Proton conductive metal phosphonate frameworks

Facile room-temperature synthesis of layered transition metal phosphonates via hitherto unknown alkali metal tert-butyl phosphonates

A facile room-temperature synthetic method is presented to produce alkali metal salts of tert-butyl phosphonic acid. The reaction between equimolar amounts of alkali metal carbonates and tert-butyl phosphonic acid in methanol results in the formation of [(tBuPO3H)Li(H2O)3·(H2O)] (1), [(tBuPO3)Na2(H2O)4]n (2), and [(tBuPO3H)K(H2O)]n (3). Solid-state structures of these compounds have been confirmed by single-crystal X-ray diffraction studies and further validated using numerous spectroscopic and analytical techniques. Compounds 1-3 are polymeric solids that are predominantly made up of a 1-D polymeric metal phosphonate chain. This synthetic approach leads to the formation of network structures/polymers in the solid state that otherwise are absent in solution due to the ionic nature of the interaction between the alkali metal ions and phosphonate anions. Apart from the multidentate nature of the phosphonate ligands, additional hydrogen bonding interactions involving water molecules, free P-OH groups, and PO moieties allow these chains to be propagated into 2-D sheets. We have further utilized the completely metalated sodium phosphonate 2 to synthesize layered metal phosphonates [(tBuPO3)Ca(H2O)]n (4), [(tBuPO3)Mn(H2O)]n (5) and [(tBuPO3)Co(H2O)]n (6) via a simple metathesis reaction.

Multidimensional Hybrid Metal Phosphonate Coordination Networks as Synergistic Anticorrosion Coatings

In the technologically important field of anticorrosion coatings, it is imperative to form well-defined and characterized films to protect the metal surface from corrosion. Phosphonate-based corrosion mitigation approaches are currently being exploited. Herein, the synergistic action of alkaline-earth metal ions and two carboxy-diphosphonates, PAIBA [N,N-bis(phosphonomethyl)-2-aminoisobutyric acid] and BPMGLY [N,N-bis(phosphonomethyl)glycine], is explored. Also, a family of four novel hybrid metal phosphonate materials is reported, Mg-PAIBA, Ca-PAIBA, Sr-PAIBA, and Sr-Na-PAIBA, whose topological analysis revealed a variety of underlying networks with the 6,10T9, unc, SP 1-periodic net (4,4)(0,2), and unique topologies. The synergistic metal/carboxy-diphosphonate blends were tested for their anticorrosion performance on carbon steel at preselected concentrations (0.1-1.0 mM) and pH values (4.0-6.0). The results showed an enhanced inhibitory performance in the presence of metal cations at higher concentrations. The inhibition of corrosion at pH 5.0 in the presence of BPMGLY, PAIBA, and their combination with Sr2+ was investigated in detail using electrochemical measurements. Enhanced inhibition was achieved with a 1:1 Sr2+/BPMGLY (or PAIBA) binary system. Polarization curves indicated that the system is a "mixed" inhibitor. This study widens the family of carboxyphosphonate coordination polymers, showing their potential as attractive hybrid coatings with anticorrosion performance.

Combined Effect of Hydrophilic Pore and the Type of Protons on Proton Conductivity in Porous Metal-Organic Frameworks: A Feasible Approach to Achieve a Super Proton Conductor under Hydrated Conditions

There has been a steady growth of interest in proton-conductive metal-organic frameworks (MOFs) due to their potential utility in proton-exchange membrane fuel cells. To attain a super proton conductivity (>1 × 10-2 S cm-1) in a MOF-based proton conductor is a key step toward practical application. Currently, most studies are focused on enhancing the proton conductivity of porous MOFs by controlling a single factor, such as the type of protons or hydrophilic pore or hydrogen bond. However, a limited contribution from a single factor cannot afford to remarkably increase the proton conductivity of the MOF and form a super proton conductor. Herein, we constructed two distinct porous MOFs, {(H3O+)4[Cu12(ci)12(OH)4(H2O)12]·3H2O·9DMF} (Cu-ci-3D, H2ci = 1H-indazole-5-carboxylic acid, DMF = N,N'-dimethylformamide) and {[Co(Hppca)2]·2HN(CH3)2·CH3OH·2H2O} (Co-ppca-2D, H2ppca = 5-(pyridin-3-yl)-1H-pyrazole-3-carboxylic acid), to tune their proton conductivities at high relative humidity (RH) using the combined effect of hydrophilic pore and the type of protons, ultimately achieving super proton conduction. Excitingly, Cu-ci-3D indeed harvests a super proton conductivity of 1.37 × 10-2 S cm-1 at 353 K and ∼97% RH, superior to some previously reported MOF-based proton conductors. The results present a unique perspective for developing high-performance MOF-based proton conductors and understanding their structure-performance relationships.

Comparative Study on the Proton Conduction Behaviors of Two Acidic Amphiphilic and Hydrophilic Coordination Compounds in Nafion Composite Membranes

The acidic amphiphilic compound H[Co(H2L1)(HL1)(phen)]·3H2O (H4(Co-L1), H3L1 = 5-(3', 5'-dicarboxylphenyl)-pyridine-2-carboxylic, phen = phenanthroline) and the hydrophilic compound [Ni(HL2)(H2O)5]·H2O (H(Ni-L2), H3L2 = 5-(3',5'-dicarboxylphenyl)-pyridine-3-carboxylic) were synthesized via hydrothermal reactions at acidic conditions. The acidity of H4(Co-L1) is stronger than of H(Ni-L2); while the hydrogen bond continuity in H4(Co-L1) extended monodirectionally, which is smaller compared to the three-directional extension observed in H(Ni-L2). The proton conduction behaviors of these two compounds as fillers of Nafion composite membranes have been investigated. The results indicate that the optimal doping amounts of H4(Co-L1) and H(Ni-L2) are 2 and 1%, respectively; the proton conductivities of H4(Co-L1)/Nafion-2 and H(Ni-L2)/Nafion-1 composite membranes are 0.243 and 0.212 S·cm-1, respectively, which are approximately 50.2 and 30.6% higher than that of pure Nafion membrane, respectively. A higher doping amount of H4(Co-L1) can be attributed to its hydrophobic phen ligand, which promotes compatibility with Nafion membrane and reduces aggregation. Hydrogen bond continuity has a more significant effect on proton conductivity than acidity at relatively low doping amounts; conversely, this relationship reverses at relatively high doping amounts.

