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

Abstract

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.

Size‐Controlled Synthesis of Luminescent Few‐Atom Silver Clusters via Electron Transfer in Isostructural Redox‐Active Porous Ionic Crystals

“…According to the observation above, we reached the idea that smaller Ag clusters can be synthesized by regulating the electron transfer from the PIC to Ag + . Therefore, we focused on [a‐P 2 W VI 18 O 62 ] 6– because [a‐P 2 W VI 18 O 62 ] 6– stores fewer electrons than [a‐P 2 Mo VI 18 O 62 ] 6– under the same reducing conditions, [ 43,53 ] and utilized K 3 [Cr 3 O(OOCH) 6 (etpy) 3 ] 3 [a‐P 2 W VI 18 O 62 ]· n H 2 O [ 45 ] [ K‐I W (0, 0) ] as a scaffold to synthesize Ag clusters. The PXRD patterns showed that K‐I Mo (0, 0) and K‐I W (0, 0) were isostructural (Figure S8, Supporting Information), and the abbreviation K‐I W ( t 1 , t 2 ) is defined in the same way as K‐I Mo ( t 1 , t 2 ) .…”

“…550 nm) color, respectively (Figure 5b,e). The Ag clusters formed in K‐I W (60, 60) can be assigned mainly to [Ag 4 ] 2+ according to the position of the maximum in the PL spectrum, [ 45 ] which is largely blue‐shifted from that of K‐I Mo (60, 60) (maximum at ca. 715 nm) containing [Ag 6 ] 2+ , consistent with our expectation.…”

“…500 nm (Figure 5f), suggesting that NH 4 ‐I Mo (5, 60) may contain Ag 3 . [ 45,46 ] The EXAFS analysis revealed that the silver atoms in NH 4 ‐I Mo (120, 60) and NH 4 ‐I Mo (5, 60) have average CN Ag‐Ag of 4.7 ± 0.1, and 2.8 ± 0.2, respectively (Figure 6c,f, Table 1). The CN Ag‐Ag smaller than 3 is in line with the result of PL, suggesting the preferential formation of Ag 3 .…”

“…Specifically, the electrons stored in the reduced POMs are transferred to Ag + followed by aggregation of Ag 0 and Ag + , the Ag clusters (mainly [Ag 4 ] 2+ ) are stabilized in the pores as countercations of POMs, and the small pores inhibit further growth of the Ag clusters. [44,45] This approach realizes a convenient ligand-free synthesis of air-stable fewatom Ag clusters with high yield, while size-control has remained as our next challenge.…”

Honeycomb‐Layered Oxides With Silver Atom Bilayers and Emergence of Non‐Abelian SU(2) Interactions

“…[ 30,36,64,65 ] Indeed, this is the case for $$\rm Ag_3^{1+}Ni_2^{2+}Bi^{5+}O_6^{2-}$$, whereby the $$\rm Ag$$ atom is monovalent, and the lattice is not only monolayered but triangular, [ 38 ] in contrast to the present material reported herein. Meanwhile, the $$\rm Ag$$ sub‐valent state $$1/3+$$ remains conspicuously absent in reported layered materials [ 30,36,64 ] and other silver‐rich oxides, [ 41–43,71–77 ] despite its presence reported, e.g., in $$\rm Ag_3^{1/3+}O^{2-}H^{1+}$$. [ 78 ] Finally, although the sub‐valency of silver remains unascertained in the present material via direct XPS measurements of $$\rm Ag$$ binding energies, the fact that the $$\rm Ag$$‐rich domains are bilayered with an apparent bifurcated bipartite honeycomb lattice demonstrates that such domains fit well within the aforementioned class of bilayered sub‐valent $$\rm Ag$$‐based frameworks, thus corroborating the charge neutrality argument provided earlier for the sub‐valency of $$\rm Ag$$.…”