Nitrate-Anion-Induced Formation of a 2D Metal-Organic Framework with an Unconventional Kagomé Lattice Exhibiting Methanol-Mediated Ketone Catalysis

Metal-organic frameworks (MOFs) with kagomé lattice are attractive for their unique physical and chemical properties, but little attention has been paid to their catalytic properties. Herein, we report a 2D MOF based on a phosphonato-amino-carboxylate ligand (NaHL), i. e., [Na0.33Co(L)(CH3OH)2](NO3)0.33 (2), which exhibits an unconventional kagomé lattice. The formation of this kagomé lattice is caused by the selective recognition of the NO3 - anion by the phenolato group of L2- as evidenced by theoretical calculations. Compound 2 can be utilized for the α-methoxymethylation and aminomethylation of aromatic ketones using methanol as a C1 source. Interestingly, compound 2 can be exfoliated in-situ into nanosheets with one-layer thickness under catalytic reaction conditions, which improves the catalytic efficiency. Based on the results of experiments and theoretical calculations, we proposed possible pathways for the catalytic reaction.

Porous Metal Phosphonate Frameworks: Construction and Physical Properties

ConspectusPorous metal phosphonate frameworks (PMPFs) as a subclass of metal-organic frameworks (MOFs) have promising applications in the fields of gas adsorption and separation, ion exchange and storage, catalysis, sensing, etc. Compared to the typical carboxylate-based MOFs, PMPFs exhibit higher thermal and water stability due to the strong coordination ability of the phosphonate ligands. Despite their robust frameworks, PMPFs account for less than 0.51% of the porous MOFs reported so far. This is because metal phosphonates are highly susceptible to the formation of dense layered or pillared-layered structures, and they precipitate easily and are difficult to crystallize. There is a tendency to use phosphonate ligands containing multiple phosphonate groups and large organic spacers to prevent the formation of dense structures and generate open frameworks with permanent porosity. Thus, many PMPFs are composed of chains or clusters of inorganic metal phosphonates interconnected by organic spacers. Using this feature, a wide range of metal ions and organic components can be selected, and their physical properties can be modulated. However, limited by the small number of PMPFs, there are still relatively few studies on the physical properties of PMPFs, some of which merely remain in the description of the phenomena and lack in-depth elaboration of the structure-property relationship. In this Account, we review the strategies for constructing PMPFs and their physical properties, primarily based on our own research. The construction strategies are categorized according to the number (n = 1-4) of phosphonate groups in the ligand. The physical properties include proton conduction, electrical conduction, magnetism, and photoluminescence properties. Proton conductivity of PMPFs can be enhanced by increasing the proton carrier concentration and mobility. The former can be achieved by adding acidic groups such as -POH and/or introducing acidic guests in the hydrophilic channels. The latter can be attained by introducing conjugate acid-base pairs or elevating the temperature. Semiconducting PMPFs, on the other hand, can be obtained by constructing highly conjugated networks of coordination bonds or introducing large conjugated organic linkers π-π stacked in the lattice. In the case of magnetic PMPFs, long-range magnetic ordering occurs at very low temperatures due to very weak magnetic exchange couplings propagated via O-P-O and/or O(P) units. However, lanthanide compounds may be interesting candidates for single-molecule magnets because of the strong single-ion magnetic anisotropy arising from the spin-orbit coupling and large magnetic moments of lanthanide ions. The luminescent properties of PMPFs depend on the metal ions and/or organic ligands. Emissive PMPFs containing lanthanides and/or uranyl ions are promising for sensing and photonic applications. We conclude with an outlook on the opportunities and challenges for the future development of this promising field.

Metal-organic frameworks as photocatalysts in energetic and environmental applications

Metal-organic frameworks (MOFs) are an exciting new class of porous materials with great potential for photocatalytic applications in the environmental and energy sectors. MOFs provide significant advantages over more traditional materials when used as photocatalysts due to their high surface area, adaptable topologies, and functional ability. In this article, we summarize current developments in the use of MOFs as photocatalysts for a variety of applications, such as CO2 reduction, water splitting, pollutant degradation, and hydrogen production. We discuss the fundamental properties of MOFs that make them ideal for photocatalytic applications, as well as strategies for improving their performance. The opportunities and challenges presented by this rapidly expanding field are also highlighted.

