Preparation, characterization and hydrogenation activity of multiply-reduced silicotungstic acid

Silicotungstic acid (H₄SiW₁₂O₄₀) demonstrates some fascinating reversible redox behavior that in recent years has been exploited both for decoupled electrolysis and for the selective hydrogenation of nitroarenes to their aniline derivatives without the need to use hydrogen gas or precious metal catalysts. Specifically, silicotungstic acid can be reduced electrochemically to its two-electron reduced form (H₆SiW₁₂O₄₀), which displays good stability and which can be induced to cycle multiple times between H₆SiW₁₂O₄₀ and H₄SiW₁₂O₄₀ on account of its reversible reduction and re-oxidation. In contrast, studies on more deeply-reduced states of silicotungstic acid are rarely addressed in the literature, although the potential for developing more powerful, yet still selective, hydrogenation agents if more deeply-reduced states could be accessed reliably offers an incentive to explore their formation. Herein, we report the controlled and reproducible formation of silicotungstic acid in its previously uncharacterized four-electron reduced form by judicious selection of electrochemical reduction parameters. We obtain and compare the crystal structures of two- and four-electron reduced silicotungstic acid for the first time and show that four-electron reduced silicotungstic acid is indeed a more effective hydrogenation agent for the reduction of 4-cyanobenzaldehyde and 4-(trifluoromethyl)benzaldehyde than the two-electron reduced form. Our results suggest that careful optimization of reduction conditions can allow polyoxometalates in oxidation states hitherto considered rather exotic to be produced reliably, and subsequently deployed to produce reaction outcomes that are demonstrably different from those occasioned by their less-reduced analogues.