Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes

Halide clusters have not been used as catalysts. Hexanuclear molecular halide clusters of niobium, tantalum, molybdenum, and tungsten possessing an octahedral metal framework are chosen as catalyst precursors. The prepared clusters have no metal-metal multiple bonds or coordinatively unsaturated sites and therefore required activation. In a hydrogen or helium stream, the clusters are treated at increasingly higher temperatures. Above 150-250 degree C, catalytically active sites develop, and the cluster framework is retained up to 350-450 degree C. One of the active sites is a Broensted acid resulting from a hydroxo ligand that is produced by the elimination of hydrogen halide from the halogen and aqua ligands. The other active site is a coordinatively unsaturated metal, which can be isoelectronic with the platinum group metals by taking two or more electrons from the halogen ligands. In the case of the rhenium chloride cluster Re3Cl9, the cluster framework is stable at least up to 300 degree C under inert atmosphere; however, it is reduced to metallic rhenium at 250-300 degree C under hydrogen. The activated clusters are characterized by X-ray diffraction analyses, Raman spectrometry, extended X-ray absorption fine structure analysis, thermogravimetry-differential thermal analysis, infrared spectrometry, acid titration with Hammett indicators, and elemental analyses.