A redox-active ionic crystal based on a Dawson-type polyoxometalate (POM) [α-P2WVI18O62]6− is utilized to form and stabilize small mixed-valence luminescent silver clusters without the aid of protecting ligands at room temperature. A reduced ionic crystal of Cs3H5[Cr3O(OOCH)6(etpy)3]3[α-P2WV5WVI13O62] ⋅ 5H2O (etpy=4-ethylpyridine) is formed by a cation-coupled electron-transfer (CCET) reaction with sodium ascorbate as a reducing reagent to provide electrons and Cs+ as counter cations of POM. Then, silver is introduced via ion-exchange between Cs+ and Ag+ jointly with CCET reaction: X-ray photoelectron spectroscopy and elemental analysis show that the ionic crystal is oxidized via electron-transfer from the POM (W5+) to Ag+, and [Ag01.5AgI1.5]H4.5[Cr3O(OOCH)6(etpy)3]3[α-P2WV3WVI15O62] ⋅ 7H2O is formed. Photoluminescence and X-ray absorption fine structure suggest that the silver species exist as mixed-valence luminescent clusters with an average formula of [Ag4]2+ probably in a tetrahedral geometry.
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