Experimental and Theoretical Investigation of Anisotropic Proton Conduction in Two-Dimensional Metal–Organic Frameworks

Two-dimensional (2D) materials are known for their potential to exhibit anisotropic transport properties due to their layered structures. However, the anisotropic ion conduction of 2D metal-organic frameworks (MOFs) has been rarely explored. In this study, we investigated the anisotropic proton conduction along the in-plane and stacking directions of two analogs of undulating 2D MOFs: [Mn(salen)]₂[Pt(CN)₄]·H₂O (<b>MnPt</b>) and [Mn(salen)]₂[PtI₂(CN)₄]·H₂O (<b>MnPtI</b>). This investigation was conducted using both experimental methods, involving single crystals, and theoretical calculations. Compared to the relatively isotropic proton conduction of <b>MnPt</b> at 85 °C and 95% relative humidity (RH), with a stacking direction conductivity (σ_stacking) of 1.8 × 10^-5 S/cm, which is approximately 2.9 times the in-plane conductivity (σ_in-plane), <b>MnPtI</b> exhibited highly anisotropic proton conduction. The σ_stacking of <b>MnPtI</b> under the same conditions (85 °C, 95% RH) was 1.5 × 10^-4 S/cm, which is 83 times higher than its σ_in-plane. Additionally, the activation energy for proton conduction in <b>MnPtI</b> ranged from 0.65 to 0.73 eV, which is higher than the 0.48 eV observed for <b>MnPt</b>. Theoretical calculations confirmed that slight differences in local structures, including node distortions between <b>MnPt</b> and <b>MnPtI</b>, significantly influenced the activation energies for water migration. This was attributed to the formation of hydrogen bonds between layers and water molecules.