Variable catalytic behavior of Nb, Mo, Ta, W, and Re halide clusters: Isomerization of alkynes to conjugated dienes under nitrogen and hydrogenation to alkenes under hydrogen

When a tantalum chloride cluster [(Ta6Cl12)Cl2(H2O)4]·4H2O (1) was treated in a nitrogen stream at temperatures above 300 °C, a catalytic activity developed for the isomerization of pentynes and hexynes to the corresponding conjugated dienes with smaller amounts of allenes. Niobium and tungsten clusters with the same metal framework also catalyzed the reactions. In a hydrogen stream, niobium, molybdenum, and tungsten clusters, and a trinuclear rhenium cluster catalyzed the partial hydrogenation of alkynes to the corresponding alkenes. The isomerization activity was ascribed to the Brønsted acidity of hydroxo species of the activated cluster complexes, and the hydrogenation activity was attributed to the nature of the metal atoms incorporated in the cluster.

Graphical abstractWhen treated in flowing nitrogen, Nb, Ta, and W halide clusters exhibit a catalytic isomerization reaction of alkynes to conjugated dienes. In flowing hydrogen, Nb, Mo, and W clusters catalyze partial hydrogenation reactions to form alkenes. Hydroxo ligands are involved in the isomerization reaction mechanism, whereas the hydrogenation reactions proceed via a mechanism where the metal atom releases its halogen ligand. Figure optionsDownload full-size imageDownload as PowerPoint slide