Formation of Mixed‐Valence Luminescent Silver Clusters via Cation‐Coupled Electron‐Transfer in a Redox‐Active Ionic Crystal Based on a Dawson‐type Polyoxometalate with Closed Pores

A redox‐active ionic crystal based on a Dawson‐type polyoxometalate (POM) [α‐P 2 W VI 18 O 62 ] 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 Cs 3 H 5 [Cr 3 O(OOCH) 6 (etpy) 3 ] 3 [α‐P 2 W V 5 W VI 13 O 62 ] ⋅ 5H 2 O (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 (W 5+ ) to Ag + , and [Ag 0 1.5 Ag I 1.5 ]H 4.5 [Cr 3 O(OOCH) 6 (etpy) 3 ] 3 [α‐P 2 W V 3 W VI 15 O 62 ] ⋅ 7H 2 O 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 [Ag 4 ] 2+ probably in a tetrahedral geometry.