of CuCl₂ and H₃ L. UPC‐16 exhibits similar properties for gas sorption, dye separation, and catalytic activity. However, the magnetic behaviors for UPC‐15 and UPC‐16 are distinct due to the metal‐specific properties. Below 47 K, UPC‐15 exhibits a ferromagnetic coupling but UPC‐16 shows a dominant antiferromagnetic behavior.     

Metal–Organic Perovskites: Synthesis,Structures, and Magnetic Properties of [C(NH2)3][MII(HCOO)3] (M=Mn,Fe, Co,Ni, Cu,and Zn; C(NH2)3= Guanidinium)

We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal–organic perovskite ABX₃, [C(NH₂)₃][M^II(HCOO)₃], in which A=C(NH₂)₃ is guanidinium, B=M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO^?. The compounds could be synthesized by either diffusion or hydrothermal methods from water or water‐rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna2₁. In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti–anti formate bridges, thus forming the anionic NaCl‐type [M(HCOO)₃]^? frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn–Teller effect of Cu²⁺ results in a distorted anionic Cu–formate framework that can be regarded as Cu–formate chains through short basal Cu? O bonds linked by the long axial Cu? O bonds. These materials show higher thermal stability than other metal–organic perovskite series of [AmineH][M(HCOO)₃] templated by the organic monoammonium cations (AmineH⁺) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin‐canted antiferromagnetism, with a Néel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6 K (Cu). In addition to the general spin‐canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin‐flop and a spin‐flip to the paramagnetic phase) within 50 kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics.     

Versatile Tailoring of Paddle‐Wheel ZnII Metal–Organic Frameworks through Single‐Crystal‐to‐Single‐Crystal Transformations

A new tetracarboxylate ligand having short and long arms formed 2D layer Zn^II coordination polymer 1 with paddle‐wheel secondary building units under solvothermal conditions. The framework undergoes solvent‐specific single crystal‐to‐single crystal (SC‐SC) transmetalation to produce 1_Cu . With a sterically encumbered dipyridyl linker, the same ligand forms non‐interpenetrated, 3D, pillared‐layer Zn^II metal–organic framework (MOF) 2 , which takes part in SC‐SC linker‐exchange reactions to produce three daughter frameworks. The parent MOF 2 shows preferential incorporation of the longest linker in competitive linker‐exchange experiments. All the 3D MOFs undergo complete SC‐SC transmetalation with Cu^II, whereby metal exchange in different solvents and monitoring of X‐ray structures revealed that bulky solvated metal ions lead to ordering of the shortest linker in the framework, which confirms that the solvated metal ions enter through the pores along the linker axis.     

Robust Ultramicroporous Metal–Organic Frameworks with Benchmark Affinity for Acetylene

Highly selective separation and/or purification of acetylene from various gas mixtures is a relevant and difficult challenge that currently requires costly and energy‐intensive chemisorption processes. Two ultramicroporous metal–organic framework physisorbents, NKMOF‐1‐M (M=Cu or Ni), offer high hydrolytic stability and benchmark selectivity towards acetylene versus several gases at ambient temperature. The performance of NKMOF‐1‐M is attributed to their exceptional acetylene binding affinity as revealed by modelling and several experimental studies: in situ single‐crystal X‐ray diffraction, FTIR, and gas mixture breakthrough tests. NKMOF‐1‐M exhibit better low‐pressure uptake than existing physisorbents and possesses the highest selectivities yet reported for C₂H₂/CO₂ and C₂H₂/CH₄. The performance of NKMOF‐1‐M is not driven by the same mechanism as current benchmark physisorbents that rely on pore walls lined by inorganic anions.     

Metal‐Organic Niccolite: Synthesis,Structures, Phase Transition,and Magnetic Properties of [CH3NH2(CH2)2NH2CH3][M2(HCOO)6] (M=divalent Mn,Fe, Co,Ni, Cu and Zn)