Porous Metal-Organic Frameworks for Light Hydrocarbon Separation

The separation of light hydrocarbon compounds is an important process in the chemical industry. Currently, its separation methods mainly include distillation, membrane separation, and physical adsorption. However, these traditional methods or materials have some drawbacks and disadvantages, such as expensive equipment costs and high energy consumption, poor selectivity, low separation ratios, and separation efficiencies. Therefore, it is important to develop novel separation materials for light hydrocarbon separation. As a new type of organic-inorganic hybrid crystalline material, metal-organic frameworks (MOFs) are promising materials for light hydrocarbon separation due to their designability of structure and easy modulation of function. This review provides an overview of recent advances in the design, synthesis, and application of MOFs for light hydrocarbon separation in recent years, with a focus on the separation of alkane, alkene, and alkyne. We discuss strategies for improving the adsorption selectivity and capacity of MOFs, including pore size limitation, physical adsorption, and chemisorption. In addition, we discuss the advantages/disadvantages, challenges, and prospects of MOFs in the separation of light hydrocarbon.

Cage Bismuth Metal-Organic Framework Materials Based on a Flexible Triazine-Polycarboxylic Acid: Subgram Synthesis, Application for Sensing, and White Light Tuning

Bismuth-based metal-organic frameworks (MOFs) have always attracted the attention of many researchers. Here, we first report a crystalline Bi-MOF (Bi-TDPAT) based on a flexible triazine-polycarboxylic linker 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine (H₆TDPAT) and bismuth nitrate; its crystallite quality is adequately good and the diffraction data can be collected directly by single crystal X-ray diffraction rather than 3D electron diffraction. The structure of Bi-TDPAT belongs to a novel topology type btt. Notably, the synthesis scale of Bi-TDPAT can be expanded, and sub-gram synthesis can be realized. At the same time, we synthesized a microcrystalline material Bi-TATAB utilizing 2,4,6-tris(4-carboxylphenylamino)-1,3,5-triazine (H₃TATAB). The structures of the two materials were characterized by several microanalysis tools. Considering that Bi-TDPAT is a blue light-emitting material with a broad emission peak, we prepared a white light emitting composite material Eu/Tb@Bi-TDPAT by encapsulating Eu(III)/Tb(III) in Bi-TDPAT. In addition, the fluorescence sensing functions of Bi-TDPAT and Bi-TATAB were explored. The results showed that they could detect and recognize various nitrophenols, and the optimal limit of detection is as low as 0.21 μM, which can be reused even after five cycles. Energy competitive absorption (CA) and photo-induced electron transfer are the main sensing mechanisms. By comparing and analyzing the properties of these two bismuth-based crystalline materials, we believe that this work also provides inspiration for the synthesis and development of bismuth-based MOF in the future.

Synthesis, Structure, and Properties of Complexes Based on 2,4-Bis-(triazol-1-yl)benzoic Acid as a Ligand

Using 2,4-bis-(triazol-1-yl)-benzoic acid as the main ligand and terephthalic acid (TPA) as the auxiliary ligand, combined with Cd(NO₃)₂·4H₂O and Zn(NO₃)₂·6H₂O, self-assembly under solvothermal conditions gave three novel complexes: [Cd_0.5(L)(H₂O)] (1), [Cd(L)(TPA)_0.5(H₂O)]·H₂O (2), and [Zn(L)(TPA)_0.5]·H₂O (3) (TPA = terephthalic acid). The crystal structure test showed that complex 1 belongs to the triclinic crystal system and the P1̅ space group and complexes 2 and 3 belong to the monoclinic crystal system and the P2₁/c space group. Solid-state fluorescence experiments show that complexes 1, 2, and 3 all have excellent optical properties: among them, complexes 1 and 3 can selectively detect MnO₄ ^- with low detection limits (0.96 μM and 0.232 μM, respectively) and complex 2 can detect Cr₂O₇ ^2- [limit of detection (LOD) = 0.035 μM], and the most interesting thing is that all three complexes can be used as sensors for detecting Fe³⁺ (LOD = 0.76 μM, 0.657 μM, and 0.11 μM, respectively). In addition, the detection capabilities of these three complexes for different amino acids and antibiotics were also analyzed, and the results showed that all three complexes can effectively detect tetracycline hydrochloride through the quenching effect and 2 and 3 can selectively detect tryptophan via the fluorescence enhancement effect.

