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

Abstract

Constructing sulfonate-coordinated 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⁺ ion. In this study, we designed solvent-free methods to enhance Cr(III)-sulfonate coordination and further advances its reticular chemistry. For the first time, two Cr(III)-sulfonate-coordinated CPs, TGU-9 and TGU-10, were successfully constructed, along with two supramolecules, TGU-7 and TGU-8. All structures were elucidated by 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. Especially, 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 condition and over a wide pH range. They also showed high proton conductivity exceeding 10^–2 S cm^–1, ranking top two among the reported sulfonate-coordinated 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.