Tuning Proton Conductivity in Alkali Metal Phosphonocarboxylates by Cation Size-Induced and Water-Facilitated Proton Transfer Pathways

The structural and functional chemistry of a family of alkali-metal ions with racemic <i>R</i>,<i>S</i>-hydroxyphosphonoacetate (<b>M-HPAA</b>; M = Li, Na, K, Cs) are reported. Crystal structures were determined by X-ray data (Li⁺, powder diffraction following an ab initio methodology; Na⁺, K⁺, Cs⁺, single crystal). A gradual increase in dimensionality directly proportional to the alkali ionic radius was observed. [Li₃(OOCCH­(OH)­PO₃)­(H₂O)₄]·H₂O (<b>Li-HPAA</b>) shows a 1D framework built up by Li-ligand “slabs” with Li⁺ in three different coordination environments (4-, 5-, and 6-coordinated). <b>Na-HPAA</b>, Na₂(OOCCH­(OH)­PO₃H)­(H₂O)₄, exhibits a pillared layered “house of cards” structure, while <b>K-HPAA</b>, K₂(OOCCH­(OH)­PO₃H)­(H₂O)₂, and <b>Cs-HPAA</b>, Cs­(HOOCCH­(OH)­PO₃H), typically present intricate 3D frameworks. Strong hydrogen-bonded networks are created even if no water is present, as is the case in <b>Cs-HPAA</b>. As a result, all compounds show proton conductivity in the range 3.5 × 10^–5 S cm^–1 (<b>Cs-HPAA</b>) to 5.6 × 10^–3 S cm^–1 (<b>Na-HPAA</b>) at 98% RH and <i>T</i> = 24 °C. Differences in proton conduction mechanisms, Grothuss (Na⁺ and Cs⁺) or vehicular (Li⁺ and K⁺), are attributed to the different roles played by water molecules and/or proton transfer pathways between phosphonate and carboxylate groups of the ligand HPAA. Upon slow crystallization, partial enrichment in the <i>S</i> enantiomer of the ligand is observed for <b>Na-HPAA</b>, while the <b>Cs-HPAA</b> is a chiral compound containing only the <i>S</i> enantiomer

Structural Variability in Multifunctional Metal Xylenediaminetetraphosphonate Hybrids

Two new families of divalent metal hybrid derivatives from the aromatic tetraphosphonic acids 1,4- and 1,3-<i>bis</i>(aminomethyl)­benzene-<i>N</i>,<i>N</i>′-<i>bis</i>(methylenephosphonic acid), (H₂O₃PCH₂)₂–N–CH₂C₆H₄CH₂–N­(CH₂PO₃H₂)₂ (designated herein as <b><i>p</i>-H₈L</b> and <b><i>m</i>-H₈L</b>) have been synthesized by crystallization at room temperature and hydrothermal conditions. The crystal structures of M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂(H₂O)₂]·2H₂O (M = Mg, Co, and Zn), <b>M–(<i>p</i>-H₆L)</b>, and M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂]·<i>n</i>H₂O (M = Ca, Mg, Co, and Zn and <i>n</i> = 1–1.5), <b><b>M–(<i>m</i>-H₆L)</b></b>, were solved ab initio by synchrotron powder diffraction data using the direct methods and subsequently refined using the Rietveld method. The crystal structure of the isostructural <b><b>M–(<i>p</i>-H₆L)</b></b> is constituted by organic–inorganic monodimensional chains where the phosphonate moiety acts as a bidentate chelating ligand bridging two metal octahedra. <b><b>M–(<i>m</i>-H₆L)</b></b> compounds exhibit a 3D pillared open-framework with small 1D channels filled with water molecules. These channels are formed by the pillaring action of the organic ligand connecting adjacent layers through the phosphonate oxygens. Thermogravimetric and X-ray thermodiffraction analyses of <b><b>M–(<i>p</i>-H₆L)</b></b> showed that the integrity of their crystalline structures is maintained up to 470 K, without significant reduction of water content, while thermal decomposition takes place above 580 K. The utility of <b><b>M–(<i>p</i>-H₆L)</b></b> (M = Mg and Zn) hybrid materials in corrosion protection was investigated in acidic aqueous solutions. In addition, the impedance data indicate both families of compounds display similar proton conductivities (σ ∼ 9.4 × 10^–5 S·cm^–1, at 98% RH and 297 K), although different proton transfer mechanisms are involved