Coordination-Induced Band Gap Reduction in a Metal-Organic Framework

Herein, we report on the synthesis of a microporous, three-dimensional phosphonate metal-organic framework (MOF) with the composition Cu3 (H5 -MTPPA)2  ⋅ 2 NMP (H8 -MTPPA=methane tetra-p-phenylphosphonic acid and NMP=N-methyl-2-pyrrolidone). This MOF, termed TUB1, has a unique one-dimensional inorganic building unit composed of square planar and distorted trigonal bipyramidal copper atoms. It possesses a (calculated) BET surface area of 766.2 m2 /g after removal of the solvents from the voids. The Tauc plot for TUB1 yields indirect and direct band gaps of 2.4 eV and 2.7 eV, respectively. DFT calculations reveal the existence of two spin-dependent gaps of 2.60 eV and 0.48 eV for the alpha and beta spins, respectively, with the lowest unoccupied crystal orbital for both gaps predominantly residing on the square planar copper atoms. The projected density of states suggests that the presence of the square planar copper atoms reduces the overall band gap of TUB1, as the beta-gap for the trigonal bipyramidal copper atoms is 3.72 eV.

New isoreticular phosphonate MOFs based on a tetratopic linker

The tetratopic linker 1,1,2,2-tetrakis(4-phosphonophenyl)ethylene (H₈TPPE) was used to synthesize the three new porous metal-organic frameworks of composition [M₂(H₂O)₂(H₂TPPE)]·xH₂O (M = Al³⁺, Ga³⁺, Fe³⁺), denoted as M-CAU-53 under hydrothermal reaction conditions, using the corresponding metal nitrates as starting materials. The crystal structures of the compounds were determined ab initio from powder X-ray diffraction data, revealing small structural differences. Proton conductivity measurements were carried out, indicating different conductivity mechanisms. The differences in proton conductivity could be linked to the individual structures. In addition, a thorough characterization via thermogravimetry, elemental analysis, IR-spectroscopy as well as N₂- and H₂O-sorption is given.

Oxalamide-Functionalized Metal Organic Frameworks for CO2 Adsorption

Metal-organic frameworks (MOFs) have received great attention in recent years as potential adsorbents for CO₂ capture due to their unique properties. However, the high cost and their tedious synthesis procedures impede their industrial application. A series of new CO₂-philic oxalamide-functionalized MOFs have been solvothermally synthesized: {[Zn₃(μ₈-OATA)_1.5(H₂O)₂(DMF)]·5/2H₂O·5DMF}_n (Zn-OATA), {[NH₂(CH₃)₂][Cd(μ₄-HOATA)]·H₂O·DMF}_n (Cd-OATA), and {[Co₂(μ₇-OATA)(H₂O)(DMF)₂]·2H₂O·3DMF}_n (Co-OATA) (H₄OATA = N,N'-bis(3,5-dicarboxyphenyl)oxalamide). In Zn-OATA, the [Zn₂(CO₂)₄] SBUs are connected by OATA^4- ligands into a 3D framework with 4-connected NbO topology. In Cd-OATA, two anionic frameworks with a dia topology interpenetrated each other to form a porous structure. In Co-OATA, [Co₂(CO₂)₄] units are linked by four OATA^4- to form a 3D framework with binodal 4,4-connected 4²·8⁴ PtS-type topology. Very interestingly, Cu-OATA can be prepared from Zn-OATA by a facile metal ions exchange procedure without damaging the structure while the CO₂ adsorption ability can be largely enhanced when Zn(II) metal ions are exchanged to Cu(II). These new MOFs possess channels decorated by the CO₂-philic oxalamide groups and accessible open metal sites, suitable for highly selective CO₂ adsorption. Cu-OATA exhibits a significant CO₂ adsorption capacity of 25.35 wt % (138.85 cm³/g) at 273 K and 9.84 wt % (50.08 cm³/g) at 298 K under 1 bar with isosteric heat of adsorption (Q_st) of about 25 kJ/mol. Cu-OATA presents a very high selectivity of 5.5 for CO₂/CH₄ and 43.8 for CO₂/N₂ separation at 0.1 bar, 298 K. Cd-OATA exhibits a CO₂ sorption isotherm with hysteresis that can be originated from structural rearrangements. Cd-OATA adsorbs CO₂ up to 11.90 wt % (60.58 cm³/g) at 273 K and 2.26 wt % (11.40 cm³/g) at 298 K under 1 bar. Moreover, these new MOFs exhibit high stability in various organic solvents, water, and acidic or basic media. The present work opens a new opportunity in the development of improved and cost-effective MOF adsorbents for highly efficient CO₂ capture.

