Catalytic Hydration of Alkynes over Brønsted Acid Sites Developed on Halide Clusters

When a molecular tungsten halide cluster on silica gel, (H₃O)₂[(W₆Cl₈)Cl₆] · 6H₂O/SiO₂, was treated in a nitrogen stream at temperatures above 250 °C, Brønsted acidity appeared as the result of formation of a hydroxo ligand. The cluster catalyzed hydration of 1-hexyne to yield 2-hexanone. The catalytic activity increased with increasing temperature, the selectivity for 2-hexanone being 70% in the temperature range 250–400 °C. 1-Heptyne, 1-octyne, 1-nonyne, and phenylacetylene were also preferentially hydrated to produce the corresponding ketones. 2-Hexyne provided 2- and 3-hexanones in equal amounts, as happens with Brønsted acid catalysts. Halide clusters of niobium, molybdenum, and tantalum having the same octahedral metal framework were also active catalysts for the reaction.