Sulfonic Group Functionalized Mixed Ligand Coordination Polymers: Synthesis, Characterization, Water Sorption, and Proton Conduction Studies

Modulation of Water Vapor Sorption by Pore Engineering in Isostructural Square Lattice Topology Coordination Networks

We report a crystal-engineering study conducted upon a platform of three mixed-linker square lattice (sql) coordination networks of general formula [Zn(Ria)(bphy)] [bphy = 1,2-bis(pyridin-4-yl)hydrazine, H2Ria = 5-position-substituted isophthalic acid, and R = -Br, -NO2, and -OH; compounds 1-3]. Analysis of single-crystal X-ray diffraction data of 1-2 and the simulated crystal structure of 3 revealed that 1-3 are isomorphous and sustained by bilayers of sql networks linked by hydrogen bonds. Although similar pore shapes and sizes exist in 1-3, distinct isotherm shapes (linear and S shape) and uptakes (2.4, 11.6, and 13.3 wt %, respectively) were observed. Ab initio calculations indicated that the distinct water sorption properties can be attributed to the R groups, which offer a range of hydrophilicity. Calculations indicated that the significantly lower experimental uptake in compound 1 can be attributed to a constricted channel. The calculated water-binding sites provide insights into how adsorbed water molecules bond to the pore walls, with the strongest interactions, water-hydroxyl hydrogen bonding, observed for 3. Overall, this study reveals how pore engineering can result in large variations in water sorption properties in an isomorphous family of rigid porous coordination networks.

Comparative Analysis of Proton Conductivity in Two Zn-Based MOFs Featuring Sulfate and Sulfonate Functional Groups

Metal-organic frameworks (MOFs) have shown promising potential as proton-conducting materials due to their tunable structures and high porosity. In this study, two novel MOFs had been successfully synthesized, one containing sulfate groups (MOF-1; [Zn4(TIPE)2(SO4)4(H2O)]·5H2O) and the other containing sulfonate groups (MOF-2; [Zn2(TIPE)(5-sip)(NO3)0.66]·0.34NO3·17.5H2O) (TIPE = 1,1,2,2-tetrakis(4-(1H-imidazole-1-yl)phenyl)ethene, H35-sip = 5-sulfoisophthalicacid), and the effect of the two groups on the proton conductivity of Zn-based MOFs had been investigated and compared for the first time. The proton conductivity of these MOFs was systematically measured at different temperatures and humidity conditions. Remarkably, the results revealed significant differences in proton conductivity between the two sets of MOFs. At 90 °C and 98% RH, MOF-1 and MOF-2 achieved optimal proton conductivity of 4.48 × 10-3 and 5.69 × 10-2 S·cm-1, respectively. This was due to the structural differences arising from the presence of different functional groups, which subsequently affected the porosity and hydrophilicity, thereby influencing the proton conductivity. Overall, this comparative study revealed the influence of sulfate and sulfonate groups on the proton conductivity of Zn-based MOFs. This research provided a feasible idea for the development of advanced MOF materials with enhanced proton conductivity and opened up new possibilities for their application in proton devices.

Polyelectrolyte membranes based on phosphorylated-PVA/cellulose acetate for direct methanol fuel cell applications: synthesis, instrumental characterization, and performance testing

Designing and synthesis of cost-effective and improved methanol permeable and proton conductive membranes are the main challenges for preparation of polymeric electrolyte membrane (PEM). Herein, a cost-effective PEM membrane based on phosphorylated polyvinyl alcohol (PVA)-grafted-cellulose acetate (CA) was prepared by a solution-casting technique. Water and methanol uptakes of phosphorylated PVA/CA membranes were characterized as function with the molar ratio of CA. Additionally, structure and morphology of phosphorylated PVA/CA (Ph-PVA/CA) membranes were verified by FT-IR analysis, SEM investigation. Furthermore, ion exchange capacity (IEC), proton conductivity and methanol permeation of Ph-PVA/CA membranes were examined based on the concentration of OPA basically. The results manifested a perceptible improvement in proton conductivity from 0.035 to 0.05 S/cm at 25 and 70 °C, respectively using 600 μL of OPA, and IEC of 2.1 meq/g using 400 μL of OPA at ambient temperature. On the other hand, methanol permeability (P = 1.08 × 10-10 cm2/s) was lower than Nafion 117 admirably. The optimum OPA concentration was 200 μL according to conductivity measurements (at 10% PVA, 150 μL GA, and CA 7%). Finally, prepared Ph-PVA/CA membranes exhibited enhancement in critical natures such as proton conductivity and IEC combined with its low-cost materials, which make them excellent candidate as PEM for DMFCs application.