We report the synthesis, crystal structures, thermal and magnetic characterizations of a family of metal‐organic frameworks adopting the niccolite (NiAs) structure, [dmenH₂²⁺][M₂(HCOO)₆²⁻] (dmen=N,N′‐dimethylethylenediamine; M=divalent Mn, 1Mn ; Fe, 2Fe ; Co, 3Co ; Ni, 4Ni ; Cu, 5Cu ; and Zn, 6Zn ). The compounds could be synthesized by either a diffusion method or directly mixing reactants in methanol or methanol–water mixed solvents. The five members, 1Mn , 2Fe , 3Co , 4Ni , and 6Zn are isostructural and crystallize in the trigonal space group P 1c, while 5Cu crystallizes in C2/c. In the structures, the octahedrally coordinated metal ions are connected by anti–anti formate bridges, thus forming the anionic NiAs‐type frameworks of [M₂(HCOO)₆²⁻], with dmenH₂²⁺ located in the cavities of the frameworks. Owing to the Jahn–Teller effect of the Cu²⁺ ion, the 3D framework of 5Cu consists of zigzag Cu‐formate chains with Cu OCHO Cu connections through short basal Cu O bonds, further linked by the long axial Cu O bonds. 6Zn exhibits a phase transition probably as a result of the order–disorder transition of the dmenH₂²⁺ cation around 300 K, confirmed by differential scanning calorimetry and single crystal X‐ray diffraction patterns under different temperatures. Magnetic investigation reveals that the four magnetic members, 1Mn , 2Fe , 3Co , and 4Ni , display spin‐canted antiferromagnetism, with a Néel temperature of 8.6 K, 19.8 K, 16.4 K, and 33.7 K, respectively. The Mn, Fe, and Ni members show spin‐flop transitions below 50 kOe. 2Fe possesses a large hysteresis loop with a large coercive field of 10.8 kOe. The Cu member, 5Cu , shows overall antiferromagnetism (both inter‐ and intra‐chains) with low‐dimensional characteristics.     

Time‐Resolved In Situ X‐ray Diffraction Reveals Metal‐Dependent Metal–Organic Framework Formation

Versatility in metal substitution is one of the key aspects of metal‐organic framework (MOF) chemistry, allowing properties to be tuned in a rational way. As a result, it important to understand why MOF syntheses involving different metals arrive at or fail to produce the same topological outcome. Frequently, conditions are tuned by trial‐and‐error to make MOFs with different metal species. We ask: is it possible to adjust synthetic conditions in a systematic way in order to design routes to desired phases? We have used in situ X‐ray powder diffraction to study the solvothermal formation of isostructural M₂(bdc)₂dabco (M=Zn, Co, Ni) pillared‐paddlewheel MOFs in real time. The metal ion strongly influences both kinetics and intermediates observed, leading in some cases to multiphase reaction profiles of unprecedented complexity. The standard models used for MOF crystallization break down in these cases; we show that a simple kinetic model describes the data and provides important chemical insights on phase selection.     

Large‐Scale,Bottom‐Up Synthesis of Binary Metal–Organic Framework Nanosheets for Efficient Water Oxidation

Ultrathin metal–organic framework (MOF) nanosheets (NSs) offer potential for many applications, but the synthetic strategies are largely limited to top‐down, low‐yield exfoliation methods. Herein, Ni–M–MOF (M=Fe, Al, Co, Mn, Zn, and Cd) NSs are reported with a thickness of only several atomic layers, prepared by a large‐scale, bottom‐up solvothermal method. The solvent mixture of N,N‐dimethylacetamide and water plays key role in controlling the formation of these two‐dimensional MOF NSs. The MOF NSs can be directly used as efficient electrocatalysts for the oxygen evolution reaction, in which the Ni–Fe–MOF NSs deliver a current density of 10 mA cm^?2 at a low overpotential of 221 mV with a small Tafel slope of 56.0 mV dec^?1, and exhibit excellent stability for at least 20 h without obvious activity decay. Density functional theory calculations on the energy barriers for OER occurring at different metal sites confirm that Fe is the active site for OER at Ni–Fe–MOF NSs.     

Large‐Scale,Bottom‐Up Synthesis of Binary Metal–Organic Framework Nanosheets for Efficient Water Oxidation

Ultrathin metal–organic framework (MOF) nanosheets (NSs) offer potential for many applications, but the synthetic strategies are largely limited to top‐down, low‐yield exfoliation methods. Herein, Ni–M–MOF (M=Fe, Al, Co, Mn, Zn, and Cd) NSs are reported with a thickness of only several atomic layers, prepared by a large‐scale, bottom‐up solvothermal method. The solvent mixture of N,N‐dimethylacetamide and water plays key role in controlling the formation of these two‐dimensional MOF NSs. The MOF NSs can be directly used as efficient electrocatalysts for the oxygen evolution reaction, in which the Ni–Fe–MOF NSs deliver a current density of 10 mA cm^?2 at a low overpotential of 221 mV with a small Tafel slope of 56.0 mV dec^?1, and exhibit excellent stability for at least 20 h without obvious activity decay. Density functional theory calculations on the energy barriers for OER occurring at different metal sites confirm that Fe is the active site for OER at Ni–Fe–MOF NSs.     