Structural Variability in Multifunctional Metal Xylenediaminetetraphosphonate Hybrids

Two new families of divalent metal hybrid derivatives from the aromatic tetraphosphonic acids 1,4- and 1,3-<i>bis</i>(aminomethyl)­benzene-<i>N</i>,<i>N</i>′-<i>bis</i>(methylenephosphonic acid), (H₂O₃PCH₂)₂–N–CH₂C₆H₄CH₂–N­(CH₂PO₃H₂)₂ (designated herein as <b><i>p</i>-H₈L</b> and <b><i>m</i>-H₈L</b>) have been synthesized by crystallization at room temperature and hydrothermal conditions. The crystal structures of M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂(H₂O)₂]·2H₂O (M = Mg, Co, and Zn), <b>M–(<i>p</i>-H₆L)</b>, and M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂]·<i>n</i>H₂O (M = Ca, Mg, Co, and Zn and <i>n</i> = 1–1.5), <b><b>M–(<i>m</i>-H₆L)</b></b>, were solved ab initio by synchrotron powder diffraction data using the direct methods and subsequently refined using the Rietveld method. The crystal structure of the isostructural <b><b>M–(<i>p</i>-H₆L)</b></b> is constituted by organic–inorganic monodimensional chains where the phosphonate moiety acts as a bidentate chelating ligand bridging two metal octahedra. <b><b>M–(<i>m</i>-H₆L)</b></b> compounds exhibit a 3D pillared open-framework with small 1D channels filled with water molecules. These channels are formed by the pillaring action of the organic ligand connecting adjacent layers through the phosphonate oxygens. Thermogravimetric and X-ray thermodiffraction analyses of <b><b>M–(<i>p</i>-H₆L)</b></b> showed that the integrity of their crystalline structures is maintained up to 470 K, without significant reduction of water content, while thermal decomposition takes place above 580 K. The utility of <b><b>M–(<i>p</i>-H₆L)</b></b> (M = Mg and Zn) hybrid materials in corrosion protection was investigated in acidic aqueous solutions. In addition, the impedance data indicate both families of compounds display similar proton conductivities (σ ∼ 9.4 × 10^–5 S·cm^–1, at 98% RH and 297 K), although different proton transfer mechanisms are involved

Guest Molecule-Responsive Functional Calcium Phosphonate Frameworks for Tuned Proton Conductivity

We report the synthesis, structural characterization, and functionality (framework interconversions together with proton conductivity) of an open-framework hybrid that combines Ca²⁺ ions and the rigid polyfunctional ligand 5-(dihydroxyphosphoryl)­isophthalic acid (<b>PiPhtA</b>). Ca₂[(HO₃PC₆H₃COOH)₂]₂[(HO₃PC₆H₃(COO)₂H)­(H₂O)₂]·5H₂O (<b>Ca-PiPhtA-I</b>) is obtained by slow crystallization at ambient conditions from acidic (pH ≈ 3) aqueous solutions. It possesses a high water content (both Ca coordinated and in the lattice), and importantly, it exhibits water-filled 1D channels. At 75 °C, <b>Ca-PiPhtA-I</b> is partially dehydrated and exhibits a crystalline diffraction pattern that can be indexed in a monoclinic cell with parameters close to the pristine phase. Rietveld refinement was carried out for the sample heated at 75 °C, <b>Ca-PiPhtA-II</b>, using synchrotron powder X-ray diffraction data, which revealed the molecular formula Ca₂[(HO₃PC₆H₃COOH)₂]₂[(HO₃PC₆H₃(COO)₂H)­(H₂O)₂]. All connectivity modes of the “parent” <b>Ca-PiPhtA-I</b> framework are retained in <b>Ca-PiPhtA-II</b>. Upon <b>Ca-PiPhtA-I</b> exposure to ammonia vapors (28% aqueous NH₃) a new derivative is obtained (<b>Ca-PiPhtA-NH</b>_<b>3</b>) containing 7 NH₃ and 16 H₂O molecules according to elemental and thermal analyses. <b>Ca-PiPhtA-NH</b>_<b>3</b> exhibits a complex X-ray diffraction pattern with peaks at 15.3 and 13.0 Å that suggest partial breaking and transformation of the parent pillared structure. Although detailed structural identification of <b>Ca-PiPhtA-NH</b>_<b>3</b> was not possible, due in part to nonequilibrium adsorption conditions and the lack of crystallinity, FT-IR spectra and DTA-TG analysis indicate profound structural changes compared to the pristine <b>Ca-PiPhtA-I</b>. At 98% RH and <i>T</i> = 24 °C, proton conductivity, σ, for <b>Ca-PiPhtA-I</b> is 5.7 × 10^–4 S·cm^–1. It increases to 1.3 × 10^–3 S·cm^–1 upon activation by preheating the sample at 40 °C for 2 h followed by water equilibration at room temperature under controlled conditions. <b>Ca-PiPhtA-NH</b>_<b>3</b> exhibits the highest proton conductivity, 6.6 × 10^–3 S·cm^–1, measured at 98% RH and <i>T</i> = 24 °C. Activation energies (<i>E</i>_a) for proton transfer in the above-mentioned frameworks range between 0.23 and 0.4 eV, typical of a Grothuss mechanism of proton conduction. These results underline the importance of internal H-bonding networks that, in turn, determine conductivity properties of hybrid materials. It is highlighted that new proton transfer pathways may be created by means of cavity “derivatization” with selected guest molecules resulting in improved proton conductivity

