Effect of Nitrogen Substitution in V2O3 on the Metal–Insulator Transition

The effect of N‐doping on the paramagnetic–antiferromagnetic transition associated with the metal–insulator (M–I) transition of V₂O₃ at 150 K has been studied in bulk samples as well as in nanosheets. The magnetic transition temperature of V₂O₃ is lowered to ～120 K in the N‐doped samples. Electrical resistivity data also indicate a similar lowering of the M–I transition temperature. First‐principles DFT calculations reveal that anionic (N) substitution and the accompanying oxygen vacancies reduce the energy of the high‐temperature metallic corundum phase relative to the monoclinic one leading to the observed reduction in Nèel temperature. In the electronic structure of N‐substituted V₂O₃, a sub‐band of 2p states of trivalent anion (N) associated with its strong bond with the vanadium cation appears at the top of the band of O(2p) states, the 3d‐states of V being slightly higher in energy. Its band gap is thus due to crystal field splitting of the degenerate d‐orbitals of vanadium and superexchange interaction, which reduces notably (ΔE_g=?0.4 eV) due to their hybridization with the 2p states of nitrogen. A weak magnetic moment arises in the monoclinic phase of N‐substituted V₂O₃ with O‐vacancies, with a moment of ?1 μ_B/N localized on vanadium atoms in the vicinity of oxygen vacancies.     

Metal‐Ion Metathesis in Metal–Organic Frameworks: A Synthetic Route to New Metal–Organic Frameworks

A porous metal–organic framework, Mn(H₃O)[(Mn₄Cl)₃(hmtt)₈] (POST‐65), was prepared by the reaction of 5,5′,10,10′,15,15′‐hexamethyltruxene‐2,7,12‐tricarboxylic acid (H₃hmtt) with MnCl₂ under solvothermal conditions. POST‐65(Mn) was subjected to post‐synthetic modification with Fe, Co, Ni, and Cu according to an ion‐exchange method that resulted in the formation of three isomorphous frameworks, POST‐65(Co/Ni/Cu), as well as a new framework, POST‐65(Fe). The ion‐exchanged samples could not be prepared by regular solvothermal reactions. The complete exchange of the metal ions and retention of the framework structure were verified by inductively coupled plasma–atomic emission spectrometry (ICP‐AES), powder X‐ray diffraction (PXRD), and Brunauer–Emmett–Teller (BET) surface‐area analysis. Single‐crystal X‐ray diffractions studies revealed a single‐crystal‐to‐single‐crystal (SCSC)‐transformation nature of the ion‐exchange process. Hydrogen‐sorption and magnetization measurements showed metal‐specific properties of POST‐65.     

Hydrothermal Synthesis,Structure, and Optical Properties of Two Nanosized Ln26@CO3 (Ln=Dy and Tb) Cluster‐Based Lanthanide–Transition‐Metal–Organic Frameworks (Ln MOFs)

Two Ln₂₆@CO₃ (Ln=Dy and Tb) cluster‐based lanthanide–transition‐metal–organic frameworks (Ln MOFs) formulated as [Dy₂₆Cu₃(Nic)₂₄(CH₃COO)₈(CO₃)₁₁(OH)₂₆(H₂O)₁₄]Cl ? 3 H₂O ( 1 ; HNic=nicotinic acid) and [Tb₂₆NaAg₃(Nic)₂₇(CH₃COO)₆(CO₃)₁₁(OH)₂₆Cl(H₂O)₁₅] ? 7.5 H₂O ( 2 ) have been successfully synthesized by hydrothermal methods and characterized by IR, thermogravimetric analysis (TGA), elemental analysis, and single X‐ray diffraction. Compound 1 crystallizes in the monoclinic space group Cc with a=35.775(12) Å, b=33.346(11) Å, c=24.424(8) Å, β=93.993(5)°, V=29065(16) ų, whereas 2 crystallizes in the triclinic space group P with a=20.4929(19) Å, b=24.671(2) Å, c=29.727(3) Å, α=81.9990(10)°, β=88.0830(10)°, γ=89.9940(10)°, V=14875(2) ų. Structural analysis indicates the framework of 1 is a 3D perovskite‐like structure constructed out of CO₃@Dy₂₆ building units and Cu⁺ centers by means of nicotinic acid ligand bridging. In 2 , however, nanosized CO₃@Tb₂₆ units and [Ag₃Cl]²⁺ centers are connected by Nic^? bridges to give rise to a 2D structure. It is worth mentioning that this kind of 4d–4f cluster‐based MOF is quite rare as most of the reported analogous compounds are 3d–4f ones. Additionally, the solid‐state emission spectra of pure compound 2 at room temperature suggest an efficient energy transfer from the ligand Nic^? to Tb³⁺ ions, which we called the “antenna effect”. Compound 2 shows a good two‐photon absorption (TPA) with a TPA coefficient of 0.06947 cm GM^?1 (1 GM=10^?50 cm⁴ s photon^?1), which indicates that compound 2 might be a good choice for third‐order nonlinear optical materials.     