Cobalt(II)-dianthracene Frameworks: Assembly, Exfoliation and Properties

Metal-organic frameworks containing responsive organic linkers are attractive for potential applications in sensors and molecular devices. Herein we report three cobalt(II) phosphonates incorporating responsive dianthracene linkers, namely, Co₂ (amp₂ H₂ )₂ (H₂ O)₄  ⋅ 6H₂ O (MDAF-1), Co₂ (amp₂ )(H₂ O)₄  ⋅ 2H₂ O (MDAF-2) and Co(amp₂ H₂ ) ⋅ 2H₂ O ⋅ 0.5DMF (MDAF-3), where amp₂ H₄ is pre-photodimerized 9-anthrylmethylphosphonic acid. MDAF-1 shows a layer structure in which dinuclear Co₂ (PO₃ H)₂ units are inter-connected by dianthracene ligands. In MDAF-2 and MDAF-3, inorganic chains of corner-sharing {CoO₄ } (or {CoO₆ }) and {PO₃ C} are cross-linked by dianthracene ligands into 3D frameworks. All compounds underwent thermo-induced phase transitions, first the de-solvation and then the de-dimerization of dianthracene (as well as the release of the remaining solvent molecules for MDAF-2 and -3), associated with magnetic changes. MDAF-1 can be exfoliated into single-layer nanosheets in water which show light-triggered luminescent changes.

Recent progresses in luminescent metal-organic frameworks (LMOFs) as sensors for the detection of anions and cations in aqueous solution

The discharge of excessive metal ions and anions into water bodies leads to the serious pollution of water and environment, which in turn has a certain impact on industry, agriculture, and human life. Because of the unique advantages of luminescent metal-organic frameworks (LMOFs), they have been successfully explored as various fluorescent probes to quickly and effectively detect these pollutants. This perspective not only introduces the design strategy and classification of LMOFs, especially the construction methods of water-stable LMOFs, but also reports the latest progresses in some LMOFs between 2016 and 2020 as well as expounds the mechanisms of LMOFs for detecting anions and cations. Moreover, the luminescence properties of LMOFs are related to the selection of metal ions, the structure of organic ligands, the pore size, and the interaction of guest molecules. Finally, the further development of LMOFs is summarized and prospected in this field.