Cu12-cluster-based metal-organic framework as a metastable intermediate in the formation of a layered copper phosphonate

The solvothermal reaction of CuSO4·5H2O and a chiral R-pempH2 ligand (molar ratio 6 : 1) first forms the metastable intermediate [Cu24(OH)20(R-pempH)8(SO4)10(H2O)10.5]·35H2O (1), followed by the formation of the stable phase [Cu2(OH)(R-pempH)(SO4)(H2O)]·H2O (2). Compound 1 displays a novel 3D open-framework structure containing Cu12 cluster nodes and sulfate links, which can be converted to the layered compound 2. We also investigated the photothermal effects of both compounds.

High H2O-Assisted Proton Conduction in One Highly Stable Sr(II)-Organic Framework Constructed by Tetrazole-Based Imidazole Dicarboxylic Acid

Solid electrolyte materials with high structural stability and excellent proton conductivity (σ) have long been a popular and challenging research topic in the fuel cell field. This problem can be addressed because of the crystalline metal-organic frameworks' (MOFs') high structural stability, adjustable framework composition, and dense H-bonded networks. Herein, one highly stable Sr(II) MOF, {[Sr(H2tmidc)2(H2O)3]·4H2O}n (1) (H3tmidc = 2-(1H-tetrazolium-1-methylene)-1H-imidazole-4,5-dicarboxylic acid) was successfully fabricated, which was structurally characterized by single-crystal X-ray diffraction and electrochemically examined by the AC impedance determination. The results demonstrated that the σ of the compound manifested a positive dependence on temperature and humidity, and the optimal proton conductivity is as high as 1.22 × 10-2 S/cm under 100 °C and 98% relative humidity, which is at the forefront of reported MOFs with ultrahigh σ. The analysis of the proton conduction mechanism reveals that numerous tetrazolium groups, carboxyl groups, coordination, and crystallization water molecules in the framework are responsible for the high efficiency of proton transport. This work offers a fresh perspective on how to create novel crystalline proton conductive materials.

Vitrification-enabled enhancement of proton conductivity in hydrogen-bonded organic frameworks

Hydrogen-bonded organic frameworks (HOFs) are versatile materials with potential applications in proton conduction. Traditional approaches involve incorporating humidity control to address grain boundary challenges for proton conduction. This study finds vitrification as an alternative strategy to eliminate grain boundary effect in HOFs by rapidly melt quenching the kinetically stable HOF-SXU-8 to glassy state HOF-g. Notably, a remarkable enhancement in proton conductivity without humidity was achieved after vitrification, from 1.31 × 10-7 S cm-1 to 5.62× 10-2 S cm-1 at 100 °C. Long term stability test showed negligible performance degradation, and even at 30 °C, the proton conductivity remained at high level of 1.2 × 10-2 S cm-1. Molecule dynamics (MD) simulations and X-ray total scattering experiments reveal the HOF-g system is consisted of three kinds of clusters, i.e., 1,5-Naphthalenedisulfonic acid (1,5-NSA) anion clusters, N,N-dimethylformamide (DMF) molecule clusters, and H+-H2O clusters. In which, the H+ plays an important role to bridge these clusters and the high conductivity is mainly related to the H+ on H3O+. These findings provide valuable insights for optimizing HOFs, enabling efficient proton conduction, and advancing energy conversion and storage devices.

A stable Mn(II) coordination polymer demonstrating proton conductivity and quantitative sensing of oxytetracycline in aquaculture

The construction and investigation of dual-functional coordination polymers (CPs) with proton conduction and luminescence sensing is of great significance in clean energy and agricultural monitoring fields. In this work, an Mn-based coordination polymer (Mn-CP), namely, [Mn0.5(HL)] (H2L = HOOCC6H4C6H4CH2PO(OH)OCH3), was hydrothermally synthesized. Mn-CP has a one-dimensional (1D) chain structure, in which uncoordinated -COOH groups can serve as potential sites for fluorescence sensing. Moreover, Mn-CP shows good water and pH stabilities, offering the feasibility for proton conduction and sensing applications. Mn-CP displays comparatively high proton conductivity of 1.07 × 10-4 S cm-1 at 368 K and 95% relative humidity (RH), which is promising for proton conduction materials. Moreover, it can serve as a repeatable, highly selective, and visualized fluorescence sensor for detecting oxytetracycline (OTC). More importantly, Mn-CP reveals an amazing quantitative sensing of OTC in actual samples such as seawater, aquaculture freshwater, soil infiltration solutions, and tap water. This work proves the excellent application potential of dual-functional CPs in the field of clean energy and environmental protection, especially for the fluorescence detection of antibiotics in aquaculture systems.

