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
N. Haraguchi, T. Okunaga, N. Ogiwara, S. Kikkawa, S. Yamazoe, M. Inada, T. Tachikawa, S. Uchida Eur. J. Inorg. Chem., accepted.
A redox-active ionic crystal based on a Dawson-type polyoxometalate (POM) [a-P2WVI18O62]6− is utilized to form and stabilize small mixed-valence luminescent silver clusters without the aid of protecting ligands. A reduced ionic crystal of Cs3H5[Cr3O(OOCH)6(etpy)3]3[a-P2WV5WVI13O62]·nH2O (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[a-P2WV3WVI15O62]·nH2O 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.