Abstract
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 °C, catalytically active sites develop, and the cluster framework is retained up to 350–450 °C. One of the active sites is a Brønsted 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 °C under inert atmosphere; however, it is reduced to metallic rhenium at 250–300 °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.
Highlights
One of the greatest challenges in catalysis is the development of new reactions over new catalysts.Our research on the application of halide cluster complexes to catalysis is reviewed.Carbonyl clusters have been strongly associated with catalysis, as most of these clusters contain platinum group metals
150–250 °C, two kinds of catalytically active sites develop in the clusters
One is a weak Brønsted acid resulting from a hydroxo ligand that is formed by the elimination of hydrogen halide from the halogen and aqua ligands
Summary
One of the greatest challenges in catalysis is the development of new reactions over new catalysts. The electronegativities of the Groups 3 and 4 transition metals are low and it is difficult to remove the coordinated oxygen atoms of ligands from the cluster complexes. This suggests that oxygen-containing reactants cannot be applied for catalysis. The Raman spectra of the heat-treated samples are shown, in which some of the assigned peaks for the molecular cluster (H3O)2[(Mo6Cl8)Cl6]·6H2O are indicated. This C.N. of the Mo–Mo bond, corresponding to the octahedral metal framework, indicates the retention of the cluster framework throughout the reaction These results clearly show the retention of the Mo cluster framework and face-capping Cl ligands during the treatment and the catalytic reaction at 400 °C or above under helium. Retention of the cluster framework of Mo with face-capping Cl ligands in (H3O)2[(Mo6Cl8)Cl6]·6H2O has been confirmed up to 400 °C [23]
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