Design and synthesis of phosphomolybdate coordination compounds based on {P4Mo6} structural units and their proton conductivity

Developing new low-cost and efficient proton-conducting materials remains an attractive and challenging task. Herein, sodium molybdate dihydrate is used as the source of molybdenum, mixed with transition metal chloride and 2-methylimidazole (2-MI), using the "one-pot method" to synthesize two crystalline proton conducting materials based on {P4Mo6} units: H14[C4H6N2]2[M(H2O)5] [M(H2O)2]2{M[(PO3)3(PO4)Mo6O15]2}·4H2O (M=Co and Fe) (1–2). Different from the common {P4Mo6}, we use H3PO3 to adjust the pH value, resulting in two different coordination modes of P atoms in the crystal structure. The structure is expanded into three-dimensional network by metal ions. At 75 °C and 98% relative humidity, the proton conductivity of compounds 1 and 2 are 1.33 × 10–2 S·cm−1 and 1.03 × 10–2 S·cm−1, respectively. The high proton conductivity is mainly attributed to the free state of 2-methylimidazole as the proton carrier, which has a fast migration rate. At the same time, 2-methylimidazole, coordination water, and {P4Mo6} anion form a hydrogen bond network to provide multiple pathways for the transmission of protons.