Mixed-valence compounds based on heterometallic-oxo-clusters containing Sb(III,V): crystal structures and proton conduction

Two isostructural Co(Cd)-antimony-oxo tartrate cluster-based compounds with a one-dimensional (1-D) belt-like structure, namely H9.2[Co(H2O)6]{M0.5(H2O)3.5{M'(H2O)4[SbVO6[Co4.2(H2O)5SbIII6(μ3-O)2(tta)6]]}}2·nH2O (M = Co, M' = Co, n = 9 (1); M = Cd0.39/Co0.61, M' = Cd0.24/Co0.76, n = 7 (2); H4tta = tartaric acid), have been synthesized by solvothermal methods. It is noteworthy that the relatively rare mixed-valence Sb(III,V) exists in the structures. The anionic clusters in both compounds appear to be in a sandwich configuration; the top and bottom layers are based on {Sb3(μ3-O)(tta)3} brackets, and the intermediate layer is occupied by {SbVO6[Co4.2(H2O)5]}. The title compounds have been characterized by single-crystal X-ray diffraction, powder X-ray diffraction, elemental analyses, thermogravimetric analyses, and UV-Vis spectroscopy. We chose compound 2 as a representative to test its proton conductivity, and the results show that the conductivity can reach 1.42 × 10-3 S cm-1 at 85 °C under 98% relative humidity.

Morphological control for high proton conduction in robust Co3O4-diethylmethylamine (metal-organic framework) membrane

Metal-organic framework (MOF) based proton conductors are synthesized by the Avrami model (time-temperature modalities). Our objective here is to obtain a material with high proton conductivity in anhydrous conditions, improved catalytic behaviour and morphology control of conductivity, band gap and catalysis. For this purpose, we try to understand the role of morphology on mass transportation using computational fluid dynamics and the experimental realisation using the synthesis of MOF membranes with high protonic conductivity. In order to alter the morphology, the membranes are synthesized from protic ionic liquid (dimethyl ethyl amine H2PO4) and metal ion (Co3O4) at different temperatures and duration. A high protonic conductivity of 0.0286 S cm-1 with a high transference number >0.99 is observed in anhydrous conditions with the change in morphology. Furthermore, catalyst properties along with high activity (Tafel slope = 39 mV decade-1) with the alteration in morphology are also investigated in detail and observed adsorption governed conduction. This adsorption governed conduction is verified using computational fluid dynamics simulations with the alteration in morphology. This study suggests that morphology not only plays a pivotal role in obtaining a robust proton exchange membrane, it also improves the catalytic functionality and stability of the membrane.

Identifying pathways to metal-organic framework collapse during solvent activation with molecular simulations

Metal-organic framework (MOF) materials are a vast family of nanoporous solids with potential applications ranging from drug delivery to environmental remediation. Application of MOFs in these scenarios is hindered, however, by difficulties in MOF 'activation' after initial synthesis - removal of the synthesis solvent from the pores to make the pore space accessible - often leading to framework collapse if improperly performed. While experimental studies have correlated collapse to specific solvent properties and conditions, the mechanism of activation-collapse is currently unknown. Developing this understanding would enable researchers to create better activation protocols for MOFs, accelerating discovery and process intensification. To achieve this goal, we simulated solvent removal using grand-canonical Monte Carlo and free energy perturbation methods. By framing activation as a fluid desorption problem, we investigated activation processes in the isoreticular metal organic framework (IRMOF) family of MOFs for different solvents. We identified two pathways for solvent activation - the solvent either desorbs uniformly from each individual pore or forms coexisting phases during desorption. These mesophases in turn lead to large capillary stresses within the framework, corroborating experimental hypotheses for the cause of activation-collapse. Finally, we found that the activation energy of solvent removal increased with pore size and connectivity due to the increased stability of solvent mesophases, matching experimental findings. Using these simulations, it is possible to screen MOF activation procedures, enabling rapid identification of ideal solvents and conditions and thus enabling faster development of MOFs for practical applications.

Photomodulation of Proton Conductivity by Nitro-Nitroso Transformation in a Metal-Organic Framework

The design of a highly and photomodulated proton conductor is important for advanced potential applications in chemical sensors and bioionic functions. In this work, a metal-organic framework (MOF; Gd-NO2) with high proton conductivity is synthesized with a photosensitive ligand of 5-nitroisophthalic acid (BDC-NO2), and it provides remote-control photomodulated proton-conducting behavior. The proton conduction of Gd-NO2 reaches 3.66 × 10-2 S cm-1 at 98% relative humidity (RH) and 25 °C, while it decreases by ∼400 times after irradiation with a 355 nm laser. The newly generated and disappearing FT-IR characteristic peaks reveal that this photomodulated process is realized by the photoinduced transformation from BDC-NO2 to 5-nitroso-isophthalic acid (BDC-NO). According to density functional theory, the smaller electronegativity of the -NO group, the longer distance of the hydrogen bond between BDC-NO and H2O molecules, and the lower water adsorption energy of BDC-NO indicate that the irradiated sample possesses a poorer hydrophilicity and has difficulty forming rich hydrogen-bonded networks, which results in the remarkable decrease of proton conductivity.

Modulation of defects in metal organic gels to enhance anhydrous proton conduction from subzero to moderate temperature

Exploitation of solid-state proton-conducting materials with high anhydrous proton conductivity from subzero temperature (<273 K) to moderate temperature (>353 K) is a great challenge. Here, Brönsted acid-dopped zirconium-organic xerogels (Zr/BTC-xerogels) are prepared for anhydrous proton conduction from subzero to moderate temperature. Abundant acid sites and strong H-bonding interactions make the CF3SO3H (TMSA)-introduced xerogel gain high proton conductivity from 9.0 × 10-4 S cm-1 (253 K) to 1.40 × 10-2 S cm-1 (363 K) under anhydrous conditions, which are in the leading level. This provides a new possibility to develop wide-operating-temperature conductors.