A New Route to Organic Intercalates Consisting of Vanadium Pentoxide and Pyridines: (4-H2N-C5H5NH)V2O5

Light at the end of the tunnel! Intercalates of pyridine and layered inorganic compounds have intrigued chemists for at least two decades, but their structure determination has always been difficult owing to a lack of suitable single crystals. The intercalate (4-H₂N-C₅H₅NH)V₂O₅ has now been characterized by X-ray crystal structure analysis, and its electronic structure and magnetic properties have been studied in detail.     

A Highly Energetic N‐Rich Metal–Organic Framework as a New High‐Energy‐Density Material

Metal–organic framework (MOF)‐based energetic material [Cu₃(MA)₂(N₃)₃] ( 1 ; MA=melamine) was synthesized and structurally characterized (47.55 % N). The structural analysis revealed the existence of unusual multiwalled tubular channels and interweaving of single and double helical units in 1 . The standard molar enthalpy of formation was found to be 1788.73 kJ mol^?1, which is the highest value among previously reported MOF‐based energetic materials. The calculated detonation properties showed that 1 can be used as a potential explosive. Sensitivity tests revealed that 1 is insensitive and thus can function as a high‐energy‐density material with a favorable level of safety.     

[Al4(OH)2(OCH3)4(H2N‐bdc)3]⋅x H2O: A 12‐Connected Porous Metal–Organic Framework with an Unprecedented Aluminum‐Containing Brick

Al together now! A new stable aluminum aminoterephthalate system contains octameric building blocks that are connected by organic linkers to form a 12‐connected net (see picture). The structure adopts a cubic centered packing motive in which octameric units replace individual atoms, thus forming distorted octahedral (red sphere) and tetrahedral cages (green spheres) with effective accessible diameters of 1 and 0.45 nm, respectively.

     

The Metal–Organic Framework MIL‐53(Al) Constructed from Multiple Metal Sources: Alumina,Aluminum Hydroxide,and Boehmite

Three aluminum compounds, namely alumina, aluminum hydroxide, and boehmite, are probed as the metal sources for the hydrothermal synthesis of a typical metal–organic framework MIL‐53(Al). The process exhibits enhanced synthetic efficiency without the generation of strongly acidic byproducts. The time‐course monitoring of conversion from different aluminum sources into MIL‐53(Al) is achieved by multiple characterization that reveals a similar but differentiated crystallinity, porosity, and morphology relative to typical MIL‐53(Al) prepared from water‐soluble aluminum salts. Moreover, the prepared MIL‐53(Al) constructed with the three insoluble aluminum sources exhibit an improved thermal stability of up to nearly 600 °C and enhanced yields. Alumina and boehmite are more preferable than aluminum hydroxide in terms of product porosity, yield, and reaction time. The adsorption performances of a typical environmental endocrine disruptor, dimethyl phthalate, on the prepared MIL‐53(Al) samples are also investigated. The improved structural stability of MIL‐53(Al) prepared from these alternative aluminum sources enables double‐enhanced adsorption performance (up to 206 mg g^?1) relative to the conventionally obtained MIL‐53(Al).     

