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 {[Fe(btb)(HO)(SiWO)]·3HO} (), {[Cd(btb)(HO)(HPMoMoO)]·2HO} (), {[Co(OH)(btb)(HO)(HPMoMoO)]·7HO} (), and {[Cu(OH)(btb)(HO)(HPMoO)]·6HO} () have been prepared using the V-type 1,3-bis(4-1,2,4-triazole-4-yl)benzene (btb) ligand. Compounds and feature similar two-dimensional (2D) structures, derived from the binuclear FeN and CdN subunits connected by tridentate btb ligands. Meanwhile, in compound , hexanuclear Co(OH) units are bound by quadridentate btb ligands forming a 2D layer with the same 4-c topology simplification as compounds and . In compound , Keggin-type polyoxoanions are monodentate-coordinated to metal ions and suspended on the 2D structure, while, in compounds and , 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 , trinuclear Cu(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, 1.24 × 10, 3.24 × 10, and 2.57 × 10 S cm, respectively. Interestingly, by mixing with Nafion, the composite membranes of compounds and exhibit enhanced proton conductivities, measuring at 4.87 × 10 and 1.28 × 10 S cm, respectively, at 358 K and 98% RH, which suggests excellent potential for applications. In addition, compounds , , and 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.