Porphyrin and phthalocyanine-based metal organic frameworks beyond metal-carboxylates

Given the ubiquitous role of porphyrins in natural systems, these molecules and related derivatives such as phthalocyanines are fascinating building units to achieve functional porous materials. Porphyrin-based MOFs have been developed over the past three decades, yet chemically robust frameworks, necessary for applications, have been achieved much more recently and this field is expanding. This progress is partially driven by the development of porphyrins and phthalocyanines bearing alternative coordinating groups (phosphonate, azolates, phenolates…) that allowed moving the related MOFs beyond metal-carboxylates and achieving new topologies and properties. In this perspective article we first give a brief outline of the synthetic pathways towards simple porphyrins and phthalocyanines bearing these complexing groups. The related MOF compounds are then described; their structural and textural properties are discussed, as well as their stability and physical properties. An overview of the resulting nets and topologies is proposed, showing both the similarities with metal-carboxylate phases and the peculiarities related to the alternative coordinating groups. Eventually, the opportunities offered by this recent research topic, in terms of both synthesis pathways and modulation of pore size and shape, stability and physical properties, are discussed.

Synthesis and CO2 photoreduction of two 3d–4f heterometal–organic frameworks

Two 3d–4f heterometal–organic frameworks with similar structures were synthesized by a steam-assisted conversion method and exhibited high activity and selectivity for the photoreduction of CO2.

A Nanotubular Metal-Organic Framework with a Narrow Bandgap from Extended Conjugation*

A one-dimensional nanotubular metal-organic framework (MOF) [Ni(Cu-H4 TPPA)]⋅2 (CH3 )2 NH2 + (H8 TPPA=5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin) constructed by using the arylphosphonic acid H8 TPPA is reported. The structure of this MOF, known as GTUB-4, was solved by using single-crystal X-ray diffraction and its geometric accessible surface area was calculated to be 1102 m2  g-1 , making it the phosphonate MOF with the highest reported surface area. Due to the extended conjugation of its porphyrin core, GTUB-4 possesses narrow indirect and direct bandgaps (1.9 eV and 2.16 eV, respectively) in the semiconductor regime. Thermogravimetric analysis suggests that GTUB-4 is thermally stable up to 400 °C. Owing to its high surface area, low bandgap, and high thermal stability, GTUB-4 could find applications as electrodes in supercapacitors.

Crystal structure of 4,4′-bipyridin-1,1′-dium poly[bis(μ4-benzene-1,3,5-triyltris(hydrogen phosphonato-κ4 O:O′:O′′:O′′′))zinc(II)], C11H11NO9P3Zn

C11H11NO9P3Zn, monoclinic, P21/n (no. 14), a = 12.619(2) Å, b = 8.4948(12) Å, c = 13.954(2) Å, β = 90.588(3)°, V = 1495.7(4) Å3, Z = 4, R gt(F) = 0.0413, wR ref(F 2) = 0.0965, T = 120(2) K.

Semiconductive microporous hydrogen-bonded organophosphonic acid frameworks

Herein, we report a semiconductive, proton-conductive, microporous hydrogen-bonded organic framework (HOF) derived from phenylphosphonic acid and 5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin (GTUB5). The structure of GTUB5 was characterized using single crystal X-ray diffraction. A narrow band gap of 1.56 eV was extracted from a UV-Vis spectrum of pure GTUB5 crystals, in excellent agreement with the 1.65 eV band gap obtained from DFT calculations. The same band gap was also measured for GTUB5 in DMSO. The proton conductivity of GTUB5 was measured to be 3.00 × 10-6 S cm-1 at 75 °C and 75% relative humidity. The surface area was estimated to be 422 m2 g-1 from grand canonical Monte Carlo simulations. XRD showed that GTUB5 is thermally stable under relative humidities of up to 90% at 90 °C. These findings pave the way for a new family of organic, microporous, and semiconducting materials with high surface areas and high thermal stabilities.

Phosphonate Metal-Organic Frameworks: A Novel Family of Semiconductors

Herein, the first semiconducting and magnetic phosphonate metal-organic framework (MOF), TUB75, is reported, which contains a 1D inorganic building unit composed of a zigzag chain of corner-sharing copper dimers. The solid-state UV-vis spectrum of TUB75 reveals the existence of a narrow bandgap of 1.4 eV, which agrees well with the density functional theory (DFT)-calculated bandgap of 1.77 eV. Single-crystal conductivity measurements for different orientations of the individual crystals yield a range of conductances from 10^-3 to 10³ S m^-1 at room temperature, pointing to the directional nature of the electrical conductivity in TUB75. Magnetization measurements show that TUB75 is composed of antiferromagnetically coupled copper dimer chains. Due to their rich structural chemistry and exceptionally high thermal/chemical stabilities, phosphonate MOFs like TUB75 may open new vistas in engineerable electrodes for supercapacitors.

