Solvent-free construction of Cr(iii)-sulfonate coordination polymers

Abstract

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.