Synthesis Strategy Guided by Decision Tree for Morphology Control of Metal Phosphonates

The morphology control of metal phosphonates is always a difficulty because there are many challenges derived from the complexity of crystallization and the multivariable synthesis system. Responding to challenges, we propose a synthesis strategy guided by a decision tree for morphology control of metal phosphonates, through which directional design of the morphology-controlled synthesis can be realized. Specifically, any one synthetic condition involving the synthesis of metal phosphonates can be regarded as a decision problem to construct a binary decision tree. By means of the classification principle of the binary decision tree, the samples synthesized under the boundary value of each synthesis condition are classified based on crystal phase and morphology. The key synthetic conditions determining crystal phase and morphology can be precisely screened out to serve as decision nodes for the binary decision tree and are also rapidly optimized by the recursion level by level, whereas others cannot. Here, the β-polymorph of copper phenylphosphonate (β-CuPP) is selected as an example to elaborate the decision-tree-guided synthesis strategy for morphology control of metal phosphonates. From the constructed binary decision tree, it is clear that the right amount of methanol in the solvent is vital to obtain β-phase of CuPP, whereas the reactant concentration, pH value, and reaction time are important for morphology and phase transformation. Under the optimal synthetic conditions screened out by the binary decision tree, β-CuPP can thus be controlled to be hierarchically flower-like microsphere morphology through either the direct synthesis route or the solid-to-solid phase transformation route. This research work confirms that the decision-tree-guided synthesis is highly efficacious for the morphology control of metal phosphonates. Furthermore, the morphology-controlled synthesis guided by a decision tree may provide some valuable inspiration for morphology control of metal-organic frameworks (MOFs) and even coordinate compounds.

Mechanochemical Phosphorylation of Acetylides Using Condensed Phosphates: A Sustainable Route to Alkynyl Phosphonates

In pursuit of a more sustainable route to phosphorus-carbon (P-C) bond-containing chemicals, we herein report that phosphonates can be prepared by mechanochemical phosphorylation of acetylides using polyphosphates in a single step, redox-neutral process, bypassing white phosphorus (P4) and other high-energy, environmentally hazardous intermediates. Using sodium triphosphate (Na5P3O10) and acetylides, alkynyl phosphonates 1 can be isolated in yields of up to 32%, while reaction of sodium pyrophosphate (Na4P2O7) and sodium carbide (Na2C2) engendered, in an optimized yield of 63%, ethynyl phosphonate 2, an easily isolable compound that can be readily converted to useful organophosphorus chemicals. Highly condensed phosphates like Graham's salt and bioproduced polyphosphate were also found to be compatible after reducing the chain length by grinding with orthophosphate. These results demonstrate the possibility of accessing organophosphorus chemicals directly from condensed phosphates and may offer an opportunity to move toward a "greener" phosphorus industry.

Dual-Functional Eu-Metal-Organic Framework with Ratiometric Fluorescent Broad-Spectrum Sensing of Benzophenone-like Ultraviolet Filters and High Proton Conduction

The construction of attractive dual-functional lanthanide-based metal-organic frameworks (Ln-MOFs) with ratiometric fluorescent detection and proton conductivity is significant and challenging. Herein, a three-dimensional (3D) Eu-MOF, namely, [Eu4(HL)2(SBA)4(H2O)6]·9H2O, has been hydrothermally synthesized with a dual-ligand strategy, using (4-carboxypiperidyl)-N-methylenephosphonic acid (H3L = H2O3PCH2-NC5H9-COOH) and 4-sulfobenzoic acid monopotassium salt (KHSBA = KO3SC6H4COOH) as organic linkers. Eu-MOF showed ratiometric fluorescent broad-spectrum sensing of benzophenone-like ultraviolet filters (BP-like UVFs) with satisfactory sensitivity, selectivity, and low limits of detection in water/ethanol (1:1, v/v) solutions and real urine systems. A portable test paper was prepared for the convenience of actual detection. The potential sensing mechanisms were thoroughly analyzed by diversified experiments. The synergistic effect of the forbidden energy transfer from the ligand to Eu3+, the internal filtration effect (IFE), the formation of a complex, and weak interactions between the KHSBA ligand and BP-like UVFs is responsible for the ratiometric sensing effect. Meanwhile, Eu-MOF displayed relatively high proton conductivity of 2.60 × 10-4 S cm-1 at 368 K and 95% relative humidity (RH), making it a potential material for proton conduction. This work provides valuable guidance for the facile and effective design and construction of multifunctional Ln-MOFs with promising performance.

Syntheses and High Proton Conductivities of Two 3D Zr(IV)/Hf(IV)-MOFs from Furandicarboxylic Acid

With the gradual progress of research on proton-conducting metal-organic framework (MOFs), it has become a challenging task to find MOF materials that are easy to prepare and have low toxicity, high stability, and splendid proton conductivity. With the abovementioned objectives in mind, we selected the non-toxic organic ligand 2,5-furandicarboxylic acid and the low toxic quadrivalent metals zirconium(IV) or hafnium(IV) as starting materials and successfully obtained 2 three-dimensional porous MOFs, [M6O4(OH)4(FDC)4(OH)4(H2O)4] [M = ZrIV (1) and HfIV (2)], with ultrahigh water stability using a rapid and green synthesis approach. Their proton conductive ability is remarkable, thanks to the large number of Lewis acidic sites contained in their porous frameworks and the abundant H-bonding network, hydroxyl groups, as well as coordination and crystalline water molecules. The positive correlation of their proton conductivity with relative humidity (RH) and the temperature was observed. Notably, their optimized proton conductivities are 2.80 × 10-3 S·cm-1 of 1 and 3.38 × 10-3 S·cm-1 of 2 under 100 °C/98% RH, which are at the forefront of Zr(IV)/Hf(IV) MOFs with prominent proton conductivity. Logically, their framework features, nitrogen/water adsorption/desorption data, and activation energy values are integrated to deduce their proton conductivity and conducting mechanism differences.

Crystalline versus Amorphous: High-Performance Hafnium Phosphonate Framework for the Separation of Uranium and Transuranium Elements

Metal phosphonate frameworks (MPFs) consisting of tetravalent metal ions and aryl-phosphonate ligands feature a large affinity for actinides and excellent stabilities in harsh aqueous environments. However, it remains elusive how the crystallinity of MPFs influences their performance in actinide separation. To this end, we prepared a new category of porous, ultrastable MPF with different crystallinities for uranyl and transuranium separation. The results demonstrated that crystalline MPF was generally a better adsorbent for uranyl than the amorphous counterpart and ranked as the top-performing one for uranyl and plutonium in strong acidic solutions. A plausible uranyl sequestration mechanism was unveiled by using powder X-ray diffraction in tandem with vibrational spectroscopy, thermogravimetry, and elemental analysis.