A novel three-dimensional zinc(II) coordination polymer based on 3,3'-{[1,3-phenylenebis(methylene)]bis(oxy)}dibenzoic acid and 1,4-bis(pyridin-4-yl)benzene: synthesis, crystal structure and photocatalytic properties

A novel three-dimensional (3D) Zn^II coordination polymer, namely, poly[[[1,4-bis(pyridin-4-yl)benzene](μ₃-3,3'-{[1,3-phenylenebis(methylene)]bis(oxy)}dibenzoato)zinc(II)] 1,4-bis(pyridin-4-yl)benzene], {[Zn(C₂₂H₁₆O₆)(C₁₆H₁₂N₂)]·C₁₆H₁₂N₂}_n or {[Zn(PMBD)(DPB)]·DPB}_n, 1, where H₂PMBD is 3,3'-{[1,3-phenylenebis(methylene)]bis(oxy)}dibenzoic acid and DPB is 1,4-bis(pyridin-4-yl)benzene, has been synthesized by self-assembly using zinc nitrate, a semi-rigid dicarboxylic acid and a nitrogen-containing ligand. The single-crystal X-ray structure determination indicates that 1 possesses an intriguing 3D architecture with a 4-connected uninodal cds topology, which is constructed from dinuclear {Zn₂} clusters and V-shaped PMBD^2- linkers. Compound 1 exhibits excellent photocatalytic activity on the degradation of the organic dyes Rhodamine B (RhB), Rhodamine 6G (Rh6G) and Methyl Red (MR).

Permanent porosity and role of sulfonate groups in coordination networks constructed from a new polyfunctional phosphonato-sulfonate linker molecule

The new linker molecule (H2O3PCH2)2N-CH2C6H4SO3H, (4-{[bis(phosphonomethyl)amino]methyl}benzene-sulfonic acid, H5L), bearing both phosphonic and sulfonic acid groups, was employed for the synthesis of new coordination polymers (CPs). Four new CPs of composition [Mg(H3L)(H2O)2]·H2O (1), [Mg2(HL)(H2O)6]·2H2O (2), [Ba(H3L)(H2O)]·H2O (3) and [Pb2(HL)]·H2O (4), were discovered using high-throughput methods and all structures were determined by single-crystal X-ray diffraction (SCXRD). With increasing ionic radius of the metal ion, an increase in coordination number from CN = 6 (Mg2+) to CN = 9 (Ba2+) and an increase in the dimensionality of the network from 1D to 3D is observed. This is reflected in the composition of the IBU and the number of metal ions that are connected by each linker molecule, i.e. from three in 1 to ten in 4. The connection of the IBUs leads to 1D and 2D structures in 1 and 2 with non-coordinating sulfonate groups, while 3 and 4 crystallise in MOF-type structures and coordination of the sulfonate groups is observed. The compounds exhibit thermal stabilities between 200 (2) and 345 °C (4) as proven by variable temperature powder X-ray diffraction (VT-PXRD) measurements. Title compound 4 contains micropores of 4 × 2 Å and reversible H2O uptake of 50 mg g-1 was demonstrated by vapour sorption measurements, making it the first porous metal phosphonatosulfonate. Detailed characterisation, i.e. CHNS and TG analysis as well as NMR and IR spectroscopy measurements confirm the phase purity of the title compounds.

Investigating the effect of positional isomerism on the assembly of zirconium phosphonates based on tritopic linkers

Combination of the novel linker 2,4,6-tris[3-(phosphonomethyl)phenyl]-1,3,5-triazine and Zr(iv) afforded a layered compound that lacks extended inorganic connectivity and displays good proton conductivity.

