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 (PiPhtA). Ca-2[(HO3PC6H3COOH)(2)](2)[(HO3PC6H3(COO)(2)H)(H2O)]center dot 5H(2)O (Ca-PiPh-tA-1) is obtained by slow crystallization at ambient conditions from acidic (pH approximate to 3) aqueous solutions. It possesses a high water content (both Ca coordinated and in the lattice), and importantly, it exhibits water-filled ID channels. At 75 degrees 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 degrees C, Ca-PiPhtA-II, using synchrotron powder X-ray diffraction data, which revealed the molecular formula Ca-2[(HO3PC6H3COOH)(2)](2)[(HO3PC6H3(COO)(2)H)(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 angstrom 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 degrees C, proton conductivity, a, for Ca-PiPhtA-I is 5.7 X 10(-4) S center dot cm(-1). It increases to 1.3 X 10(-3) S center dot cm(-1) upon activation by preheating the sample at 40 degrees C for 2 h followed by water equilibration at room temperature under controlled conditions. Ca-PiPhtA-NH3 exhibits the highest proton conductivity, 6.6 x 10(-3) S center dot cm(-1), measured at 98% RH and T = 24 degrees C. Activation energies (E-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.