In Situ Modification of Metal–Organic Frameworks in Mixed‐Matrix Membranes

Processable films of metal–organic frameworks (MOFs) have been long sought to advance the application of MOFs in various technologies from separations to catalysis. Herein, MOF–polymer mixed‐matrix membranes (MMMs) are described, formed on several substrates using a wide variety of MOF materials. These MMMs can be delaminated from their substrates to create free‐standing MMMs that are mechanically stable and pliable. The MOFs in these MMMs remain highly crystalline, porous, and accessible for further chemical modification through postsynthetic modification (PSM) and postsynthetic exchange (PSE) processes. Overall, the findings here demonstrate a versatile approach to preparing stable functional MMMs that should contribute significantly to the advancement of these materials.     

Multi‐frequency (S,X, Q and W‐band) EPR and ENDOR Study of Vanadium(IV) Incorporation in the Aluminium Metal–Organic Framework MIL‐53

Doping the well‐known metal–organic framework MIL‐53(Al) with vanadium(IV) ions leads to significant changes in the breathing behaviour and might have repercussions on the catalytic behaviour as well. To understand the properties of such a doped framework, it is necessary to determine where dopant ions are actually incorporated. Electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) are applied to reveal the nearest environment of the paramagnetic vanadium(IV) dopant ions. EPR spectra of as‐synthesised vanadium‐doped MIL‐53 are recorded at S‐, X‐, Q‐ and W‐band microwave frequencies. The EPR spectra suggest that at low dopant concentrations (1.0–2.6 mol %) the vanadium(IV) ions are well dispersed in the matrix. Varying the vanadium dopant concentration within this range or the dopant salt leads to the same dominant EPR component. In the ENDOR spectra, hyperfine (HF) interactions with ¹H, ²⁷Al and ⁵¹V nuclei are observed. The HF parameters extracted from simulations strongly suggest that the vanadium(IV) ions substitute Al in the framework.     

Controlled Construction of Metal–Organic Frameworks: Hydrothermal Synthesis,X‐ray Structure,and Heterogeneous Catalytic Study

The role of pH in the formation of metal–organic frameworks (MOFs) has been studied for a series of magnesium‐based carboxylate framework systems. Our investigations have revealed the formation of five different zero‐dimensional (0D) to three‐dimensional (3D) ordered frameworks from the same reaction mixture, merely by varying the pH of the medium. The compounds were synthesized by the hydrothermal method and characterized by single‐crystal X‐ray diffraction. Increase of the pH of the medium led to abstraction of the imine hydrogen from the ligand and a concomitant increase in the OH^? ion concentration in the solution, facilitating the construction of higher dimensional framework compounds. A stepwise increase in pH resulted in a stepwise increase in the dimensionality of the network, ultimately leading to the formation of a 3D porous solid. A gas adsorption study of the 3D framework compound confirmed its microporosity with a BET surface area of approximately 450 m² g^?1. Notably, the 3D framework compound catalyzes aldol condensation reactions of various aromatic aldehydes with acetone under heterogeneous conditions.     

Guest Modulation of Spin‐Crossover Transition Temperature in a Porous Iron(II) Metal–Organic Framework: Experimental and Periodic DFT Studies