New Mechanistic Insight into Stepwise Metal‐Center Exchange in a Metal–Organic Framework Based on Asymmetric Zn4 Clusters

Herein, a mechanism of stepwise metal‐center exchange for a specific metal–organic framework, namely, [Zn₄(dcpp)₂(DMF)₃(H₂O)₂]_n (H₄dcpp=4,5‐bis(4′‐carboxylphenyl)phthalic acid), is disclosed for the first time. The coordination stabilities between the central metal atoms and the ligands as well as the coordination geometry are considered to be dominant factors in this stepwise exchange mechanism. A new magnetic analytical method and a theoretical model confirmed that the exchange mechanism is reasonable. When the metathesis reaction occurs between Cu^II ions and framework Zn^II ions, the magnetic exchange interaction of each pair of Cu^II centers gradually strengthens with increasing amount of framework Cu^II ions. By analyzing the changes of coupling constants in the Cu‐exchanged products, it was deduced that Zn4 and Zn3 are initially replaced, and then Zn1 and Zn2 are replaced later. The theoretical calculation further verified that Zn4 is replaced first, Zn3 next, then Zn1 and Zn2 last, and the coordination stability dominates the Cu/Zn exchange process. For the Ni/Zn and Co/Zn exchange processes, besides the coordination stability, the preferred coordination geometry was also considered in the stepwise‐exchange behavior. As Ni^II and Co^II ions especially favor octahedral coordination geometry in oxygen‐ligand fields, Ni^II ions and Co^II ions could only selectively exchange with the octahedral Zn^II ions, as was also confirmed by the experimental results. The stepwise metal‐exchange process occurs in a single crystal‐to‐single crystal fashion.     

Mechanical Alloying of Metal–Organic Frameworks

The solvent‐free mechanical milling process for two distinct metal–organic framework (MOF) crystals induced the formation of a solid solution, which is not feasible by conventional solution‐based syntheses. X‐ray and STEM‐EDX studies revealed that performing mechanical milling under an Ar atmosphere promotes the high diffusivity of each metal ion in an amorphous solid matrix; the amorphous state turns into the porous crystalline structure by vapor exposure treatment to form a new phase of a MOF solid solution.     

The Role of Multiwall Carbon Nanotubes in Cu‐BTC Metal‐Organic Frameworks for CO2 Adsorption

The discovery of natural gas fields with a high content of CO₂ in world gas reservoirs poses new challenges for CO₂ capture. This work investigates the use of the metal‐organic framework (MOF) Cu‐BTC and hybrid MWCNTs@Cu‐BTC for CO₂ adsorption. Cu‐BTC and hybrid MWCNTs@Cu‐BTC were synthesized by the solvothermal method. The results of imaging of intact MOF pores in Cu‐BTC and hybrid MWCNTs@Cu‐BTC nanocrystals by high‐resolution transmission electron microscopy (HRTEM) under liquid nitrogen conditions are presented. Physical characterizations of the solid adsorbents were made by using a selection of different techniques, including field‐emission scanning electron microscopy (FESEM), X‐ray powder diffraction (XRD), Fourier transform infrared (FT‐IR) spectroscopy, thermogravimetric analysis (TGA), Brunauer–Emmet–Teller (BET) surface area, and CO₂ adsorption and physisorption measurements. HRTEM and FESEM confirmed that Cu‐BTC has an octahedral shape and that the surface morphology of Cu‐BTC changes by the intercalation of MWCTNs. The results show that the modified Cu‐BTC improved the CO₂ adsorption compared to pure Cu‐BTC. The increase in the CO₂ uptake capabilities of hybrid MWCNTs@Cu‐BTC was ascribed to the intercalation of MWCNTs with Cu‐BTC crystals. The CO₂ sorption capacities of Cu‐BTC and hybrid MWCNTs@Cu‐BTC were found to increase from 1.91701 to 3.25642 mmol/g at ambient conditions.     

