Solvent-free construction of Cr(<scp>iii</scp>)-sulfonate coordination polymers

Constructing sulfonate coordination polymers (CPs) with high stability remains a significant challenge due to the relatively weak coordination ability of the sulfonate group, especially when paired with highly inert Cr³⁺ ions. In this study, we designed solvent-free methods to enhance Cr(iii)-sulfonate coordination and further advance its reticular chemistry. For the first time, two Cr(iii)-sulfonate CPs, TGU-9 and TGU-10, were successfully constructed, along with two supramolecules, TGU-7 and TGU-8. All structures were elucidated using the 3D electron diffraction technique. Through solvent-free methods, Cr(iii)-sulfonate coordination was achieved by a double displacement reaction between Cr salts and -SO₃H groups. In particular, this method resulted in a counterintuitive coordination reversal from -COO^- > -SO₃ ^- to -SO₃ ^- > -COO^-. Reaction mechanism analysis revealed that the higher acidity of the -SO₃H group, compared to the -COOH group, leads to its preferential deprotonation, thereby facilitating the kinetics of Cr-sulfonate self-assembly. Furthermore, both TGU-9 and TGU-10 exhibited exceptional long-term stability under ambient conditions and over a wide pH range. They also showed high proton conductivity exceeding 10^-2 S cm^-1, ranking in the top two among the reported sulfonate CPs. The designed solvent-free method demonstrated a generally applicable and simple strategy in designing novel metal-ligand coordination and constructing reticular chemistry, beyond the limitations of conventional solvent-based methods.