Properties and Applications of Metal Phosphates and Pyrophosphates as Proton Conductors.

We review the progress in metal phosphate structural chemistry focused on proton conductivity properties and applications. Attention is paid to structure-property relationships, which ultimately determine the potential use of metal phosphates and derivatives in devices relying on proton conduction. The origin of their conducting properties, including both intrinsic and extrinsic conductivity, is rationalized in terms of distinctive structural features and the presence of specific proton carriers or the factors involved in the formation of extended hydrogen-bond networks. To make the exposition of this large class of proton conductor materials more comprehensive, we group/combine metal phosphates by their metal oxidation state, starting with metal (IV) phosphates and pyrophosphates, considering historical rationales and taking into account the accumulated body of knowledge of these compounds. We highlight the main characteristics of super protonic CsH2PO4, its applicability, as well as the affordance of its composite derivatives. We finish by discussing relevant structure-conducting property correlations for divalent and trivalent metal phosphates. Overall, emphasis is placed on materials exhibiting outstanding properties for applications as electrolyte components or single electrolytes in Polymer Electrolyte Membrane Fuel Cells and Intermediate Temperature Fuel Cells.

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.

Structural and proton conductivity studies of fibrous π-Ti2O(PO4)2·2H2O: application in chitosan-based composite membranes.

Although the fibrous polymorphic modification of titanium phosphate, π-Ti2O(PO4)2·2H2O (π-TiP) has been known for decades, its crystal structure has remained unsolved. Herewith, we report the crystal structure of π-TiP at room temperature, as determined from synchrotron radiation powder X-ray diffraction, and corroborated by 31P solid state NMR and accurate density functional theory calculations. In contrast to the previously reported ρ-TiP polymorph, the as-synthesized hydrated phase crystallizes in the monoclinic system (P21/c, a = 5.1121(2) Å, b = 14.4921(9) Å, c = 12.0450(11), ß = 115.31(1)°, Z = 4), and is composed of corner-sharing titanium octahedra and phosphate units arranged in a pattern that is unique to the ρ-TiP polymorph. The unit cell was confirmed by electron diffraction, while the formation of planar packing imperfections and stacking faults along the [101] plane was revealed by HRTEM analysis. An in situ dehydration study of π-TiP, monitored by high-temperature powder X-ray diffraction, led to a new anhydrous monoclinic (P21/c, a = 5.1187(13) Å, b = 11.0600(21) Å, c = 14.4556(26), ß = 107.65(2)°, Z = 4) phase that crystallizes at 500 °C. The latter resembles the packing fashion of the parental π-TiP, albeit titanium atoms are present in both distorted tetrahedral and octahedral coordination environments. Anhydrous π-TiP was found to partially rehydrate at room temperature, reversibly adopting the structure of the initial phase. The studies carried out under different conditions of leaching and impregnation with H3PO4 showed that π-TiP exhibits an extrinsic proton conductivity (1.3 × 10-3 S cm-1 at 90 °C and 95% RH) due to the presence of the protonated phosphate species bound on the particles surface, as revealed by 31P MAS-NMR spectroscopy data. The composite membranes of Chitosan (CS) matrices filled with H3PO4-impregnated π-TiP solid show an increment of proton conductivity up to 4.5 × 10-3 S cm-1, at 80 °C and 95% RH, which is 1.8-fold higher than those of the bare CS membranes.

Synthesis and electrochemical properties of metal(ii)-carboxyethylphenylphosphinates.