Cooperative Cluster Metalation and Ligand Migration in Zirconium Metal–Organic Frameworks

Cooperative cluster metalation and ligand migration were performed on a Zr‐MOF, leading to the isolation of unique bimetallic MOFs based on decanuclear Zr₆M₄ (M=Ni, Co) clusters. The M²⁺ reacts with the μ₃‐OH and terminal H₂O ligands on an 8‐connected [Zr₆O₄(OH)₈(H₂O)₄] cluster to form a bimetallic [Zr₆M₄O₈(OH)₈(H₂O)₈] cluster. Along with the metalation of Zr₆ cluster, ligand migration is observed in which a Zr–carboxylate bond dissociates to form a M–carboxylate bond. Single‐crystal to single‐crystal transformation is realized so that snapshots for cooperative cluster metalation and ligand migration processes are captured by successive single‐crystal X‐ray structures. In³⁺ was metalated into the same Zr‐MOF which showed excellent catalytic activity in the acetaldehyde cyclotrimerization reaction. This work not only provides a powerful tool to functionalize Zr‐MOFs with other metals, but also structurally elucidates the formation mechanism of the resulting heterometallic MOFs.     

Solvent Molecule Controlled Zinc(II) Metal‐Organic Frameworks with Different Topology

Two zinc(II) coordination polymers, namely [Zn₂(bptc)(DMF)₂(H₂O)]_n ( 1 ) and [Zn(bptc)_0.5(DMA)]_n ( 2 ) (H₄bptc = biphenyl‐3,3′,5,5′‐tetracarboxylic acid, DMF = N,N′‐dimethylformamide, DMA = N,N′‐dimethylacetamide), were obtained under solvothermal conditions by varying the reaction solvents. Single crystal X‐ray diffraction analyses revealed that compound 1 features a 3D PtS type framework based on dinuclear [Zn₂O(COO)₂] subunits and compound 2 features a 3D lvt type framework based on paddle‐wheel shaped [Zn₂(COO)₄] subunits. Moreover, the luminescent and thermal stabilities of these two compounds were investigated.     

A Polyoxometalate‐Encapsulating Cationic Metal–Organic Framework as a Heterogeneous Catalyst for Desulfurization

A new cationic triazole‐based metal–organic framework encapsulating Keggin‐type polyoxometalates, with the molecular formula [Co(BBPTZ)₃][HPMo₁₂O₄₀]?24 H₂O [compound 1 ; BBPTZ=4,4′‐bis(1,2,4‐triazol‐1‐ylmethyl)biphenyl] is hydrothermally synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X‐ray diffraction, and single‐crystal X‐ray diffraction. The structure of compound 1 contains a non‐interpenetrated 3D CdSO₄ (cds)‐type framework with two types of channels that are interconnected with each other; straight channels that are occupied by the Keggin‐type POM anions, and wavelike channels that contain lattice water molecules. The catalytic activity of compound 1 in the oxidative desulfurization reaction indicates that it is not only an effective and size‐selective heterogeneous catalyst, but it also exhibits distinct structural stability in the catalytic reaction system.     

A Series of Lanthanide Metal–Organic Frameworks with Interesting Adjustable Photoluminescence Constructed by Helical Chains

Based on the isonicotinic acid (HIN=pyridine‐4‐carboxylic acid), seven lanthanide metal–organic frameworks (MOFs) with the formula [Ln(IN)₂L] (Ln=Eu ( 1 ), Tb ( 2 ), Er ( 3 ), Dy ( 4 ), Ho ( 5 ), Gd ( 6 ), La ( 7 ), L=OCH₂CH₂OH) have been synthesized by mixing Ln₂O₃ with HIN under solvothermal conditions, and characterized by single‐crystal X‐ray diffraction, powder X‐ray diffraction, infrared spectroscopy, and fluorescence spectroscopy. Crystal structural analysis shows that compounds 1–6 are isostructural, crystallize in a chiral space group P2₁2₁2₁, whereas compound 7 crystallizes in space group C2/c. Nevertheless, they all consist of new intertwined chains. Simultaneously, on the basis of the above‐mentioned compounds, we have realized a rational design strategy to form the doped Ln MOFs [(Eu_xTb_1?x)(IN)₂L] (x=0.35 ( 8 ), x=0.19 ( 9 ), x=0.06 ( 10 )) by utilizing Tb^III as the second “rare‐earth metal”. Interestingly, the photoluminescence of [(Eu_xTb_1?x)(IN)₂L] are not only adjustable by the ratios of Eu/Tb, but also temperature or excitation wavelength.     

