A novel oxalate-based three-dimensional coordination polymer showing magnetic ordering and high proton conductivity

A novel three-dimensional (3D) coordination polymer with the formula (C₃N₂H₅)₄[MnCr₂(ox)₆]·5H₂O (2), where ox = oxalate and C₃N₂H₅ = imidazolium cation, is reported. Single crystal X-ray diffraction reveals that this porous coordination polymer adopts a chiral three-dimensional quartz-like architecture, with the guest imidazolium cations and water molecules being hosted in its pores. This novel multifunctional material exhibits both a ferromagnetic ordering at T_C = 3.0 K, related to the host MnCr₂ network, and high proton conductivity [1.86 × 10^-3 S cm^-1 at 295 K and 88% relative humidity (RH)] due to the presence of the acidic imidazolium cations and free water molecules. The similarity of the structure of compound 2 to that of the previously reported analogous compound (NH₄)₄[MnCr₂(ox)₆]·4H₂O, (1), also allows us to analyse, to a certain extent, the effect of the acidity of the proton donating guest molecules on proton conduction properties. 2 hosts, in one-dimensional (1D) channels, imidazolium cations, which are more acidic than the ammonium ones in 1 and, as a consequence, 2 shows higher proton conduction than 1, highlighting the effect of the pK_a of the proton donating guest molecules on proton conductivity.