High Proton Conductivity in a Flexible, Cross-Linked, Ultramicroporous Magnesium Tetraphosphonate Hybrid Framework

Multifunctional materials, especially those combining two or more properties of interest, are attracting immense attention due to their potential applications. MOFs, metal organic frameworks, can be regarded as multifunctional materials if they show another useful property in addition to the adsorption behavior. Here, we report a new multifunctional light hybrid, MgH₆ODTMP·2H₂O­(DMF)_0.5 (<b>1</b>), which has been synthesized using the tetraphosphonic acid H₈ODTMP, octamethylenediamine-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis­(methylenephosphonic acid), by high-throughput methodology. Its crystal structure, solved by Patterson-function direct methods from synchrotron powder X-ray diffraction, was characterized by a 3D pillared open framework containing cross-linked 1D channels filled with water and DMF. Upon H₂O and DMF removal and subsequent rehydration, MgH₆ODTMP·2H₂O (<b>2</b>) and MgH₆ODTMP·6H₂O (<b>3</b>) can be formed. These processes take place through crystalline–quasi-amorphous–crystalline transformations, during which the integrity of the framework is maintained. A water adsorption study, at constant temperature, showed that this magnesium tetraphosphonate hybrid reversibly equilibrates its lattice water content as a function of the water partial pressure. Combination of the structural study and gas adsorption characterization (N₂, CO₂, and CH₄) indicates an ultramicroporous framework. High-pressure CO₂ adsorption data are also reported. Finally, impedance data indicates that <b>3</b> has high proton conductivity σ = 1.6 × 10^–3 S cm^–1 at <i>T</i> = 292 K at ∼100% relative humidity with an activation energy of 0.31 eV

High Proton Conductivity in a Flexible, Cross-Linked, Ultramicroporous Magnesium Tetraphosphonate Hybrid Framework

Multifunctional materials, especially those combining two or more properties of interest, are attracting immense attention due to their potential applications. MOFs, metal organic frameworks, can be regarded as multifunctional materials if they show another useful property in addition to the adsorption behavior. Here, we report a new multifunctional light hybrid, MgH₆ODTMP·2H₂O­(DMF)_0.5 (<b>1</b>), which has been synthesized using the tetraphosphonic acid H₈ODTMP, octamethylenediamine-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis­(methylenephosphonic acid), by high-throughput methodology. Its crystal structure, solved by Patterson-function direct methods from synchrotron powder X-ray diffraction, was characterized by a 3D pillared open framework containing cross-linked 1D channels filled with water and DMF. Upon H₂O and DMF removal and subsequent rehydration, MgH₆ODTMP·2H₂O (<b>2</b>) and MgH₆ODTMP·6H₂O (<b>3</b>) can be formed. These processes take place through crystalline–quasi-amorphous–crystalline transformations, during which the integrity of the framework is maintained. A water adsorption study, at constant temperature, showed that this magnesium tetraphosphonate hybrid reversibly equilibrates its lattice water content as a function of the water partial pressure. Combination of the structural study and gas adsorption characterization (N₂, CO₂, and CH₄) indicates an ultramicroporous framework. High-pressure CO₂ adsorption data are also reported. Finally, impedance data indicates that <b>3</b> has high proton conductivity σ = 1.6 × 10^–3 S cm^–1 at <i>T</i> = 292 K at ∼100% relative humidity with an activation energy of 0.31 eV

