Studies on the Genesis and Proton Conductivity of Imidazole-Based Linear and Open-Framework Zinc Phosphites
Ravi Shankar, Ekta Jakhar, Archishmati Dubey, Priyanka Chauhan, Pankaj Tiwari, Suddhasatwa Basu
Abstract
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 (Ea = 0.22–0.35 eV). On the contrary, the conduction property is completely impaired in 2 under similar conditions.