Palladium(II) Containing γ-Keggin Silicodecatungstate That Efficiently Catalyzes Hydration of Nitriles

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Abstract
A mixture of Pd(OAc)2 and TBA4[γ-SiW10O34(H2O)2] (TBA-SiW10, TBA = [(n-C4H9)4N]+) showed high catalytic activities for hydration of various kinds of structurally diverse nitriles including aromatic, aliphatic, heteroaromatic, and double bond-containing ones. For hydration of 3-cyanopyridine, the turnover frequency was 860 h–1, and the turnover number reached up to 670. A dipalladium-substituted γ-Keggin silicodecatungstate, [γ-H2SiW10O36Pd2(OAc)2]4– (I), was successfully synthesized by the reaction of [γ-SiW10O34(H2O)2]4– with Pd(OAc)2 in a mixed solvent of acetone and water. The crystal structure of I was a monomeric, dipalladium-substituted, γ-Keggin silicodecatungstate with bidentate acetate ligands. Compound I showed similar activities and selectivities to those of a simple mixture of Pd(OAc)2 and TBA-SiW10. The kinetic, mechanistic, and density functional theory calculation studies show that the dipalladium site plays an important role in the present hydration, and the nucleophilic attack of a hydroxide or water to the nitrile carbon atom is included in the rate-determining step.

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