Integration of fluorescent probes into metal–organic frameworks for improved performances

Recent years have witnessed a rapid development of fluorescent probes in both analytical sensing and optical imaging. Enormous efforts have been devoted to the regulation of fluorescent probes during their development, such as improving accuracy, sensitivity, selectivity, recyclability and overcoming the aggregation-caused quenching effect. Metal–organic frameworks (MOFs) as a new class of crystalline porous materials possess abundant host–guest chemistry, based on which they display a great application potential in regulating fluorescent probes. This review summarized the research works on the regulation of fluorescent probes using MOFs, with emphasis on the methods of integrating fluorescent probes into MOFs, the regulation effects of MOFs on fluorescent probes, the superiorities of MOFs in regulating fluorescent probes, and the outlook of this subject. It is desirably hoped that this review can provide a useful reference for the researchers interested in this field.

This review surveyed the research works for the regulation of fluorescent probes with metal–organic frameworks based on host–guest chemistry.

A fast and highly selective Congo red adsorption material based on a cadmium-phosphonate network

This cadmium-phosphonate network exhibited rapid and efficient adsorption of Congo red dye, as well as excellent structural stability and adsorptive recyclability.

Flexible bifunctional monoethylphosphonate/carboxylates of Zn(ii) and Co(ii) reinforced with DABCO co-ligand: paradigmatic structural organization with topology

The prototypal [M₂(EtBCP)₂(DABCO)_0.5] MOFs, compliant with isoreticular expansion in two-dimensions, show flexibility manifested by a two-step CO₂ adsorption isotherm, which might be associated to the “lever-action” of the metal-phosphonate moieties.

Analysis of Electrocatalytic Metal-Organic Frameworks

The electrochemical analysis of molecular catalysts for the conversion of bulk feedstocks into energy-rich clean fuels has seen dramatic advances in the last decade. More recently, increased attention has focused on the characterization of metal-organic frameworks (MOFs) containing well-defined redox and catalytically active sites, with the overall goal to develop structurally stable materials that are industrially relevant for large-scale solar fuel syntheses. Successful electrochemical analysis of such materials draws heavily on well-established homogeneous techniques, yet the nature of solid materials presents additional challenges. In this tutorial-style review, we cover the basics of electrochemical analysis of electroactive MOFs, including considerations of bulk stability, methods of attaching MOFs to electrodes, interpreting fundamental electrochemical data, and finally electrocatalytic kinetic characterization. We conclude with a perspective of some of the prospects and challenges in the field of electrocatalytic MOFs.

Phosphonate Decomposition-Induced Polyoxomolybdate Dumbbell-Type Cluster Formation: Structural Analysis, Proton Conduction, and Catalytic Sulfoxide Reduction

The reaction of MoO₄^2- with a number of phosphonic acids [bis(phosphonomethyl)glycine, R,S-hydroxyphosphonoacetic acid, 1-hydroxyethane-1,1-diphosphonic acid, phenylphosphonic acid, aminotris(methylene phosphonic acid), and 1,2-ethylenediphosphonic acid] under oxidizing (H₂O₂) hydrothermal conditions at low pH leads to rupture of the P-C bond, release of orthophosphate ions, and generation of the octanuclear, phosphate-bridged, polyoxometalate molybdenum cluster (NH₄)₅[Mo₈(OH)₂O₂₄(μ₈-PO₄)](H₂O)₂ (POMPhos). This cluster has been fully characterized and its structure determined. It was studied as a proton conductor, giving moderate values of σ = 2.13 × 10^-5 S·cm^-1 (25 °C) and 1.17 × 10^-4 S·cm^-1 (80 °C) at 95% relative humidity, with E_a = 0.27 eV. The POMPhos cluster was then thermally treated at 310 °C, yielding (NH₄)_2.6(H₃O)_0.4(PO₄Mo₁₂O₃₆) together with an amorphous impurity containing phosphate and molybdenum oxide. This product was also studied for its proton conductivity properties, giving rise to an impressively high value of σ = 2.43 × 10^-3 S·cm^-1 (25 °C) and 6.67 × 10^-3 S·cm^-1 (80 °C) at 95% relative humidity, 2 orders of magnitude higher than those corresponding to the "as-synthesized" solid. The utilization of POMPhos in catalytic reduction of different sulfoxides was also evaluated. POMPhos acts as an efficient homogeneous catalyst for the reduction of diphenyl sulfoxide to diphenyl sulfide, as a model reaction. Pinacol was used as a low-cost, environmentally friendly, and highly efficient reducing agent. The effects of different reaction parameters were investigated, namely the type of solvent and reducing agent, presence of acid promoter, reaction time and temperature, loading of catalyst and pinacol, allowing to achieve up to 84-99% yields of sulfide products under optimized conditions. Substrate scope was tested on the examples of diaryl, alkylaryl, dibenzyl, and dialkyl sulfoxides and excellent product yields were obtained.