Multifunctional Luminescent and Proton-Conducting Lanthanide Carboxyphosphonate Open-Framework Hybrids Exhibiting Crystalline-to-Amorphous-to-Crystalline Transformations

The chemistry of metal phosphonates has been progressing fast with the addition of new materials that possess novel structural features and new properties, occasionally in a cooperative manner. In this paper, we report a new family of functional lanthanide-carboxyphosphonate materials. Specifically, the lanthanide is La, Ce, Pr, Sm, Eu, Gd, Tb, or Dy and the carboxyphosphonate ligand is 2-hydroxyphosphonoacetic acid (H₃HPA). All reported LnHPA compounds, Ln₃(H_0.75O₃PCHOHCOO)₄·<i>x</i>H₂O (<i>x</i> = 15–16), crystallize in the orthorhombic system. Two types of structures were isolated: series I and II polymorphs. For both series, the three-dimensional (3D) open frameworks result from the linkage of similar organo-inorganic layers, in the <i>ac</i>-plane, by central lanthanide cations, which yield trimeric units also found in other metal-HPA hybrids. Large oval-shaped 1D channels are formed by the spatial separation of the layers along the <i>b</i>-axis and filled with lattice water molecules. LnHPA materials undergo remarkable crystalline-to-amorphous-to crystalline transformations upon dehydration and rehydration cycles, as confirmed by thermodiffraction and NMR spectroscopy. The highest proton conductivity was observed for GdHPA (series II), 3.2 × 10^–4 S cm^–1 at 98% RH and <i>T</i> = 21 °C. The dehydration–rehydration chemistry was also followed by photoluminescence spectroscopy. It was shown that loss and reuptake of water molecules are accompanied by clear changes in the photoluminescence spectra and lifetimes of the Eu analog (series II). Our present results reveal a wide family of well-characterized, multifunctional lanthanide-based phosphonate 3D-structured metal–organic frameworks (MOFs) that show reversible crystalline-to-amorphous-to-crystalline transformations and, at the same time, exhibit high proton conductivity

Multifunctional Luminescent and Proton-Conducting Lanthanide Carboxyphosphonate Open-Framework Hybrids Exhibiting Crystalline-to-Amorphous-to-Crystalline Transformations

The chemistry of metal phosphonates has been progressing fast with the addition of new materials that possess novel structural features and new properties, occasionally in a cooperative manner. In this paper, we report a new family of functional lanthanide-carboxyphosphonate materials. Specifically, the lanthanide is La, Ce, Pr, Sm, Eu, Gd, Tb, or Dy and the carboxyphosphonate ligand is 2-hydroxyphosphonoacetic acid (H₃HPA). All reported LnHPA compounds, Ln₃(H_0.75O₃PCHOHCOO)₄·<i>x</i>H₂O (<i>x</i> = 15–16), crystallize in the orthorhombic system. Two types of structures were isolated: series I and II polymorphs. For both series, the three-dimensional (3D) open frameworks result from the linkage of similar organo-inorganic layers, in the <i>ac</i>-plane, by central lanthanide cations, which yield trimeric units also found in other metal-HPA hybrids. Large oval-shaped 1D channels are formed by the spatial separation of the layers along the <i>b</i>-axis and filled with lattice water molecules. LnHPA materials undergo remarkable crystalline-to-amorphous-to crystalline transformations upon dehydration and rehydration cycles, as confirmed by thermodiffraction and NMR spectroscopy. The highest proton conductivity was observed for GdHPA (series II), 3.2 × 10^–4 S cm^–1 at 98% RH and <i>T</i> = 21 °C. The dehydration–rehydration chemistry was also followed by photoluminescence spectroscopy. It was shown that loss and reuptake of water molecules are accompanied by clear changes in the photoluminescence spectra and lifetimes of the Eu analog (series II). Our present results reveal a wide family of well-characterized, multifunctional lanthanide-based phosphonate 3D-structured metal–organic frameworks (MOFs) that show reversible crystalline-to-amorphous-to-crystalline transformations and, at the same time, exhibit high proton conductivity