Crystal structure and luminescence spectrum of a one-dimensional nickel(II) coordination polymer incorporating 1,4-bis-[(2-methyl-imidazol-1-yl)meth-yl]benzene and adamantane-1,3-di-carboxyl-ate co-ligands

An NiII coordination polymer, namely, poly[(μ2-adamantane-1,3-di-carboxyl-ato-κ4 O 1,O 1':O 3,O 3')[μ2-1,4-bis-(2-methyl-imidazol-1-ylmeth-yl)benzene-κ2 N 3:N 3']nickel(II)], [Ni(C12H14O4)(C16H18N4)]n or [Ni(adc)(bmib)]n, (I) [adc = adamantane-1,3-di-carboxyl-ate, C12H14O4 2- and bmib = 1,4-bis-(2-methyl-imidazol-1-ylmeth-yl)benzene, C16H18N4] was synthesized and characterized. It exhibits a one-dimensional extended structure built up from alternating [Ni2(bmib)2] 26-membered rings and [Ni2(adc)2] 16-membered rings. The nickel atom lies on a crystallographic twofold axis and both ligands are completed by mirror symmetry. The solid-state luminescence spectra of (I) and the bmib ligand show strong emissions at 442 and 410 nm, respectively.

High Proton Conductivity of Magnetically Ordered 2D Oxalate-Bridged [MnIICrIII] Coordination Polymers with Irregular Topology

Two heterometallic coordination polymers {[NH(CH₃)₂(C₂H₅)]₈[Mn₄Cl₄Cr₄(C₂O₄)₁₂]}_n (1) and {[NH(CH₃)-(C₂H₅)₂]₈[Mn₄Cl₄Cr₄(C₂O₄)₁₂]}_n (2) were obtained by slow evaporation of an aqueous solution containing the building block [A]₃[Cr(C₂O₄)₃] [A = (CH₃)₂(C₂H₅)NH⁺ or (CH₃)(C₂H₅)₂NH⁺] and MnCl₂·2H₂O. The isostructural compounds comprise irregular two-dimensional (2D) oxalate-bridged anionic layers [Mn₄Cl₄Cr₄(C₂O₄)₁₂]_n^8n- with a Shubnikov plane net fes topology designated as (4·8²), interleaved by the hydrogen-bonded templating cations (CH₃)₂(C₂H₅)NH⁺ (1) or (CH₃)(C₂H₅)₂NH⁺ (2). They exhibit remarkable humidity-sensing properties and very high proton conductivity at room temperature [1.60 × 10^-3 (Ω·cm)^-1 at 90% relative humidity (RH) of 1 and 9.6 × 10^-4 (Ω·cm)^-1 at 94% RH of 2]. The layered structure facilitates the uptake of water molecules, which contributes to the enhancement of proton conductivity at high RH. The better proton transport observed in 1 compared to that in 2 can be tentatively attributed to the higher hydrophilicity of the cations (CH₃)₂(C₂H₅)NH⁺, which is closely related to their affinity for water molecules. The original topology of the anionic networks in both compounds leads to the development of interesting magnetic phases upon cooling. The magnetically ordered ground state can be described as the coupling of ferromagnetic spin chains in which Mn²⁺ and Cr³⁺ ions are bridged by bis(bidentate) oxalate groups into antiferromagnetic planes through monodentate-bidentate oxalate bridges in the layers, which are triggered to long-range order below temperature 4.45 K via weaker interlayer interactions.

Two Stable Cd-MOFs as Dual-Functional Materials with Luminescent Sensing of Antibiotics and Proton Conduction

Construction and investigation of dual-functional metal-organic frameworks (MOFs) with luminescent sensing and proton conduction provide widespread applications in clean energy and environmental monitoring fields. By selecting a phosphonic acid ligand 4-pyridyl-CH₂N(CH₂PO₃H₂)₂ (H₄L) and coligand 2,2'-biimidazole (H₂biim), two cadmium-based MOFs [Cd_1.5(HL)(H₂biim)_0.5] (1) and (H₄biim)_0.5·[Cd₂(L)(H₂biim)Cl] (2) with different structures and properties have been hydrothermally synthesized by controlling reaction temperature. Based on the excellent thermal and chemical stabilities, and good luminescent stabilities in water solution, 1 and 2 can serve as luminescent sensors of chloramphenicol (CAP) with different quenching constant (K_SV) values and detection limits (LODs) in water, simulated environmental system, and real fish water system. Meanwhile, different sensing effects and possible sensing mechanisms are analyzed in detail. Moreover, 1 and 2 can also serve as good proton-conducting materials. The proton conductivities can reach up to 1.41 × 10^-4 S cm^-1 for 1 and 1.02 × 10^-3 S cm^-1 for 2 at 368 K and 95% relative humidity (RH). Among them, 2 shows better luminescent sensing and proton conduction performance than 1, which indicates that different crystal structures have a great impact on the properties of MOFs. Through the discussion of the relationship between structures and properties in detail, the possible reasons for the differences in properties are obtained, which can provide theoretical guidance for the rational design of this kind of dual-functional MOFs in the future.