Alkali Phosphonate Metal-Organic Frameworks

A new family of porous metal-organic frameworks (MOFs), namely alkali phosphonate MOFs, is reported. [Na₂ Cu(H₄ TPPA)]⋅(NH₂ (CH₃ )₂ )₂ (GTUB-1) was synthesized using the tetratopic 5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin (H₈ -TPPA) linker with planar X-shaped geometrical core. GTUB-1 is composed of rectangular void channels with BET surface area of 697 m²  g^-1 . GTUB-1 exhibits exceptional thermal stability. The toxicity analysis of the (H₈ -TPPA) linker indicates that it is well tolerated by an intestinal cell line, suggesting its suitability for creating phosphonate MOFs for biological applications.

Platonic Relationships in Metal Phosphonate Chemistry: Ionic Metal Phosphonates

Phosphonate ligands demonstrate strong affinity for metal ions. However, there are several cases where the phosphonate is found non-coordinated to the metal ion. Such compounds could be characterized as salts, since the interactions involved are ionic and hydrogen bonding. In this paper we explore a number of such examples, using divalent metal ions (Mg2+, Ca2+, Sr2+ and Ni2+) and the phosphonic acids: p-aminobenzylphosphonic acid (H2PABPA), tetramethylenediamine-tetrakis(methylenephosphonic acid) (H8TDTMP), and 1,2-ethylenediphosphonic acid (H4EDPA). The compounds isolated and structurally characterized are [Mg(H2O)6]·[HPABPA]2·6H2O, [Ca(H2O)8]·[HPABPA]2, [Sr(H2O)8]·[HPABPA]2, [Mg(H2O)6]·[H6TDTMP], and [Ni(H2O)6]·[H2EDPA]·H2O. Also, the coordination polymer {[Ni(4,4’-bpy)(H2O)4]·[H2EDPA]·H2O}n was synthesized and characterized, which contains a bridging 4,4’-bipyridine (4,4’-bpy) ligand forming an infinite chain with the Ni2+ cations. All these compounds contain the phosphonate anion as the counterion to charge balance the cationic charge originating from the metal cation.

New Directions in Metal Phosphonate and Phosphinate Chemistry

In September 2018, the First European Workshop on Metal Phosphonates Chemistry brought together some prominent researchers in the field of metal phosphonates and phosphinates with the aim of discussing past and current research efforts and identifying future directions. The scope of this perspective article is to provide a critical overview of the topics discussed during the workshop, which are divided into two main areas: synthesis and characterisation, and applications. In terms of synthetic methods, there has been a push towards cleaner and more efficient approaches. This has led to the introduction of high-throughput synthesis and mechanochemical synthesis. The recent success of metal–organic frameworks has also promoted renewed interest in the synthesis of porous metal phosphonates and phosphinates. Regarding characterisation, the main advances are the development of electron diffraction as a tool for crystal structure determination and the deployment of in situ characterisation techniques, which have allowed for a better understanding of reaction pathways. In terms of applications, metal phosphonates have been found to be suitable materials for several purposes: they have been employed as heterogeneous catalysts for the synthesis of fine chemicals, as solid sorbents for gas separation, notably CO2 capture, as materials for electrochemical devices, such as fuel cells and rechargeable batteries, and as matrices for drug delivery.