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

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.