We report herein the synthesis, structural characterization and electrocatalytic properties of three new coordination polymers, resulting from the combination of divalent metal (Ca2+, Cd2+ or Co2+) salts with (2-carboxyethyl)(phenyl)phosphinic acid. In addition to the usual hydrothermal procedure, the Co2+ derivative could also be prepared by microwave-assisted synthesis, in much shorter times. The crystal structures were solved by ab initio calculations, from powder diffraction data. Compounds MII[O2P(CH2CH2COOH)(C6H5)]2 {M = Cd (1) or Ca (2)} crystallize in the monoclinic system and display a layered topology, with the phenyl groups pointing toward the interlayer space in a interdigitated fashion. Compound Co2[(O2P(CH2CH2COO)(C6H5)(H2O)]2·2H2O (3) presents a 1D structure composed of zig-zag chains, formed by edge-sharing cobalt octahedra, with the phenyl groups pointing outside. Packing of these chains is favored by hydrogen bond interactions via lattice water molecules. In addition, H-bonds along the chains are established with the participation of the water molecules and the hydrophilic groups from the ligand. However, the solid exhibits a low proton conductivity, attributed to the isolation of the hydrophilic regions caused by the arrangement of hydrophobic phenyl groups. Preliminary studies on the electrocatalytic performance for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) have been conducted for compound 3 and its pyrolytic derivatives, which were previously thoroughly characterized. By comparison, another Co2+ phosphinate, 4, obtained by microwave-assisted synthesis, but with distinct stoichiometry and a known structure was also tested. For the OER, the best performance was achieved with a derivative of 3, prepared by heating this compound in N2 at 200 °C. This derivative showed overpotential (339 mV, at a current density of 10 mA cm-2) and Tafel slope (51.7 mV dec-1) values comparable to those of other Co2+ related materials.

Phase Transformation Dynamics in Sulfate-Loaded Lanthanide Triphosphonates. Proton Conductivity and Application as Fillers in PEMFCs.

Phase transformation dynamics and proton conduction properties are reported for cationic layer-featured coordination polymers derived from the combination of lanthanide ions (Ln3+) with nitrilo-tris(methylenephosphonic acid) (H6NMP) in the presence of sulfate ions. Two families of materials are isolated and structurally characterized, i.e., [Ln2(H4NMP)2(H2O)4](HSO4)2·nH2O (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb; n = 4-5, Series I) and [Ln(H5NMP)]SO4·2H2O (Ln = Pr, Nd, Eu, Gd, Tb; Series II). Eu/Tb bimetallic solid solutions are also prepared for photoluminescence studies. Members of families I and II display high proton conductivity (10-3 and 10-2 S·cm-1 at 80 °C and 95% relative humidity) and are studied as fillers for Nafion-based composite membranes in PEMFCs, under operating conditions. Composite membranes exhibit higher power and current densities than the pristine Nafion membrane working in the range of 70-90 °C and 100% relative humidity and with similar proton conductivity.

NH3/H2O-mediated proton conductivity and photocatalytic behaviour of Fe(ii)-hydroxyphosphonoacetate and M(ii)-substituted derivatives.

Synthesis redesign and derivatisation of Fe(ii)-hydroxyphosphonoacetate, incorporating different ammonia loads and M(ii) isomorphic substitutions (M = Mn, Co and Zn), have been implemented. The NH3 adsorption led to materials with enhanced proton conductivity, up to ∼10-3 S cm-1, although it caused a progressive amorphization. The Pair Distribution Function (PDF) analysis for this material confirmed the loss of crystallinity but the local order appeared to be maintained. The parent compound was shown to be an efficient photocatalyst for phenol, 4-chlorophenol and methylene blue even under mild conditions, with TOC removal of 75-90% at 50-150 min of reaction. The M(ii)-substituted derivatives displayed similar behaviour in properties, and therefore their multifunctional character, as the parent compound, although with slightly reduced capabilities.

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

The reaction of MoO42- 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 Ea = 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.

Luminescent and Proton Conducting Lanthanide Coordination Networks Based On a Zwitterionic Tripodal Triphosphonate.

The synthesis, structural characterization, luminescence properties, and proton conduction performance of a new family of isostructural cationic 2D layered compounds are reported. These have the general formula [Ln(H4NMP)(H2O)2]Cl·2H2O [Ln = La(3+), Pr(3+), Sm(3+), Eu(3+), Gd(3+), Tb(3+), Dy(3+), Ho(3+), H6NMP = nitrilotris(methylphosphonic acid)], and contain Cl(-) as the counterion. In the case of Ce(3+), a 1D derivative, [Ce2(H3NMP)2(H2O)4]·4.5H2O, isostructural with the known lanthanum compound has been isolated by simply crystallization at room temperature. The octa-coordinated environment of Ln(3+) in 2D compounds is composed by six oxygen atoms from three different ligands and two oxygens from each bound water. Two of the three phosphonate groups act as both chelating and bridging linkers, while the third phosphonate group acts solely as a bridging moiety. The materials are stable at low relative humidity at less at 170 °C. However, at high relative humidity transform to other chloride-free phases, including the 1D structure. The proton conductivity of the 1D materials varies in a wide range, the highest values corresponding to the La derivative (σ ≈ 2 × 10(-3) S·cm(-1) at RH 95% and 80 °C). A lower proton conductivity, 3 × 10(-4) S·cm(-1), was measured for [Gd(H4NMP)(H2O)2]Cl·2H2O at 80 °C, which remains stable under the work conditions used. Absorption and luminescence spectra were recorded for selected [Ln(H4NMP)(H2O)2]Cl·2H2O compounds. In all of them, the observed transitions are attributed solely to f-f transitions of the lanthanide ions present, as the H4NMP(2-) organic group has no measurable absorption or luminescence properties.

