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Metals 2014, 4(2), 84-107; doi:10.3390/met4020084

Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes

1
Organometallic Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako City, Saitama 351-0198, Japan
2
Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako City, Saitama 351-0198, Japan
3
Graduate School of Science and Engineering, Saitama University, Shimo-Okubo, Sakura-Ku, Saitama City, Saitama 338-8570, Japan
*
Author to whom correspondence should be addressed.
Received: 9 January 2014 / Revised: 4 March 2014 / Accepted: 19 March 2014 / Published: 1 April 2014
(This article belongs to the Special Issue Metal and Molecular Clusters)

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.
Keywords: niobium and tantalum; molybdenum and tungsten; rhenium; chloride and bromide; inorganic cluster complex; Brønsted acid; coordinatively unsaturated site; bi-functional catalyst niobium and tantalum; molybdenum and tungsten; rhenium; chloride and bromide; inorganic cluster complex; Brønsted acid; coordinatively unsaturated site; bi-functional catalyst
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This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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Kamiguchi, S.; Nagashima, S.; Chihara, T. Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes. Metals 2014, 4, 84-107.

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