Synthesis and In Situ Monitoring of Mechanochemical Preparation of Highly Proton Conductive Hydrogen-Bonded Metal Phosphonates

Crystalline porous materials are recognized as promising proton conductors for the proton exchange membrane (PEM) in fuel cell technology owing to their tunable framework structure. However, it is still a challenging bulk synthesis for real-world applications of these materials. Herein, we report the mechanochemical gram-scale synthesis of two isostructural metal hydrogen-bonded organic frameworks (MHOFs) of Co(II) and Ni(II) based on 1-hydroxyethylidenediphosphonic acid (HEDPH₄) with 2,2'-bipyridine (2,2'-bipy): Co(HEDPH₃)₂(2,2'-bipy)·H₂O (1) and Ni(HEDPH₃)₂(2,2'-bipy)·H₂O (2). In situ monitoring of the mechanochemical synthesis using different synchrotron-based techniques revealed a one-step mechanism - the starting materials are directly converted to the product. With the existence of extensive hydrogen bonds with amphiprotic uncoordinated phosphonate hydroxyl and oxygen atoms, both frameworks exhibited proton conduction in the range of 10^-4 S cm^-1 at room temperature under humid conditions. This study demonstrates the potential of green mechanosynthesis for bulk material preparation of framework-based solid-state proton conductors.

A Heniconuclear {Mn21} Cluster-Based Coordination Polymer with Manganese(II) Linkers Showing High Proton Conductivity

An unprecedented metal-linked cluster-based coordination polymer, composed of heniconuclear {Mn₂₁} clusters and Mn(II) ions as the nodes and linkers, respectively, was self-assembled from a facile aqueous synthesis. The structural analysis reveals that the compound possesses a rare 3D 8-connected hex framework topology. Significantly, the compound demonstrates a high proton conductivity of 1.06 mS cm^-1 at 373 K and 98% RH and exhibits a magnetocaloric effect with a magnetic entropy change of -9.94 J kg^-1 K^-1 at H = 80 kOe and T = 6.0 K.

Synthesis, Structure, and Catalytic Activities of Two Multi-Rh-Decorated Polyoxometalates

Two multi-Rh-incorporated polyoxometalates [NH₂(CH₃)₂]₁₀[Na₄(H₂O)₈]H₃[As₄W₄₂O₁₄₂(OH)₄(CH₃COO)₂Rh₃(H₂O)₄]·13H₂O·4[NH(CH₃)₂] (1) and [K₄Na(H₂O)₆]KH₁₀[As₄W₄₀O₁₄₀Rh₄(H₂O)₄]·34H₂O (2) have been synthesized in acetate buffer solution. Polyanion 1a is built up atop of an acetate-modified rectangular framework [As₄W₄₂O₁₄₂(OH)₄(CH₃COO)₂]^26-, while polyanion 2a contains a pure inorganic cryptand [As₄W₄₀O₁₄₀]^28-. All Rh atoms of these two compounds share the same hexa-coordinate distorted-octahedral geometry and are embedded into their cavities through As-Rh bonding with a bond length around 2.304(4)-2.436(5) Å. Besides, they not only represent structural novelty but also demonstrate controllable proton conduction properties. Catalysts 1 and 2 can catalyze cycloaddition of epoxides with CO₂ in a solvent-free system in conjunction with 1-ethyl-1-methylpyrrolidinium bromide.

Mechanochemical Synthesis of Phosphonate-Based Proton Conducting Metal-Organic Frameworks