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 Ca(2+) ions and the rigid polyfunctional ligand 5-(dihydroxyphosphoryl)isophthalic acid (PiPhtA). Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H)(H2O)2]·5H2O (Ca-PiPhtA-I) 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, Ca-PiPhtA-I 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, Ca-PiPhtA-II, 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" Ca-PiPhtA-I framework are retained in Ca-PiPhtA-II. Upon Ca-PiPhtA-I exposure to ammonia vapors (28% aqueous NH3) a new derivative is obtained (Ca-PiPhtA-NH3) containing 7 NH3 and 16 H2O molecules according to elemental and thermal analyses. Ca-PiPhtA-NH3 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 Ca-PiPhtA-NH3 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 Ca-PiPhtA-I. At 98% RH and T = 24 °C, proton conductivity, σ, for Ca-PiPhtA-I 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. Ca-PiPhtA-NH3 exhibits the highest proton conductivity, 6.6 × 10(-3) S·cm(-1), measured at 98% RH and T = 24 °C. Activation energies (Ea) 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.

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-bis(aminomethyl)benzene-N,N'-bis(methylenephosphonic acid), (H2O3PCH2)2-N-CH2C6H4CH2-N(CH2PO3H2)2 (designated herein as p-H8L and m-H8L) 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), M-(p-H6L), and M[(HO3PCH2)2N(H)CH2C6H4CH2N(H)(CH2PO3H)2]·nH2O (M = Ca, Mg, Co, and Zn and n = 1-1.5), M-(m-H6L), 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 M-(p-H6L) is constituted by organic-inorganic monodimensional chains where the phosphonate moiety acts as a bidentate chelating ligand bridging two metal octahedra. M-(m-H6L) 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 M-(p-H6L) 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 M-(p-H6L) (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.

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, MgH(6)ODTMP·2H(2)O(DMF)(0.5) (1), which has been synthesized using the tetraphosphonic acid H(8)ODTMP, octamethylenediamine-N,N,N',N'-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 H(2)O and DMF removal and subsequent rehydration, MgH(6)ODTMP·2H(2)O (2) and MgH(6)ODTMP·6H(2)O (3) 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 (N(2), CO(2), and CH(4)) indicates an ultramicroporous framework. High-pressure CO(2) adsorption data are also reported. Finally, impedance data indicates that 3 has high proton conductivity σ = 1.6 × 10(-3) S cm(-1) at T = 292 K at ~100% relative humidity with an activation energy of 0.31 eV.

Multifunctional lanthanum tetraphosphonates: flexible, ultramicroporous and proton-conducting hybrid frameworks.

A new flexible ultramicroporous solid, La(H(5)DTMP)·7H(2)O (1), has been crystallized at room temperature using the tetraphosphonic acid H(8)DTMP, hexamethylenediamine-N,N,N',N'-tetrakis(methylenephosphonic acid). Its crystal structure, solved by synchrotron powder X-ray diffraction, is characterised by a 3D pillared open-framework containing 1D channels filled with water. Upon dehydration, a new related crystalline phase, La(H(5)DTMP) (2) is formed. Partial rehydration of 2 led to La(H(5)DTMP)·2H(2)O (3). These new phases contain highly corrugated layers showing different degrees of conformational flexibility of the long organic chain. The combination of the structural study and the gas adsorption characterization (N(2) and CO(2)) suggests an ultramicroporous flexible framework. NO isotherms are indicative of a strong irreversible adsorption of NO within the pores. Impedance data indicates that 1 is a proton-conductor with a conductivity of 8 × 10(-3) S cm(-1) at 297 K and 98% of relative humidity, and an activation energy of 0.25 eV.

