Electrochemically\nAssisted Single Crystal Growth of\nReduced Preyssler Polyoxometalates Decorated with <i>M</i><sup>2+</sup> (<i>M</i> = Co, Ni) and Cubane–Like\nNi<sub>4</sub>O<sub>4</sub>

Polyoxometalates (POMs) are of great interest to the\nscientific\ncommunity, and their reduction and nucleation have been well–established\nby multi–step techniques. The present study develops an electrochemical\napproach for simultaneous reduction and nucleation of polyoxometalate–containing\nsolids. Herein we report crystal growth of reduced Preyssler polyoxotungstate–based\n(anionic formula [NaP<sub>5</sub>W<sub>30</sub>O<sub>110</sub>]<sup>14–</sup>) new crystalline solids made of Preyssler anions\ninterlinked by Co<sup>2+</sup> and Ni<sup>2+</sup> ions. Crystal nucleation\nand in situ reduction were achieved at room temperature using a two\nsilver wire electrode setup in various aqueous solutions under constant\napplied potentials. The POM material was deposited on the cathode,\nand its structure was characterized by X–ray diffraction techniques.\nThe primary structure type observed involves POMs decorated by disordered\nCo<sup>2+</sup>/Ni<sup>2+</sup> octahedra and fused into 1–D\npillars by additional Co<sup>2+</sup>/Ni<sup>2+</sup> octahedra. A\nsecondary phase was observed in the Ni–based reactions, where\nreduced Preyssler anions are decorated by Ni<sub>4</sub>O<sub>4</sub> cubane–like units. To understand the electrochemical process,\npolarization curves of the electrolyte solutions are presented, suggesting\nan applied potential best suited for crystal growth. The work highlights\nthe effectiveness of an electrochemical pathway where nucleation and\nsimultaneous reduction of POMs can make novel reduced POM solids.