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.

Ag clusters with a controlled number of atoms have received significant interest because they show size‐dependent catalytic, optical, electronic, or magnetic properties. However, the synthesis of size‐controlled, ligand‐free, and air‐stable Ag clusters with high yields has not been well‐established. Herein, it is shown that isostructural porous ionic crystals (PICs) with redox‐active polyoxometalates (POMs) can be used to synthesize Ag clusters via electron transfer from POMs to Ag+. Ag clusters with average numbers of three, four, or six atoms emitting blue, green, or red colors, respectively, are formed and stabilized in the PICs under ambient conditions without any protecting ligands. The cluster size solely correlates with the degree of electron transfer, which is controlled by the reduction time and types of ions or elements of the PICs. Thus, advantages have been taken of POMs as electron sources and PICs as scaffolds to demonstrate a convenient method to obtain few‐atom Ag clusters.

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The Front Cover shows a redox‐active ionic crystal based on a Dawson‐type polyoxometalate (POM), which can form and stabilize small mixed‐valence luminescent silver clusters via a cation‐coupled electron‐transfer reaction between the reduced POM and Ag+. The silver cluster can stably exist without the aid of protecting ligands at room temperature. X‐ray photoelectron spectroscopy, photoluminescence, and X‐ray absorption fine structure measurements suggest that the silver species exist mainly as [Ag4]2+ in a tetrahedral geometry in the closed pores of the ionic crystal. More information can be found in the Communication by S. Uchida and co‐workers.

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