Divalent metal vinylphosphonate layered materials: compositional variability, structural peculiarities, dehydration behavior, and photoluminescent properties.

A family of M-VP (M = Ni, Co, Cd, Mn, Zn, Fe, Cu, Pb; VP = vinylphosphonate) and M-PVP (M = Co, Cd; PVP = phenylvinylphosphonate) materials have been synthesized by hydrothermal methods and characterized by FT-IR, elemental analysis, and thermogravimetric analysis (TGA). Their structures were determined either by single crystal X-ray crystallography or from laboratory X-ray powder diffraction data. The crystal structure of some M-VP and M-PVP materials is two-dimensional (2D) layered, with the organic groups (vinyl or phenylvinyl) protruding into the interlamellar space. However, the Pb-VP and Cu-VP materials show dramatically different structural features. The porous, three-dimensional (3D) structure of Pb-VP contains the Pb center in a pentagonal pyramid. A Cu-VP variant of the common 2D layered structure shows a very peculiar structure. The structure of the material is 2D with the layers based upon three crystallographically distinct Cu atoms; an octahedrally coordinated Cu(2+) atom, a square planar Cu(2+) atom and a Cu(+) atom. The latter has an unusual co-ordination environment as it is 3-coordinated to two oxygen atoms with the third bond across the double bond of the vinyl group. Metal-coordinated water loss was studied by TGA and thermodiffractometry. The rehydration of the anhydrous phases to give the initial phase takes place rapidly for Cd-PVP but it takes several days for Co-PVP. The M-VP materials exhibit variable dehydration-rehydration behavior, with most of them losing crystallinity during the process.

Structural mapping and framework interconversions in 1D, 2D, and 3D divalent metal R,S-hydroxyphosphonoacetate hybrids.

Reactions of divalent cations (Mg(2+), Co(2+), Ni(2+), and Zn(2+)) with R,S-hydroxyphosphonoacetic acid (HPAA) in aqueous solutions (pH values ranging 1.0-4.0) yielded a range of crystalline hydrated M-HPAA hybrids. One-dimensional (1D) chain compounds were formed at room temperature whereas reactions conducted under hydrothermal conditions resulted in two-dimensional (2D) layered frameworks or, in some cases, three-dimensional (3D) networks incorporating various alkaline cations. 1D phases with compositions [M{HO(3)PCH(OH)CO(2)}(H(2)O)(2)].2H(2)O (M = Mg, Co, and Zn) were isolated. These compounds were dehydrated in liquid water to yield the corresponding [M{HO(3)PCH(OH)CO(2)}(H(2)O)(2)] compounds lacking the lattice water between the 1D chains. [M{HO(3)PCH(OH)CO(2)}(H(2)O)(2)] (M = Mg, Ni, Co, Zn) compounds were formed by crystallization at room temperature (at higher pH values) or also by partial dehydration of 1D compounds with higher hydration degrees. Complete dehydration of these 1D solids at 240-270 degrees C led to 3D phases, [M{HO3PCH(OH)CO(2)}]. The 2D layered compound [Mg{HO(3)PCH(OH)CO(2)}(H(2)O)(2)] was obtained under hydrothermal conditions. For both synthesis methods, addition of alkali metal hydroxides to adjust the pH usually led to mixed phase materials, whereas direct reactions between the metal oxides and the hydroxyphosphonoacetic acid gave single phase materials. On the other hand, adjusting the pH with acetate salts and increasing the ratio M(2+)/HPAA and/or the A(+)/M(2+) ratio (A = Na, K) resulted in 3D networks, where the alkali cations were incorporated within the frameworks for charge compensation. The crystal structures of eight new M(II)-HPAA hybrids are reported herein and the thermal behavior related to dehydration/rehydration of some compounds are studied in detail.

"Breathing" in adsorbate-responsive metal tetraphosphonate hybrid materials.

