N–N-Bridged Polynuclear POM-Based Coordination Polymers Based on a V-Type Ligand: Proton Conduction and Magnetism

The construction of polyoxometalate (POM)-based coordination polymers, in the presence of a nitrogen heterocyclic ligand, is intriguing due to the potential for obtaining diverse structures. These structures exhibit extensive application possibilities in the fields of proton conductivity and magnetism. Herein, four new POM-based polynuclear coordination polymers with the formulas of {[Fe₂(btb)₃(H₂O)₂(SiW₁₂O₄₀)]·3H₂O}_n (1), {[Cd₂(btb)₂(H₂O)₆(HPMo^VI₁₀Mo^V₂O₄₀)]·2H₂O}_n (2), {[Co₃(OH)₂(btb)₂(H₂O)₅(HPMo^VI₁₀Mo^V₂O₄₀)]·7H₂O}_n (3), and {[Cu₃(OH)(btb)₂(H₂O)(HP₂Mo₅O₂₃)]·6H₂O}_n (4) have been prepared using the V-type 1,3-bis(4H-1,2,4-triazole-4-yl)benzene (btb) ligand. Compounds 1 and 2 feature similar two-dimensional (2D) structures, derived from the binuclear Fe₂N₆ and Cd₂N₄ subunits connected by tridentate btb ligands. Meanwhile, in compound 3, hexanuclear Co₆(OH)₄ units are bound by quadridentate btb ligands forming a 2D layer with the same 4-c sql topology simplification as compounds 1 and 2. In compound 1, Keggin-type polyoxoanions are monodentate-coordinated to metal ions and suspended on the 2D structure, while, in compounds 2 and 3, they act as discrete counterions residing in the interstitial spaces between two adjacent layers, thereby extending the 2D structures into 3D structures through hydrogen bonding interactions. In compound 4, trinuclear Cu₃(OH) subunits are further constructed into a 3D framework through cooperation with four tridentate and quadridentate btb ligands as well as Strandberg-type anions. Furthermore, the proton conduction of the four compounds has been investigated. They display high proton conductivities at 358 K and 98% RH with powdered samples, which are 1.26 × 10^-3, 1.24 × 10^-3, 3.24 × 10^-4, and 2.57 × 10^-4 S cm^-1, respectively. Interestingly, by mixing with Nafion, the composite membranes of compounds 2 and 4 exhibit enhanced proton conductivities, measuring at 4.87 × 10^-2 and 1.28 × 10^-2 S cm^-1, respectively, at 358 K and 98% RH, which suggests excellent potential for applications. In addition, compounds 1, 3, and 4 display antiferromagnetic behaviors due to similar magnetic interactions. This work can provide research insights into the assembly of 2D POM-based coordination polymers with nitrogen heterocyclic ligands and Keggin-type POMs and further promote their research progress in proton conduction.