The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin‐crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)₄]} ( 1 ) with five‐membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin‐crossover transition with hysteresis loops 14–29 K wide and average critical temperatures T_c=201 K ( 1?fur ), 167 K ( 1?pyr ), and 114.6 K ( 1?thio ) well below that of the parent compound 1 (T_c=295 K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1?fur , while 1?pyr and 1?thio show 50 % spin transition. For 1?fur the transformation between the HS and IS (middle of the plateau) phases occurs concomitantly with a crystallographic phase transition between the tetragonal space groups P4/mmm and I4/mmm, respectively. The latter space group is retained in the subsequent transformation involving the IS and the LS phases. 1?pyr and 1?thio display the tetragonal P4/mmm and orthorhombic Fmmm space groups, respectively, in both HS and IM phases. Periodic calculations using density functional methods for 1?fur , 1?pyr , 1?thio , and previously reported derivatives 1?CS₂ , 1?I, 1?bz (benzene), and 1?pz (pyrazine) have been carried out to investigate the electronic structure and nature of the host–guest interactions as well as their relationship with the changes in the LS–HS transition temperatures of 1?Guest . Geometry‐optimized lattice parameters and bond distances in the empty host 1 and 1?Guest clathrates are in general agreement with the X‐ray diffraction data. The concordance between the theoretical results and the experimental data also comprises the guest molecule orientation inside the host and intermolecular distances. Furthermore, a general correlation between experimental T_c and calculated LS–HS electronic energy gap was observed. Finally, specific host–guest interactions were studied through interaction energy calculations and crystal orbital displacement (COD) curve analysis.     

A New Strategy for Storage and Transportation of Sensitive High‐Energy Materials: Guest‐Dependent Energy and Sensitivity of 3D Metal–Organic‐Framework‐Based Energetic Compounds

Reaction of Co(II) with the nitrogen‐rich ligand N,N‐bis(1H‐tetrazole‐5‐yl)‐amine (H₂bta) leads to a mixed‐valence, 3D, porous, metal–organic framework (MOF)‐based, energetic material with the nitrogen content of 51.78%, [Co₉(bta)₁₀(Hbta)₂(H₂O)₁₀]_n?(22 H₂O)_n ( 1 ). Compound 1 was thermohydrated to produce a new, stable, energetic material with the nitrogen content of 59.85% and heat of denotation of 4.537 kcal cm^?3, [Co₉(bta)₁₀(Hbta)₂(H₂O)₁₀]_n ( 2 ). Sensitivity tests show that 2 is more sensitivity to external stimuli than 1 , reflecting guest‐dependent energy and sensitivity of 3D, MOF‐based, energetic materials. Less‐sensitive 1 can be regarded as a more safe form for storage and transformation to sensitive 2 .     

Continuous‐Flow Microwave Synthesis of Metal–Organic Frameworks: A Highly Efficient Method for Large‐Scale Production

Metal–organic frameworks are having a tremendous impact on novel strategic applications, with prospective employment in industrially relevant processes. The development of such processes is strictly dependent on the ability to generate materials with high yield efficiency and production rate. We report a versatile and highly efficient method for synthesis of metal–organic frameworks in large quantities using continuous flow processing under microwave irradiation. Benchmark materials such as UiO‐66, MIL‐53(Al), and HKUST‐1 were obtained with remarkable mass, space–time yields, and often using stoichiometric amounts of reactants. In the case of UiO‐66 and MIL‐53(Al), we attained unprecedented space–time yields far greater than those reported previously. All of the syntheses were successfully extended to multi‐gram high quality products in a matter of minutes, proving the effectiveness of continuous flow microwave technology for the large scale production of metal–organic frameworks.     

Unusually Strong Long‐Distance Metal–Metal Coupling in Bis(ferrocene)‐Containing BOPHY: An Introduction to Organometallic BOPHYs

The first organometallic BOPHY (BOPHY=bis(difluoroboron)‐1,2‐bis{(pyrrol‐2‐yl)methylene}hydrazine) containing two ferrocene substituents was prepared through a Knoevenagel condensation between tetramethyl substituted BOPHY and ferrocene carboxaldehyde. An unprecedentedly strong long‐range (≈17.2 Å) metal–metal coupling in this new complex was investigated using electrochemical, spectroelectrochemical, and chemical oxidation methods. Electrochemical data is indicative of a 200 mV separation between the first and the second ferrocene‐centered oxidation processes. Formation of the mixed‐valence states and appearance and disappearance of two NIR bands were observed during stepwise oxidation of the first organometallic BOPHY. The electronic structure and the nature of the excited states in this new chromophore were studied by DFT and TDDFT calculations.