The structures of various layered calcium tetraphosphonates (CaH6DTMP; H8DTMP=hexamethylenediamine tetrakis(methylenephosphonic acid)), have been determined. Starting from CaH6DTMP.2H2O, thermal treatment and subsequent exposure to NH3 and/or H2O vapors led to four new compounds that showed high storage capacity of guest species between the layers (up to ten H2O/NH3 molecules) and a maximum volume increase of 55 %. The basic building block for these phosphonates consists of an eight-membered ring chelating Ca2+ through two phoshonate groups, and the organic ligand is located within the layers, which are held together by hydrogen bonds. The structural analysis revealed that the uptake/removal of guest species (H2O and NH3) induces significant changes in the framework not only by changing the interlayer distances but also through important conformational changes of the organic ligand. An anisotropic breathing motion could be quantified by the changes of the unit-cell dimensions and ligand arrangements in four crystalline derivatives. Complete characterization revealed the existence of interconversion reactions between the different phases upon gas uptake and release. The observed behavior represents, to the best of our knowledge, the first example of a breathing-like mechanism in metal phosphonates that possess a 2D topology.

Layered microporous tin(IV) bisphosphonates.

This article reports the hydrothermal synthesis and characterization of two new series of porous tin(IV) phosphonophenoxyphenylphosphonates with controlled pore size distributions, using as precursor the 4-(4'-phosphonophenoxy)phenyl phosphonic acid, [H2O3P-C6H4]2-O. Supermicroporous solids (S(BET), 300-400 m2 g(-1)) were obtained employing n-alcohol (C1-C6)-water mixtures (solvents ratio 1 : 1), in the presence of hydrofluoric acid. X-Ray powder diffraction shows that these compounds are semi-crystalline and the local environments around the phosphorus and tin elements have been studied by 31P and 119Sn MAS-NMR spectroscopy, respectively. The microstructure (particle sizes and shapes) of these phosphonates has been analyzed by scanning and transmission electron microscopy. This study shows that the microstructures of single-ligand (for instance tin(IV) phenylphosphonate) and cross-linked tin(IV) bisphosphonates are different. Tin(IV) phenylphosphonate crystallizes as micron-sized spheres, theta approximately 1-2 microm, formed by the aggregation of nanospheres, whereas tin(IV) bisphosphonates crystallize as microparticles larger than 20 microm. The textural properties of these porous solids were characterized by N2 and CO2 sorption isotherms. The key result of this work is that maxima of pore size distributions smoothly shift from 12 to 16 angstroms upon increasing the chain length of the alcohol. The microporosity of tin(IV) bisphosphonates is compatible with a double role played by the phosphonate groups acting as a pillar between adjacent layers and as a component of the hybrid organic-inorganic layers.

Layered and pillared metal carboxyethylphosphonate hybrid compounds.

A series of carboxyethylphosphonate hybrid materials has been prepared: Mn(II)(O3PCH2CH2COOH) *H2O (1), Mn(III)(OH)(O3PCH2CH2COOH)*H2O (2), Al3(III)(OH)3(O3PCH2CH2CO2)2 *3H2O (3) and Cr2(III)(OH)3(O3PCH2CH2CO2) *3H2O (4). Compounds 1 and 2 were synthesized from Mn(III)(CH3COO)3 *2H2O under hydrothermal, or refluxing treatments, respectively. The crystal structures of the manganese-bearing solids have been solved ab initio from laboratory X-ray powder diffraction data and refined by the Rietveld method. 1 crystallises in a orthorhombic cell and 2 in monoclinic symmetry. Both solids have inorganic 2D layered structures with the acid carboxylic groups pointing towards the interlayer space, and the layers linked only through hydrogen bonds. The inorganic layers of these compounds are formed by manganese atoms in distorted octahedral environments linked together by the phosphonate groups. The crystal structure of 3 has been solved ab initio from synchrotron X-ray powder diffraction data. This solid shows a pillared structure with the phosphonate and carboxylate groups cross-linking the inorganic layers. These layers contain chains of aluminium octahedra running parallel to each other. 4 is amorphous and the IR-UV-VIS spectra suggest a framework with Cr(III) cations in octahedral environments. Thermal, spectroscopic and magnetic data for manganese and chromium compounds as well as the structural details of these solids are discussed.