New Type of Nanocomposite CsH2PO4-UiO-66 Electrolyte with High Proton Conductivity

New (1−x)CsH2PO4−xUiO-66 electrolytes with high proton conductivity and thermal stability at 230−250 °C were developed. The phase composition and proton conductivity of nanocomposites (x = 0−0.15) were investigated in detail. As shown, the UiO-66 matrix is thermally and chemically suitable for creating composites based on CsH2PO4. The CsH2PO4 crystal structure remains, but the degree of salt crystallinity changes in nanocomposites. As a result of interface interaction, dispersion, and partial salt amorphization, the proton conductivity of the composite increases by two orders of magnitude in the low-temperature range (up to 200 °C), depending on the UiO-66 fraction, and goes through a maximum. At higher temperatures, up to 250 °C, the conductivity of nanocomposites is close to the superprotonic values of the original salt at low UiO-66 values; then, it decreases linearly within one order of magnitude and drops sharply at x > 0.07. The stability of CsH2PO4-UiO-66 composites with high proton conductivity was shown. This creates prospects for their use as proton membranes in electrochemical devices.

An alternative C–P cross-coupling route for the synthesis of novel V-shaped aryldiphosphonic acids

The synthesis of phosphonate esters is a topic of interest for various fields, including the preparation of phosphonic acids to be employed as organic linkers for the construction of metal phosphonate materials. We report an alternative method that requires no solvent and involves a different order of addition of reactants to perform the transition-metal-catalyzed C–P cross-coupling reaction, often referred to as the Tavs reaction, employing NiCl2 as a pre-catalyst in the phosphonylation of aryl bromide substrates using triisopropyl phosphite. This new method was employed in the synthesis of three novel aryl diphosphonate esters which were subsequently transformed to phosphonic acids through silylation and hydrolysis.

Remarkable proton conducting behavior driven by synergistic effects in acid-base conjugated MOFs impregnated with sulphuric acid molecules

The Coulomb force between protons and negative charges usually leads to low proton diffusion and poor proton conductivity. Herein, acid-base conjugated MOFs of UiO-66-SO₃^--NH₃⁺ were constructed by linking sulfonic acid and amine groups to an organic skeleton. According to the XPS and elemental analysis, the ratio of 2-aminoterephthalic acid/2-sulfoterephthalic acid was ∼1.9 in UiO-66-SO₃^--NH₃⁺. After introducing sulphuric acid molecules, the MOF-based electrolyte exhibited a remarkable proton conductivity of 5.40 × 10^-1 S cm^-1 and low activation energy of 0.15 eV at 100% RH and 90 °C. This remarkable proton conducting behavior was generated by the synergistic system in the MOF, which contained certain synergistic effects such as between -SO₃^-⋯H⁺⋯SO₄^2- and NH₃⁺⋯SO₄^2-, therefore possessing a Grotthuss mechanism, which facilitates facile proton transport.

Photoresponsive proton conduction in Zr-based metal-organic frameworks using the photothermal effect

A highly stable and porous MOF [Zr₂(H₄TPPP)(OH/F)₂]·xH₂O (1) containing a metal-free porphyrin-phosphonate ligand is reported. It shows high proton conductivity of 1.2 × 10^-3 S·cm^-1 at 25 °C and 95% RH, a photothermal effect over a wide spectral range from UV-vis to NIR, and photo-enhanced and switchable proton conductivity.

Layered Uranyl Phosphonates Encapsulating Co(II)/Mn(II)/Zn(II) Ions: Exfoliation into Nanosheets and Its Impact on Magnetic and Luminescent Properties

Layered heterometallic 5f-3d uranyl phosphonates can exhibit unique luminescent and/or magnetic properties, but the fabrication and properties of their 2D counterparts have not been investigated. Herein we report three heterobimetallic uranyl phosphonates, namely, [(UO₂ )₃ M(2-pmbH)₄ (H₂ O)₄ ] ⋅ 2H₂ O [MU, M=Co(II), CoU; Mn(II), MnU; Zn(II), ZnU; 2-pmbH₃ =2-(phosphonomethyl)benzoic acid]. They are isostructural and display two-dimensional layered structures where the M(II) centers are encapsulated inside the windows generated by the diamagnetic uranyl phosphonate layer. Each M(II) has an octahedral geometry filled with four water molecules in the equatorial positions and two phosphonate oxygen atoms in the axial positions. The uranium atoms adopt UO₇ pentagonal bipyramidal and UO₆ square bipyramidal geometries. The lattice and coordination water molecules can be released by thermal treatment and reabsorbed in a reversible manner, accompanied with changes of magnetic dynamics. Interestingly, the bulk samples of MU can be exfoliated in acetone via freezing and thawing processes forming nanosheets with single-layer or two-layer thickness (MU-ns). Magnetic studies revealed that the CoU and MnU systems exhibited field-induced slow magnetization relaxation at low temperature. Compared with crystalline CoU, the magnetic relaxation of the CoU-ns aggregates is significantly accelerated. Moreover, photoluminescence measured at 77 K showed slight red-shift of the five characteristic uranyl emission bands for ZnU-ns in comparison with those of the crystalline ZnU. This work gives the first examples of 2D materials based on 5f-3d heterometallic uranyl phosphonates and illustrates the impact of dimension reduction on their magnetic/optical properties.

Insight into the High Proton Conductivity of One-/Two-Dimensional Cadmium Phosphites

The study describes the synthesis and structural attributes of two new cadmium phosphites, [Cd{OP(O)(OH)H}₂(4,4'-bipy)] (1) and [H₂pip][Cd(HPO₃)₂(H₂O)]·H₂O (2). The structure of 1 adopts a two-dimensional motif featuring alternate [Cd-μ₂-O]₂ and [Cd-O-P-O]₂-cyclic rings, while the inorganic chains are held together by 4,4'-bipyridine. The presence of strong hydrogen bonding interactions between the appended H₂PO₃ groups (O---O = 2.55 Å) provides a facile proton conduction pathway and results in a proton conductivity of 3.2 × 10^-3 S cm^-1 at 75 °C under 77% relative humidity (RH). Compound 2 comprises an anionic framework formed by vertex-shared [Cd-O-P-O]₂-cyclic rings, while the [H₂pip] cations between the adjacent chains assist a well-directed O-H---O hydrogen-bonded network between coordinated water, lattice water, and phospite groups. The bulk proton conductivity value under conditions as in 1 reaches 4.3 × 10^-1 S cm^-1. For both 1 and 2, the proton conductivity remains practically unchanged under ambient temperatures (25-35 °C), suggesting their potential in low-temperature fuel cells.

