Cation-Anion Interactions and Synergistic Catalysis by Supramolecular Polyoxometalate Complexes [C₁₀H₁₈N]_n[XM₁₂O₄₀]

A series of novel organic ammonium salts of Keggin [XM₁₂O₄₀]ⁿ⁻ (X=P, Si, B; M=W, Mo) was synthesized and fully characterized using ESI-MS and X-ray crystallography. The solid-state structure of the title compounds [C₁₀H₁₈N]_n[XM₁₂O₄₀] was unveiled by X-ray crystallography that hydrogen bonds are formed between the nitrogen center of amantadine and the bridging oxygen atoms of POM in addition to Coulombic attractions and by MS/MS that consecutive proton-transfer reactions from [C₁₀H₁₈N]⁺ to [XM₁₂O₄₀]ⁿ⁻ were observed in the gas-phase fragmentations of the dimeric clusters {(C₁₀H₁₈N)_m[XM₁₂O₄₀]₂}^(2n−m)−. A comparative study on the catalytic property of [XM₁₂O₄₀]ⁿ⁻ in methyl phenyl sulfide (MPS) oxidation with aqueous hydrogen peroxide in MeCN was performed and the effect of counterions ([C₁₀H₁₈N]⁺ and TBA⁺, etc) on the catalytic performance has been investigated. The best catalytic activity of [C₁₀H₁₈N]₃[PMo₁₂O₄₀] in MPS oxidation illustrates the explicit role of cations on promoting catalytic activity of polyoxoanions.