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

Abstract

When a tantalum chloride cluster [(Ta 6Cl 12)Cl 2(H 2O) 4]·4H 2O ( 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.