Water-stable metal-organic frameworks (MOFs) with proton-conducting behavior have attracted great attention as promising materials for proton-exchange membrane fuel cells. Herein, we report the mechanochemical gram-scale synthesis of three new mixed-ligand phosphonate-based MOFs, {Co(H₂PhDPA)(4,4'-bipy)(H₂O)·2H₂O}_n (BAM-1), {Fe(H₂PhDPA)(4,4'-bipy) (H₂O)·2H₂O}_n (BAM-2), and {Cu(H₂PhDPA)(dpe)₂(H₂O)₂·2H₂O}_n (BAM-3) [where H₂PhDPA = phenylene diphosphonate, 4,4'-bipy = 4,4'-bipyridine, and dpe = 1,2-di(4-pyridyl)ethylene]. Single-crystal X-ray diffraction measurements revealed that BAM-1 and BAM-2 are isostructural and possess a three-dimensional (3D) network structure comprising one-dimensional (1D) channels filled with guest water molecules. Instead, BAM-3 displays a 1D network structure extended into a 3D supramolecular structure through hydrogen-bonding and π-π interactions. In all three structures, guest water molecules are interconnected with the uncoordinated acidic hydroxyl groups of the phosphonate moieties and coordinated water molecules by means of extended hydrogen-bonding interactions. BAM-1 and BAM-2 showed a gradual increase in proton conductivity with increasing temperature and reached 4.9 × 10^-5 and 4.4 × 10^-5 S cm^-1 at 90 °C and 98% relative humidity (RH). The highest proton conductivity recorded for BAM-3 was 1.4 × 10^-5 S cm^-1 at 50 °C and 98% RH. Upon further heating, BAM-3 undergoes dehydration followed by a phase transition to another crystalline form which largely affects its performance. All compounds exhibited a proton hopping (Grotthuss model) mechanism, as suggested by their low activation energy.

Proton-conducting metal–organic frameworks with linkers containing anthracenyl and sulfonate groups

Co(dia)1.5(Hsip)(H2O)·H2O (1) and Zn2(μ-OH)(dia)2(sip)·2H2O (2) were prepared from the same set of ligand precursors. They exhibited bnn and dia topologies, respectively. Factors that contributed to the higher proton conductivity of 1 were presented.

Dual-Functional Coordination Polymer with High Proton Conductivity and a Low-Detection-Limit Fluorescent Probe

A coordination polymer with dual functions of high proton conductivity and highly sensitive fluorescent sensors demonstrates a great application potential. In this work, a cadmium-based coordination polymer (denoted as CP 1) with hydrothermal stability was synthesized. The abundant coordination water, lattice water, and amino groups make an extended hydrogen-bonding pathway for efficient proton migration, which endows CP 1 with the highest proton conductivity of 2.41 × 10^-3 S·cm^-1 at 353 K and 98% RH. Especially, the proton conductivity of the chitosan (CS) hybrid membrane containing CP 1 reaches a maximum value of 2.62 × 10^-2 S·cm^-1 under 343 K and 98% RH, which increases almost 7 times higher than that of the pure CS membrane due to the host-guest collaboration. Furthermore, luminescence studies revealed that CP 1 is a high-sensitivity and good-selectivity fluorescent probe for the detection of trace amounts of l-histidine with a lowest detection limit of 1.0 × 10^-8 M.

Bent-bis(triazolyl)-based coordination polymers tuned by dicarboxylate ligands: syntheses, structures and properties

Seven new coordination polymers based on the bent 1,1'-(oxybis(1,4-phenylene))-bis(1H-1,2,4-triazole) ligand, with diverse structures and novel topologies, that are directed by the dicarboxylate ligands.

Structural, optical, dielectric and electrical transport properties of a [Mg(H2O)6]2+-templated proton conducting, semiconducting and photoresponsive 3D hydrogen bonded supramolecular framework

{[Co(2,5-Pdc)2,(H2O)2]2−·[Mg(H2O)6]2+·4(H2O)} (where 2,5-pdc = 2,5-pyridinedicarboxylate): a proton conducting semiconducting photoresponsive [Mg(H2O)6]2+ templated 3D hydrogen bonded supramolecular framework (HSF).

Auxiliary ligand-modulated trisimidazole-based coordination polymers: syntheses, structures and photoluminescence properties

Five coordination polymers having 1,3,5-tris(1-imidazolyl)benzene and the varied O-donor auxiliary ligands were designed and synthesized. Further, the auxiliary ligands modulated these complexes with structural diversities and novel topologies.

Proton-Conductive Coordination Polymers Based on Diphenylsulfone-3,3'-disulfo-4,4'-dicarboxylate with Well-Defined Hydrogen Bonding Networks

