A redox-active inorganic crown ether based on a polyoxometalate capsule.

Cation-uptake has been long researched as an important topic in materials science. Herein we focus on a molecular crystal composed of a charge-neutral polyoxometalate (POM) capsule [Mo VI 72 Fe III 30 O 252 (H 2 O) 102 (CH 3 CO 2 ) 15 ] 3+ encapsulating a Keggin-type phosphododecamolybdate anion [α-PMo VI 12 O 40 ] 3- . Cation-coupled electron-transfer reaction occurs by treating the molecular crystal in an aqueous solution containing CsCl and ascorbic acid as a reducing reagent. Specifically, multiple Cs + ions and electrons are captured in crown-ether-like pores {Mo VI 3 Fe III 3 O 6 }, which exist on the surface of the POM capsule, and Mo atoms, respectively. The locations of Cs + ions and electrons are revealed by single-crystal X-ray diffraction and density functional theory studies. Highly selective Cs + ion uptake is observed from an aqueous solution containing various alkali metal ions. Cs + ions can be released from the crown-ether-like pores by the addition of aqueous chlorine as an oxidizing reagent. These results show that the POM capsule functions as an unprecedented "redox-active inorganic crown ether", clearly distinguished from the non-redox-active organic counterpart.