Organoamine Templated Multifunctional Hybrid Metal Phosphonate Frameworks: Promising Candidates for Tailoring Electrochemical Behaviors and Size-Selective Efficient Heterogeneous Lewis Acid Catalysis

The successful discovery of novel multifunctional metal phosphonate framework materials that incorporate newer organoamines and their utilization as a potential electroactive material for energy storage applications (supercapacitors) and as efficient heterogeneous catalysts are the most enduring challenges at present. From this perspective, herein, four new inorganic-organic hybrid zinc organodiphosphonate materials, namely, [C₅H₁₄N₂]₂[Zn₆(hedp)₄] (I), [C₅H₁₄N₂]_0.5[Zn₃(Hhedp) (hedp)]·2H₂O (II), [C₆H₁₆N₂][Zn₃(hedp)₂] (III), and [C₁₀H₂₄N₄][Zn₆(Hhedp)₂(hedp)₂] (IV) (H₄hedp = 1-hydroxyethane 1,1-diphosphonic acid), have been synthesized through the introduction of different organoamines and then structurally analyzed using various techniques. The compounds (I-IV) possess a three-dimensional network through alternate connectivity of zinc ions and diphosphonate ligands, as confirmed using single-crystal X-ray diffraction. The investigations of electrochemical charge storage behaviors of the present compounds indicate that compound III exhibits a high specific capacitance of 190 F g^-1 (76 C g^-1) at 1 A g^-1, while compound II shows an excellent cycling stability of 90.11% even after 5000 cycles at 5 A g^-1 in the 6 M KOH solution. Further, the present materials have also been utilized as active heterogeneous Lewis acid catalysts in the ketalization reaction. The screening of various substrate scopes during the catalytic process confirms the size-selective heterogeneous catalytic nature of the framework compounds. To our utmost knowledge, such a size-selective heterogeneous Lewis acid catalytic behavior has been observed for the first time in the amine templated inorganic-organic hybrid framework family. Moreover, the excellent size-selective catalytic efficiencies with the d¹⁰ metal system and recyclability performances make the compounds (I-IV) more efficient and promising Lewis acid heterogeneous catalysts.

Switched Proton Conduction in Metal-Organic Frameworks

Stimuli-responsive materials can respond to external effects, and proton transport is widespread and plays a key role in living systems, making stimuli-responsive proton transport in artificial materials of particular interest to researchers due to its desirable application prospects. On the basis of the rapid growth of proton-conducting porous metal-organic frameworks (MOFs), switched proton-conducting MOFs have also begun to attract attention. MOFs have advantages in crystallinity, porosity, functionalization, and structural designability, and they can facilitate the fabrication of novel switchable proton conductors and promote an understanding of the comprehensive mechanisms. In this Perspective, we highlight the current progress in the rational design and fabrication of stimuli-responsive proton-conducting MOFs and their applications. The dynamic structural change of proton transfer pathways and the role of trigger molecules are discussed to elucidate the stimuli-responsive mechanisms. Subsequently, we also discuss the challenges and propose new research opportunities for further development.

Hybrid Photochromic Lanthanide Phosphonate with Multiple Photoresponsive Functionalities

Hybrid photochromic materials (HPMs) with specific photoresponsive functionality have applications in many fields. The photoinduced electron-transfer (ET) strategy has been proved to be effective in the synthesis of HPMs with diverse photomodulated properties. The exploitation of new electron acceptors (EAs) is meaningful for promoting the development of HPMs. In this work, we introduced a rigid tetraimidazole derivative, 3,3,5,5-tetra(imidazol-1-yl)-1,1-biphenyl (TIBP) as a potential EA, into a metal-diphosphonate (1-hydroxyethylidene-1,1-diphosphonic acid, H₄-HEDP) system to explore HPMs and finally obtained a hybrid metal phosphonate (H₄-TIBP)_0.5·[Dy(H-HEDP) (H₂-HEDP)]·H₂O (1). 1 features anionic chains composed of diphosphonate and Dy³⁺ ions. The extra charge is balanced by protonated TIBP cations, which exist in the void of adjacent chains and form H-bonds with O_phosphonate (N-H···O). Upon photostimulation with a Xe lamp (300 W), the crystalline sample 1 exhibited coloration by changing from colorless to pale yellow because of the presence of photoinduced radicals that originated from the ET from O_phosphonate to N_TIBP. Along with the coloration, photomodulated fluorescence, magnetism, and proton conductivity were also detected in the photoactivated samples. Different from the reported HPMs based on polypyridine derivatives and photoactive species such as pyridinium and naphthalimide derivatives as EAs, our study provides a new category of EA units to yield HPMs with fascinating photoresponsive functionality via the assembly of polyimidazole derivatives and phosphonate-based supramolecular building blocks.

Phosphinate MOFs Formed from Tetratopic Ligands as Proton-Conductive Materials

Metal-organic frameworks (MOFs) are attracting attention as potential proton conductors. There are two main advantages of MOFs in this application: the possibility of rational design and tuning of the properties and clear conduction pathways given by their crystalline structure. We hereby present two new MOF structures, ICR-10 and ICR-11, based on tetratopic phosphinate ligands. The structures of both MOFs were determined by 3D electron diffraction. They both crystallize in the P3̅ space group and contain arrays of parallel linear pores lined with hydrophilic noncoordinated phosphinate groups. This, together with the adsorbed water molecules, facilitates proton transfer via the Grotthuss mechanism, leading to a proton conductivity of up to 4.26 × 10^-4 S cm^-1 for ICR-11. The presented study demonstrates the high potential of phosphinate MOFs for the fabrication of proton conductors.

