Tuning Proton Conductivity in A Multifunctional Calcium Phosphonate Hybrid Framework

Abstract

Multi-chelating phosphonate ligands are organic linkers alternative to polycarboxylates and other ligands providing synthetic access to a number of thermally- and chemically-stable MOFs [1]. Metal phosphonates are amenable for accessing suitable and highly conducting materials both by tuning structural features and post-synthesis treatment [2,3]. For instance, proton conductivity values in the order of 10−2 S cm–1 have been reported for several phosphonate-based compounds [4]. We report hereby the synthesis, structural characterization and proton conductivity of an open framework hybrid, Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H).(H2O)2].5H2O, that combines Ca2+ ions and the rigid polyfunctional ligand 5-(dihydroxyphosphoryl)isophthalic acid. This compound was obtained by slow crystallization at ambient conditions at pH 3. Its complex pillared layered structure, solved by single crystal X-ray analysis, contains hydrophilic 1D channels filled with both water and acidic phosphonate and carboxylate groups creating a hydrogen-bonded network. Partial removal of the lattice water at 75 °C causes a monoclinic structural distortion but still retaining the initial conductivity properties (5.7x10-4 Sxcm-1). Exposure of the sample to ammonia vapor from a concentrated aqueous solution led to major structural changes resulting in a new layered material containing seven NH3 and sixteen H2O molecules per formula. This solid exhibits enhanced conductivity, reaching 6.6x10-3 S.cm-1, as measured at 98 % RH and T = 24 °C. Activation energies were between 0.23 and 0.40 eV, typical of a Grothuss mechanism of proton conduction.