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

摘要

A mixture of Pd(OAc)_2 and TBA_4[γ-SiW_(10)O_(34)(H_2O)_2] (TBA-SiW_(10), TBA = [(n-C_4H_9)_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-H_2SiW_(10)O_(36)Pd_2(OAc)_2]~4 (I), was successfully synthesized by the reaction of [γ-SiW_(10)O)(34)-(H_2O)_2] ~ 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-SiWIO. 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.