The role of sulfonate groups and hydrogen bonding in the proton conductivity of two coordination networks

The proton conductivity of two coordination networks, [Mg(H₂O)₂(H₃L)]·H₂O and [Pb₂(HL)]·H₂O (H₅L = (H₂O₃PCH₂)₂-NCH₂-C₆H₄-SO₃H), is investigated by AC impedance spectroscopy. Both materials contain the same phosphonato-sulfonate linker molecule, but have clearly different crystal structures, which has a strong effect on proton conductivity. In the Mg-based coordination network, dangling sulfonate groups are part of an extended hydrogen bonding network, facilitating a "proton hopping" with low activation energy; the material shows a moderate proton conductivity. In the Pb-based metal-organic framework, in contrast, no extended hydrogen bonding occurs, as the sulfonate groups coordinate to Pb²⁺, without forming hydrogen bonds; the proton conductivity is much lower in this material.

Colodrero RM, Demadis KD. Exploiting the Multifunctionality of M2+/Imidazole-Etidronates for Proton Conductivity (Zn2+) and Electrocatalysis (Co2+, Ni2+) toward the HER, OER, and ORR

This work deals with the synthesis and characterization of one-dimensional (1D) imidazole-containing etidronates, [M2(ETID)(Im)3]·nH2O (M = Co2+ and Ni2+; n = 0, 1, 3) and [Zn2(ETID)2(H2O)2](Im)2, as well as the corresponding Co2+/Ni2+ solid solutions, to evaluate their properties as multipurpose materials for energy conversion processes. Depending on the water content, metal ions in the isostructural Co2+ and Ni2+ derivatives are octahedrally coordinated (n = 3) or consist of octahedral together with dimeric trigonal bipyramidal (n = 1) or square pyramidal (n = 0) environments. The imidazole molecule acts as a ligand (Co2+, Ni2+ derivatives) or charge-compensating protonated species (Zn2+ derivative). For the latter, the proton conductivity is determined to be ∼6 × 10-4 S·cm-1 at 80 °C and 95% relative humidity (RH). By pyrolyzing in 5%H2-Ar at 700-850 °C, core-shell electrocatalysts consisting of Co2+-, Ni2+-phosphides or Co2+/Ni2+-phosphide solid solution particles embedded in a N-doped carbon graphitic matrix are obtained, which exhibit improved catalytic performances compared to the non-N-doped carbon materials. Co2+ phosphides consist of CoP and Co2P in variable proportions according to the used precursor and pyrolytic conditions. However, the Ni2+ phosphide is composed of Ni2P exclusively at high temperatures. Exploration of the electrochemical activity of these metal phosphides toward the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) reveals that the anhydrous Co2(ETID)(Im)3 pyrolyzed at 800 °C (CoP/Co2P = 80/20 wt %) is the most active trifunctional electrocatalyst, with good integrated capabilities as an anode for overall water splitting (cell voltage of 1.61 V) and potential application in Zn-air batteries. This solid also displays a moderate activity for the HER with an overpotential of 156 mV and a Tafel slope of 79.7 mV·dec-1 in 0.5 M H2SO4. Ni2+- and Co2+/Ni2+-phosphide solid solutions show lower electrochemical performances, which are correlated with the formation of less active crystalline phases.

Superprotonic Conductivity of UiO-66 with Missing-Linker Defects in Aqua-Ammonia Vapor

The design and preparation of proton-conducting metal-organic frameworks (MOFs) with superconductivity are of significance for the proton-exchange membrane fuel cell (PEMFC). Introducing functional structural defects to enhance proton conductivity is a good approach. Here, we synthesized a series of UiO-66 (first synthesized in the University of Oslo) with missing-linker defects and investigated the effect of defect numbers on the proton conductivity of the samples. Among them, 60-UiO-66-1.8 (60 represents the synthesis temperature and 1.8 the number of defects) prepared with 3-mercaptopropionic acid as a modulator has the best proton conductivity, which is 3 × 10^-2 S cm^-1 at 100 °C and under 98% relative humidity (RH). The acidic sites induced by missing-linker defects further promote the chemisorption of ammonia molecules, resulting in the formation of a richer hydrogen-bond network and hence boosting the proton conductivity to 1.04 × 10^-1 S cm^-1 at 80 °C, which is one of the highest values among the reported MOF-based proton conductor. Therefore, this work provides a new strategy for enhancing proton conduction in MOF-based materials.

Proton Conductive Lanthanide-Based Metal-Organic Frameworks: Synthesis Strategies, Structural Features, and Recent Progress

In the fields of proton exchange membrane fuel cells as well as impedance recognition, molecular sieve, and biochemistry, the development of proton conductive materials is essential. The design and preparation of the next generation of proton conductive materials-crystalline metal-organic framework (MOF) materials with high proton conductivity and excellent water stability-are facing great challenges. Due to the large radius and high positive charge of lanthanides, they often interact with organic ligands to exhibit high coordination numbers and flexible coordination configurations, resulting in the higher stability of lanthanide-based MOFs (Ln-MOFs) than their transition metal analogues, especially regarding water stability. Therefore, Ln-MOFs have attracted considerable attention. This review offers a view of the latest progress of proton conductive Ln-MOFs, including synthesis strategy, structural characteristics, and advantages, proton conductivity, proton conductive mechanism, and applications. More importantly, by discussing structure-property relationships, we searched for and analyzed design techniques and directions of development of Ln-MOFs in the future. The latest progress of synthesis strategy, structural characteristics, proton conductive properties and mechanism and applications on Ln-MOFs. Ln-MOFS Lanthanide-based MOFs, MOF metal-organic framework, PEMFC proton exchange membrane fuel cells.