Preyssler-Type POM-Based CPs Incorporating V-Type N-Heterocyclic Ligands as Proton Conductors

Polyoxometalate-based coordination polymers (POM-based CPs), particularly those incorporating Preyssler anions with their oxygen-rich surfaces, are promising proton-conducting materials due to their unique structural and functional advantages. Herein, we present three CPs derived from Preyssler-type POMs with the formulas of {[Zn₄(H₂O)₈(H₇P₅W₃₀O₁₁₀)(btp)₄]·6H₂O}_n (1), {[Ni₄(H₂O)₈(H₇P₅W₃₀O₁₁₀)(btp)₄]·14H₂O}_n (2), and {[Cd₅(μ₂-H₂O)(H₂O)₁₄(H₅P₅W₃₀O₁₁₀)(btp)₄]·8H₂O}_n (3) by introducing V-type N-heterocyclic ligand btp (btp = 2,6-bis(1,2,4-triazol-1-yl) pyridine). Structurally, compounds 1 and 2 exhibit analogous structural features, in which each of the four btp ligands is linked to eight metal ions, forming a window to perfectly accommodate the {P₅W₃₀} wheel, and {P₅W₃₀} units link to 1D zigzag chains to form a 2D layer structure with sql topology simplification. In compound 3, four hexanuclear Cd₆(btp)₄ units enclose into an interspace with an open window, where the {P₅W₃₀} wheel is located by connection to metal ions, ultimately expanding to a 3D framework. Meanwhile, compounds 1-3 exhibit good proton conduction and the values of conductivity can reach 2.92 × 10^-3, 1.53 × 10^-3, and 1.70 × 10^-3 S cm^-1 at 85 °C and 98% RH. Remarkably, the proton conductivity of these compounds is significantly enhanced when incorporated into composite membranes with Nafion, reaching 10^-2 S cm^-1 at 85 °C and 98% RH, demonstrating excellent application prospects.