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)­(H2O)]n·xH2O} [Ln = Gd3+ and x = 3 (1) and Dy3+ and x = 1.5 (2); H2L = mucic acid; OxH2 = 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 1, 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 a and b axes, whereas for complex 2,
the 3D framework originates from a carboxylate-bridged dysprosium-based
criss-cross-type secondary building block. Magnetic studies reveal
that 1 exhibits a significant magnetic entropy change
(−ΔSM) of 30.6 J kg–1 K–1 for ΔH= 7 T at 3 K.
Our electronic structure calculations under the framework of density
functional theory reveal that exchange interactions between Gd3+ ions are weak and of the antiferromagnetic type. Complex 2 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 × 10–4 S cm–1 for 1 and 9.06 × 10–5 S cm–1 for 2 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 1. 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 a axis) contributes more to the proton migration compared
to the other channel (along the b axis) in the framework.