Unusual Flexibility of Microporous Sulfides during Ion Exchange

Open-framework chalcogenides with ion-exchange capacity are promising materials for removing hazardous heavy-metal ions and for capturing radioactive Cs+. However, research on the exchange mechanism is limited, especially for the framework chalcogenides that have multiple bridging anions. Generally, open-framework chalcogenides that have multiple bridging anions at the window or wall of the channels are rigid during the ion-exchange process. We show here that microporous sulfides with μ3-S2– (where μ3 = triple bridging mode) at the windows exhibit framework flexibility upon ion exchange. Three new microporous sulfides Na4Cu8Ge3S12·2H2O (1), Na3(Hen)­Cu8Sn3S12 (where en = ethylenediamine) (2) and (dap)2(Hdap)4Cu8Ge3S18 (where dap = 1,2-diaminopropane) (3) were synthesized under solvothermal conditions. Compounds 1 and 2 contain a copper-rich framework composed of icosahedral [Cu8S12]16– units linked via monomeric GeS4 4– or SnS4 4– tetrahedral units, whereas compound 3 features an expanded framework composed of icosahedral [Cu8S12]16‑ units interconnected with dimeric Ge2S6 4– units. These compounds exhibit unusual ion-exchange properties. Specifically, the frameworks of 1 and 2 (with μ3-S at the small windows) show “breathing action” upon ion exchange of K+ or Rb+, which have relative large sizes, and compound 3 exhibits framework flexibility upon Cs+ ion exchange with both space group and channels changed.