Catalytic Activity of Molecular Rhenium Sulfide Clusters [Re6S8(OH)6−n(H2O)n](4−n)− (n = 0, 2, 4, 6) with Retention of the Octahedral Metal Frameworks: Dehydrogenation and Dehydration of 1,4-Butanediol

A series of molecular rhenium sulfide clusters [Re 6 S 8 (OH) 6− n (H 2 O) n ] (4− n )− ( n = 0, 2, 4, 6) catalyze dehydrogenation of alcohols, and hydrogenation of ketones and olefins in a hydrogen stream at 350 °C. The catalytic activities of the dianionic and neutral clusters ( n = 2, 4) are lower than those of tetraanionic and dicationic clusters ( n = 0, 6) for all the reactions. When 1,4-butanediol is allowed to react over K 4 [Re 6 S 8 (OH) 6 ], dehydrogenation proceeds to yield 2-hydroxytetrahydrofuran and successively γ-butyrolactone above 300 °C. Over [Re 6 S 8 (H 2 O) 6 ]SO 4 dehydration proceeds to yield tetrahydrofuran above 250 °C. The thermal activation mechanisms of these clusters were studied by powder X-ray diffraction analyses, Raman spectrometry, extended X-ray absorption fine structure spectrometry, thermogravimetry, and differential thermal analyses. The catalytically active site of K 4 [Re 6 S 8 (OH) 6 ] is an uncoordinated metal site (Lewis acid site) developed by the loss of a water molecule from two hydroxo ligands. The active site of [Re 6 S 8 (H 2 O) 6 ]SO 4 is a Brønsted acid site; the anhydrous aqua cluster dication disproportionates to a hydroxo cluster monocation and a proton. Both of the octahedral cluster frameworks are retained up to 500 °C.