The diphenylsulfone-3,3'-disulfo-4,4'-dicarboxylic acid (H₄-DPSDSDC) ligand and its coordination polymers, [K₂Zn(C₁₄H₆S₃O₁₂)(H₂O)₄]_n (1) and {[Cu₃(μ₃-OH)₂(C₁₄H₆S₃O₁₂)(H₂O)₃(DMF)]·3(H₂O)}_n (2) (C₁₄H₆S₃O₁₂ = diphenylsulfone-3,3'-disulfo-4,4'-dicarboxylate), were synthesized. The Zn(H₂O)₄ units in 1 are connected by DPSDSDC^4- ligands to generate a one-dimensional (1D) chain, which is bridged by K-O bonds associated with bridging water molecules and sulfonate groups to yield a two-dimensional (2D) layer. In 2, the 1D hydroxyl-bridging Cu(II) chains are connected by DPSDSDC^4- ligands to give a 2D layer. The 2D layers in 1 and 2 are further connected by interlayered hydrogen bonds to give three-dimensional (3D) frameworks. Compounds 1 and 2 have good conductivities of 1.57 × 10^-4 and 5.32 × 10^-5 S cm^-1, respectively. Continuous well-defined hydrogen bonding networks associated with water molecules, sulfonate groups, and carboxylate groups were observed in compounds 1 and 2. Such hydrogen bonding networks provide hydrophilic domains and effective transfer pathways for protons. Here, we present elegant examples of a precise determination of the pathways for proton transport.

Two mixed-ligand coordination polymers based on 2,5-thiophenedicarboxylic acid and flexible N-donor ligands: the protective effect on periodontitis via reducing the release of IL-1β and TNF-α

Two novel mixed-ligand coordination polymers, {[Co(tdc)(btrp)]·0.67DMF}n (1) and {[Zn2(bimb)2(tdc)2]·2H2O}n (2) involving 2,5-thiophenedicarboxylate (H2tdc), and bitopic flexible N-donor ligands, 1,3-bis(1,2,4-triazol-1-yl)propane (btrp) and 1,4-bis((1H-benzo[d]imidazol-1-yl)methyl)benzene (bimb), have been synthesized by the hydrothermal method and characterized via IR, elemental analysis, thermal analysis, and powder X-ray diffraction. The biological functional studies were performed; the treatment activity of the compounds on periodontitis and the specific mechanism was explored. First, the real-time RT-PCR was carried out to determine the inflammatory genes nf-κb and p53 relative expression in periodontal mucosal cells after treating with compounds 1 and 2. Then, the level of the inflammatory cytokine in the gingival crevicular fluid after treating with compounds was also determined by the ELISA detection kit.

Proton conduction studies on four porous and nonporous coordination polymers with different acidities and water uptake

Acidity and water absorption ability are important influencing factors on proton conducting behavior, which are determined by the protonation degree and amount of hydrophilic groups in the crystal structures, respectively.

Proton conductivity studies on five isostructural MOFs with different acidity induced by metal cations

Five isostructural MOFs display very different proton conductivities despite the same proton transfer pathway. This difference is caused by the different coordination ability between the metal cations and the ligand.

Structural features of proton-conducting metal organic and covalent organic frameworks

Proton conductivity in MOFs and COFs have been attracted due to their applicability as electrolytes in proton exchange membrane fuel cells. A short overview with recent updates on the structural features of MOFs and COFs for proton conduction are presented here.

Syntheses, characterization and properties of three coordination polymers with interpenetrating structures comprising 4,4'-(1H-1,2,4-triazol-1-yl)methylene-bis(benzonic acid)

Three new coordination polymers (CPs), {[Pb(tmdb)](H₂O)} _n (1), {[Zn(tmdb)(bimb)_0.5]} _n (2) and {[Zn₃(tmdb)₃(bpmb)_1.5](H₂O)₆} _n (3) (H₂tmdb = 4,4'-(1H-1,2,4-triazol-1-yl)methylene-bis(benzonic acid), where bpmb = 1,4-bis(pyridin-4-ylmethoxy)benzene and bimb = 1,4-bis(imidazoly-1-yl)benzene), have been solvothermally or hydrothermally synthesized. Compound 1 is a 2D network with the point symbol (4·6·8)(4·6²) and compound 2 is a 4-fold interpenetrating 3D network with spiral chains. The topological type of 2 is dmc (topos&RCSR.ttd) with the point symbol (4·8²)(4·8⁵). Compound 3 is a 3-fold interpenetrating 3D network with the point symbol (6³)₂(8·6⁵)₂(10·6²)(8·10·6⁴). The electrochemiluminescence (ECL) behaviors of 2 and 3 were studied. The applications of CP 2 and 3 in detecting ions were explored, and the results show that they can be used as fluorescent probes to selectively detect and identify Fe³⁺ ions in water. In addition, the applications of CP 2 and 3 in the adsorption and separation of dyes were researched. Furthermore, the gas adsorption of 3 was studied.

