Catalytic Reactions over Halide Cluster Complexes of Group 5-7 Metals

Halide clusters of Group 5-7 metals develop catalytic activity above 150-250 degrees C, and the activity is retained up to 350-450 degrees C by taking advantage of their thermal stability, low vapor pressure, and high melting point. Two types of active site function: the solid Bronsted acid site and a coordinatively unsaturated site that catalyzes like the platinum metals do. Various types of catalytic reactions including new reactions and concerted catalyses have been observed over the clusters: hydrogenation, dehydrogenation, hydrogenolysis, isomerization of alkene and alkyne, and alkylation of toluene, amine, phenol, and thiol. Ring-closure reactions to afford quinoline, benzofuran, indene, and heterocyclic common rings are also catalyzed. Beckmann rearrangement, S-acylation of thiol, and dehydrohalogenation are also catalyzed. Although the majority of the reactions proceed over conventional catalysts, closer inspection shows some conspicuous features, particularly in terms of selectivity. Halide cluster catalysts are characterized by some aspects: cluster counter anion is too large to abstract counter cation from the protonated reactants, cluster catalyst is not poisoned by halogen and sulfur atoms. Among others, cluster catalysts are stable at high temperatures up to 350-450 degrees C. At high temperatures, apparent activation energy decreases, and hence weak acid can be a catalyst without decomposing reactants.