Synthesis and Characterization of a Metal‐Organic Coordination Polymer

A novel energetic microporous metal‐organic coordination polymer {[Ni(tnbpdc)(bpy)(H₂O)₂] · 1.5(DMF)}_n ( 1 ) (tnbpdc = 2, 2′,6, 6′‐tetranitro‐4, 4′‐biphenyl dicarboxylate, bpy = 4, 4′‐bipyridine) was prepared solvothermally and characterized by elemental, IR spectroscopic, and single‐crystal X‐ray diffraction analyses. The X‐ray crystal structure of 1 revealed a rectangular‐shaped grid constructed with tnbpdc linkers and bpy linkers, with the free tunnel size estimated as 11 × 15 Ų. The thermal stability of the compound was evaluated by differential scanning calorimetry and thermogravimetric analysis. Such complexes may find application as novel heat‐resistant energetic materials.     

Direct Structural Identification of Gas Induced Gate‐Opening Coupled with Commensurate Adsorption in a Microporous Metal–Organic Framework

Gate‐opening is a unique and interesting phenomenon commonly observed in flexible porous frameworks, where the pore characteristics and/or crystal structures change in response to external stimuli such as adding or removing guest molecules. For gate‐opening that is induced by gas adsorption, the pore‐opening pressure often varies for different adsorbate molecules and, thus, can be applied to selectively separate a gas mixture. The detailed understanding of this phenomenon is of fundamental importance to the design of industrially applicable gas‐selective sorbents, which remains under investigated due to the lack of direct structural evidence for such systems. We report a mechanistic study of gas‐induced gate‐opening process of a microporous metal–organic framework, [Mn(ina)₂] (ina=isonicotinate) associated with commensurate adsorption, by a combination of several analytical techniques including single crystal X‐ray diffraction, in situ powder X‐ray diffraction coupled with differential scanning calorimetry (XRD‐DSC), and gas adsorption–desorption methods. Our study reveals that the pronounced and reversible gate opening/closing phenomena observed in [Mn(ina)₂] are coupled with a structural transition that involves rotation of the organic linker molecules as a result of interaction of the framework with adsorbed gas molecules including carbon dioxide and propane. The onset pressure to open the gate correlates with the extent of such interaction.     

Extreme Carbon Dioxide Sorption Hysteresis in Open‐Channel Rigid Metal–Organic Frameworks

A systematic study is presented of three closely related microporous metal‐organic frameworks the pore dimensions of which vary according to the choice of 4,4′‐bipyridyl linker. The tunable linker allows exploration of the effect of increasing pore dimensions on the sorption behavior of the frameworks. The MOFs described capture CO₂ under supercritical conditions and continue to sequester the gas under ambient conditions. Gas sorption isotherms for CO₂ are compared with thermogravimetric data, and the CO₂ molecules in the channels of the frameworks could be modeled using single‐crystal X‐ray diffraction analysis. Crystallographic data were used to construct a theoretical model based on DFT methods to calculate framework electrostatic potential maps with a view to understanding the nature of the sorbate–sorbent interactions.     

Formation of Nickel Sulfide Nanoframes from Metal–Organic Frameworks with Enhanced Pseudocapacitive and Electrocatalytic Properties

Nanoframe‐like hollow structures with unique three‐dimensional (3D) open architecture hold great promise for various applications. Current research efforts mainly focus on frame‐like noble metals and metal oxides. However, metal sulfides with frame‐like nanostructures have been rarely reported. Starting from metal–organic frameworks (MOFs), we demonstrate a novel structure‐induced anisotropic chemical etching/anion exchange method to transform Ni‐Co Prussian blue analogue (PBA) nanocubes into NiS nanoframes with tunable size. The reaction between Ni‐Co PBA nanocube templates and Na₂S in solution leads to the formation of well‐defined NiS nanoframes. The different reactivity between the edges and the plane surface of the Ni‐Co PBA nanocubes is found to be the key factor for the formation of NiS nanoframes. Benefitting from their structural merits including 3D open structure, small size of primary nanoparticles, high specific surface area, and good structural robustness, the as‐derived NiS nanoframes manifest excellent electrochemical performance for electrochemical capacitors and hydrogen evolution reaction in alkaline electrolyte.