Harnessing the Hard-Soft Acid-Base Principle in Sulfur-Dense Flexible Covalent Organic Frameworks for Targeted Gold...

Efficient gold (Au) recovery from e-waste and industrial effluents is crucial for sustainable resource management, yet building crystalline covalent organic frameworks (COFs) with dense sulfur soft sites under Hard-Soft Acid-Base (HSAB) guidance is severely hindered by the intrinsic rotational freedom of thioether linkages, which typically precipitates amorphous networks. Herein, we report the rational synthesis of sulfur-bridged flexible COFs by tailoring solvothermal kinetics to enable the construction of crystalline topological framework isomers (sql and kgm nets) from an identical tetracyclic C-S bond-enriched monomer. The kgm framework was further functionalized with pendant methyl sulfide groups, generating abundant soft binding sites. Consequently, the resulting electron-rich framework delivers an exceptional Au uptake capacity of 6420 mg g^-1, ranking among the highest values reported to date for COFs under dark conditions, alongside rapid uptake kinetics and excellent selectivity. Combined spectroscopic and theoretical investigations reveal that sulfur sites play a central role in Au capture by providing abundant active sites, facilitating the coordination of Au species, and promoting electron transfer, thereby favoring the reduction of Au(III) to Au(0). This paradigm underscores the potency of the HSAB principle as a robust blueprint for designing high-performance adsorbents aimed at the sustainable precious metal recovery.