A new type of lanthanide–sodium metalloring organic framework featuring high proton conduction in a wide temperature range and detection of Fe³⁺ ions

We present the design and synthesis of two novel isostructural metalloring organic frameworks (MROFs) {[(Me₂NH₂)_1.25(H₃O)_4.25Na_1.5X₂(μ₂-OH)(H₂O)(SIP)₄]_<i>n</i>·<i>x</i>sol} (FUT-2-X, X = Eu and Sm), which are composed of the sulfonate-carboxylate ligand 5-sulfoisophthalic acid monosodium salt (NaH₂SIP) and an unprecedented lanthanide-sodium metalloring [X₄Na₄(H₂O)(SIP)₂, X = Eu and Sm]. The two MROFs possess channel walls decorated with uncoordinated sulfonic acid groups and filled with abundant guest molecules residing within the framework, which support the proton conductivity of the materials by expanding the intermolecular hydrogen bonding network. FUT-2-Eu exhibits exceptional proton conductivity over a wide temperature range, achieving conductivity from 1.91 × 10^-5 S cm^-1 (-40 °C) to 2.65 × 10^-3 S cm^-1 (90 °C). Thanks to the dominant role of the additional guest H₂O molecules in FUT-2-Eu's channels, which facilitate the formation of hydrogen-bonded networks for ultra-fast proton transfer with low energy barriers, FUT-2-Eu outperforms FUT-2-Sm in both the operating temperature range and proton conductivity. It is worth noting that FUT-2-Eu has the widest operating temperature range among proton conduction MROF materials. Furthermore, FUT-2-Eu can be considered as an excellent luminescence sensor with high sensitivity (<i>K</i>_SV = 1.66 × 10⁴ L mol^-1) and a low detection limit (3.64 μM) for detecting Fe³⁺.