Studies on the Genesis and Proton Conductivity of Imidazole-Based Linear and Open-Framework Zinc Phosphites

Three new zinc phosphites, [HIm]₂[Zn₃(HPO₃)₄] (1), [Zn₂(HPO₃)₂Im₂] (2), and [Zn(HPO₃)Im] (3) (Im = imidazole), have been synthesized from the hydro/solvothermal reaction of zinc acetate, dimethyl phosphite, and imidazole by varying the temperature and solvent of the reaction medium. The structure of 1 is built from vertex-sharing of four HPO₃-capped Zn₃P₃ units and adopts an open framework with 12-ring channels stabilized by HIm cations via N-H···O hydrogen bonds. For 2, the inorganic skeleton is comprised of alternating ZnO₄ and HPO₃ tetrahedra, while the coordinatively associated ZnN₂O₂ fragments occupy the 12-ring hexagonal channels. Compound 3 adopts a ladder-type one-dimensional structure and exhibits N-H···O hydrogen-bonding interactions to afford a supramolecular assembly. A plausible rationale on the genesis of 1-3 has been put forth by reacting the preformed inorganic zinc phosphites Zn{OP(O)(OMe)H}₂ or [Zn₂(HPO₃)₂(H₂O)₄]·H₂O with imidazole as the structure-directing ligand. Alternating-current impedance measurements reveal that 1 and 3 exhibit proton conductivities on the order of 10^-3-10^-4 S cm^-1 between 25 and 100 °C under 35 and 77% relative humidity in repeated impedance cycles (E_a = 0.22-0.35 eV). On the contrary, the conduction property is completely impaired in 2 under similar conditions.