Proton conduction via lattice water molecules in oxalato-bridged lanthanide porous coordination polymers.

The proton conducting properties of two different structural types of porous coordination polymers [La ₂ (ox) ₃ (H ₂ O) ₆ ]·4H ₂ O (1) and [Er ₂ (ox) ₃ (H ₂ O) ₆ ]·12H ₂ O (2), where ox ^2- = oxalate, were investigated. 1 has a two-dimensional layered structure, whereas 2 has a three-dimensional structure. Both 1 and 2 have hydrophilic one-dimensional channels filled by lattice water molecules with hydrogen-bonding networks. The coordinated H ₂ O molecules are Lewis acidic due to the lanthanoid ions donating protons to lattice-filling H ₂ O molecules, thereby forming efficient proton conduction pathways. Alternating-current impedance analyses of 1 and 2 indicated significant proton conduction (σ = 3.35 × 10 ^-7 S cm ^-1 at 368 K for 1, 1.79 × 10 ^-6 S cm ^-1 at 363 K for 2 under RH = 100%, with E _a = 0.35 eV for 1 and 0.47 eV for 2), which was attributed to the Grotthuss mechanism via the lattice H ₂ O molecules.