Heteroatom Effect on Site-Selective Oxygen–Sulfur Substitution Reactions of Keggin-Type Polyoxotungstates

Sulfur, a group 16 element, can substitute the oxygen sites of metal oxides, potentially providing them with unique properties and enabling new applications. Polyoxometalates (POMs) are anionic metal oxide clusters with wide structural diversity owing to arbitrary selection of their constituting metal atoms. However, substitution of the oxygen sites of POMs with sulfur atoms has been rarely explored. Recently, we reacted a Keggin-type POM [XW₁₂O₄₀]^4- (I^X; X = Si and Ge) with a sulfurizing reagent (Lawesson's reagent), synthesizing Keggin-type polyoxothiometalates (POTMs) [XW₁₂O₂₈S₁₂]^4- (II^X, X = Si and Ge) in which all 12 terminal oxygen atoms of I^X were site-selectively substituted with sulfur atoms. Although the selected heteroatoms (X) and anion charges substantially influence the properties and reactivities of POMs, oxygen-sulfur substitution of Keggin-type POMs with other heteroatoms and anion charges has not been attempted. Herein, we report the oxygen-sulfur substitution reaction of Keggin-type POMs [XW₁₂O₄₀]^n- (I^X; X = Al, Si, and P; n = 5, 4, and 3, respectively) with various heteroatoms, yielding the corresponding Keggin-type POTMs [XW₁₂O₂₈S₁₂]^n- (II^X). The oxygen-sulfur substitution reaction depended on the heteroatom: POMs with higher anion charges reacted more strongly with the sulfurizing reagents. We also investigated the reaction mechanism of POMs and sulfurizing reagents.