Electrochemically Assisted Single Crystal Growth of Reduced Preyssler Polyoxometalates Decorated with M²⁺ (M = Co, Ni) and Cubane-Like Ni₄O₄ Units

Polyoxometalates (POMs) are of great interest to the scientific community, and their reduction and nucleation have been well-established by multi-step techniques. The present study develops an electrochemical approach for simultaneous reduction and nucleation of polyoxometalate-containing solids. Herein we report crystal growth of reduced Preyssler polyoxotungstate-based (anionic formula [NaP₅W₃₀O₁₁₀]^14-) new crystalline solids made of Preyssler anions interlinked by Co²⁺ and Ni²⁺ ions. Crystal nucleation and in situ reduction were achieved at room temperature using a two silver wire electrode setup in various aqueous solutions under constant applied potentials. The POM material was deposited on the cathode, and its structure was characterized by X-ray diffraction techniques. The primary structure type observed involves POMs decorated by disordered Co²⁺/Ni²⁺ octahedra and fused into 1-D pillars by additional Co²⁺/Ni²⁺ octahedra. A secondary phase was observed in the Ni-based reactions, where reduced Preyssler anions are decorated by Ni₄O₄ cubane-like units. To understand the electrochemical process, polarization curves of the electrolyte solutions are presented, suggesting an applied potential best suited for crystal growth. The work highlights the effectiveness of an electrochemical pathway where nucleation and simultaneous reduction of POMs can make novel reduced POM solids.