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). 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.