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

A mixture of Pd(OAc)(2) and TBA(4)[γ-SiW(10)O(34)(H(2)O)(2)] (TBA-SiW10, TBA = [(n-C(4)H(9))(4)N](+)) 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-H(2)SiW(10)O(36)Pd(2)(OAc)(2)](4-) (I), was successfully synthesized by the reaction of [γ-SiW(10)O(34)(H(2)O)(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.