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.

Proton conduction in solids attracts great interest, not only because of possible applications in fuel cell technologies, but also because of the main role of this process in many biological mechanisms. Metal−organic frameworks (MOFs) can exhibit exceptional proton-conduction performances, because of the large number of hydrogen-bonded water molecules embedded in their pores. However, further work remains to be done to elucidate the real conducting mechanism. Among the different MOF subfamilies, bioMOFs, which have been constructed using biomolecule derivatives as building blocks and often affording water-stable materials, emerge as valuable systems to study the transport mechanisms involved in the proton-hopping dynamics. Herein, we report a versatile chiral threedimensional (3D) bioMOF, exhibiting permanent porosity, as well as high chemical, structural, and water stability. Moreover, the choice of this suitable bioligand results in proton conductivity, and allows us to propose a proton-conducting mechanism based on experimental data, which are displayed visually by means of quantum molecular dynamics simulations.

The structural and functional chemistry of a family of alkali-metal ions with racemic R,Shydroxyphosphonoacetate (M-HPAA; M = Li, Na, K, Cs) are reported. Crystal structures were determined by X-ray powder diffraction (Li + ), following an ab initio methodology, or by single crystal (Na + , K + , Cs + ). A gradual increase in dimensionality directly proportional to the alkali ionic radius was observed. [Li 3 (ΟOCCH(OH)PO 3 )(H 2 O) 4 ]⋅H 2 O (Li-HPAA) shows a 1D framework built up by Li-ligand "slabs" with Li + in three different coordination environments (4-, 5-and 6-coordinated). Na-HPAA, Na 2 (ΟOCCH(OH)PO 3 H)(H 2 O) 4 , exhibits a pillared layered "house of cards" structure, while K-HPAA, K 2 (ΟOCCH(OH)PO 3 H)(H 2 O) 2 , and Cs-HPAA, Cs(HΟOCCH(OH)PO 3 H), typically present intricate 3D frameworks. Strong hydrogen bonded networks are created even if no water is present, as is the case in Cs-HPAA. As a result, all compounds show proton conductivity in the range 3.5⋅10 -5 S⋅cm -1 (Cs-HPAA) to 5.6⋅10 -3 S⋅cm -1 (Na-HPAA) at 98 % RH and T = 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 S enantiomer of the ligand is observed for Na-HPAA, while the Cs-HPAA is a chiral compound containing only the S enantiomer.

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.

Metal phosphonates containing acidic groups exhibit a wide range of proton conduction properties, which may enhance the performance of membrane electrode assemblies (MEAs). In this work, focus is placed on proton conduction properties of coordination polymers derived from the combination of lanthanide ions with a phosphonate derivative of taurine (2-[bis(phosphonomethyl)amino]ethanesulfonic acid, H 5 SP). High-throughput hydrothermal screening (140°C) was used to reach optimal synthesis conditions and access pure crystalline phases. Seven compounds with the composition Ln[H(O 3 PCH 2) 2 −NH− (CH 2) 2 −SO 3 ]•2H 2 O were isolated and characterized, which crystallize in two different structures, monoclinic m-LaH 2 SP and orthorhombic o-LnH 2 SP (Ln = Pr, Nd, Sm, Eu, Gd, and Tb), with unit cell volumes of ∼1200 and ∼2500 Å 3 , respectively. Their crystal structures, solved ab initio from X-ray powder diffraction data, correspond to different layered frameworks depending on the Ln 3+ cation size. In the orthorhombic series, o-LnH 2 SP, the sulfonate group is noncoordinated and points toward the interlayer space, while for m-LaH 2 SP, both phosphonate and sulfonate groups coordinate to the Ln 3+ centers. As a consequence, different H-bonding networks and proton transfer pathways are generated. Proton conductivity measurements have been carried out between 25 and 80°C at 70−95% relative humidity. The Sm 3+ derivative exhibits a conductivity of ∼1 × 10 −3 S•cm −1 and activation energy characteristic of a Grotthuss-type mechanism for proton transfer. Preliminary MEA assays indicate that these layered lanthanide sulfophosphonates assist in maintaining the proton conductivity of Nafion membranes at least up to 90°C and perform satisfactorily in single proton-exchange membrane fuel cells.