Channel-Assisted Proton Conduction Behavior in Hydroxyl-Rich Lanthanide-Based Magnetic Metal–Organic Frameworks

Two new lanthanide-based 3D metal–organic frameworks (MOFs), {[Ln­(L)­(Ox)­(H<sub>2</sub>O)]<sub><i>n</i></sub>·<i>x</i>H<sub>2</sub>O} [Ln = Gd<sup>3+</sup> and <i>x</i> = 3 (<b>1</b>) and Dy<sup>3+</sup> and <i>x</i> = 1.5 (<b>2</b>); H<sub>2</sub>L = mucic acid; OxH<sub>2</sub> = oxalic acid] showing interesting magnetic properties and channel-mediated proton conduction behavior, are presented here. Single-crystal X-ray structure analysis shows that, in complex <b>1</b>, the overall structure originates from the mucate-bridged gadolinium-based rectangular metallocycles. The packing view reveals the presence the two types of hydrophilic 1D channels filled with lattice water molecules, which are strongly hydrogen-bonded with coordinated water along the <i>a</i> and <i>b</i> axes, whereas for complex <b>2</b>, the 3D framework originates from a carboxylate-bridged dysprosium-based criss-cross-type secondary building block. Magnetic studies reveal that <b>1</b> exhibits a significant magnetic entropy change (−Δ<i>S</i><sub>M</sub>) of 30.6 J kg<sup>–1</sup> K<sup>–1</sup> for Δ<i>H</i>= 7 T at 3 K. Our electronic structure calculations under the framework of density functional theory reveal that exchange interactions between Gd<sup>3+</sup> ions are weak and of the antiferromagnetic type. Complex <b>2</b> shows field-induced single-molecule-magnetic behavior. Impedance analysis shows that the proton conductivity of both complexes reaches up to the maximum value of 4.7 × 10<sup>–4</sup> S cm<sup>–1</sup> for <b>1</b> and 9.06 × 10<sup>–5</sup> S cm<sup>–1</sup> for <b>2</b> at high temperature (>75 °C) and relative humidity (RH; 95%). The Monte Carlo simulations confirm the exact location of the adsorbed water molecules in the framework after humidification (RH = 95%) for <b>1</b>. Further, the results from computational simulation also reveal that the presence of a more dense arrangement of adsorbed water molecules through hydrogen bonding in a particular type of channel (along the <i>a</i> axis) contributes more to the proton migration compared to the other channel (along the <i>b</i> axis) in the framework.