Synthesis and Applications of Porous Organosulfonate-Based Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are an emerging class of porous crystalline materials attracting attention for their vast array of topologies as well as potential applications in gas storage, heterogeneous catalysis, and molecular sensing. In most cases, organocarboxylates (or corresponding carboxylic acids) are the most common building block, achieving well-defined metal-carboxylate coordination motifs in MOF structures. However, organosulfonates (or corresponding sulfonic acids) have been less well studied in MOF chemistry, probably owing to the weak coordination tendency of the sulfonate oxygens toward metal centers. This review summarizes the research on organosulfonate-based porous crystalline MOFs in recent years. The construction of most porous organosulfonate MOFs relies on using either a second N-donor ligand or carboxylate-sulfonate bifunctional ligands. Despite occupying more confined porosity than the carboxylate counterpart, the permanent porosity in organosulfonate MOFs is often highly polar and hydrophilic. Thus, organosulfonate MOFs often exhibit improved proton/Li⁺ conductivity as well as CO₂ affinity compared with their carboxylate-based counterparts. In addition, the application of organosulfonate MOFs in molecular sensing, molecular sieving, catalysis, and anion exchange are discussed in this review as well.

MOFs containing a linear bis-pyridyl-tris-amide and angular carboxylates: exploration of proton conductivity, water vapor and dye sorptions

Four new MOFs were shown to have appreciable proton conductivities, selective adsorption of water vapor over nitrogen and a tendency to selectively adsorb cationic dyes such as methylene blue and crystal violet.

High proton conductivity in a nickel(ii) complex and its hybrid membrane

A novel 2D nickel(ii) complex (1) has been successfully synthesized using a 2,2′-bipyridyl, polycarboxylsulfonate ligand H₄SBTC and Ni²⁺ ions. Owing to the presence of abundant water molecules, hydrogen bond networks and other protons, 1 and its hybrid membranes demonstrate high proton conductivity.

Water mediated proton conductance in a hydrogen-bonded Ni(ii)-bipyridine-glycoluril chloride self-assembled framework

Proton conducting properties have been investigated in a new Ni(ii)-based hydrogen-bonded porous framework synthesized using bipyridine-glycoluril (BPG) tecton.

Microporous mixed-metal mixed-ligand metal organic framework for selective CO2 capture

Layered-pillared framework of the CO₂-loaded MOF developed using this mixed-metal mixed-ligand approach showing the multiple-adsorption sites within the MOF.

Set of Multifunctional Azo Functionalized Semiconducting Cd(II)-MOFs Showing Photoswitching Property and Selective CO2 Adsorption

Syntheses, structural characterizations, photoluminescence, and adsorption properties of three new azo-functionalized Cd(II)-MOFs, namely, {[Cd(azbpy)(msuc)]·2.5(H₂O)}_n (2), {[Cd(azbpy)(mglu)]·5(H₂O)}_n (3), and {[Cd_1.5(azbpy)₂(glu)]·(NO₃)·MeOH}_n (4) [where msuc^2- = methylsuccinate; mglut^2- = methylglutarate; glut^2- = glutarate; azbpy = 4,4'-azobispyridine] have been reported. The compounds show different structures only with the variation of aliphatic dicarboxylates. The photoswitching behavior for the above-mentioned newly synthesized Cd(II)-MOFs along with one of our previously reported other azo-functionalized Cd(II)-MOF, namely, {[Cd(azbpy)(suc)]·2(H₂O)}_n (1), has been studied extensively. At photoilluminated condition, the conductivity values can draw a clear structure-property relationship among the structures of compounds 1-4. Single crystal structural analysis reveals that all the compounds exhibit a three-dimensional (3D) framework connected by azbpy linker and respective aliphatic dicarboxylate through their bis-chelating mono/bis oxo-bridging fashion. Compounds 1-3 exhibit an iso-structural honeycomb like 3D framework showing the same coordination environments, where the metal-carboxylate 2D sheets of compounds 1-3 are pillared by N,N'-donor azbpy linkers. On the other hand, compound 4 exhibits a 2-fold interpenetrated 3D framework with a little difference in its coordination environment and the pillaring of 1D metal-carboxylate ladder by azbpy linkers. All the compounds significantly demonstrate their enhanced sensitivity under light rather than the dark condition. The gas and solvent vapor sorption studies have been performed for the synthesized compounds 2-4. Moreover, compound 2 exhibits an enhanced type IV selective CO₂ adsorption isotherm over N₂ along with the appearance of gate opening phenomena in that.