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Metals
Special Issues
Metal and Molecular Clusters
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Metal and Molecular Clusters

A special issue of Metals (ISSN 2075-4701).

Deadline for manuscript submissions: closed (30 June 2014) | Viewed by 20389

Special Issue Editor

Prof. Dr. M. Samy El-Shall

E-Mail Website
Guest Editor
Department of Chemistry, Virginia Commonwealth University, Richmond, VA 23284-2006, USA
Interests: molecular clusters; gas phase and cluster polymerization; nucleation phenomena and nanostructured materials
Special Issues, Collections and Topics in MDPI journals

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Keywords

  • molecular clusters
  • molecular beam techniques
  • bound
  • mass spectrometry
  • size-dependent spectroscopic lineshifts

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Published Papers (2 papers)

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2653 KiB  
Review
Catalytic Reactions over Halide Cluster Complexes of Group 5–7 Metals
by Sayoko Nagashima, Satoshi Kamiguchi and Teiji Chihara
Metals 2014, 4(2), 235-313; https://doi.org/10.3390/met4020235 - 23 Jun 2014
Cited by 18 | Viewed by 8130
Abstract
Halide clusters of Group 5–7 metals develop catalytic activity above 150–250 °C, and the activity is retained up to 350–450 °C by taking advantage of their thermal stability, low vapor pressure, and high melting point. Two types of active site function: the solid [...] Read more.
Halide clusters of Group 5–7 metals develop catalytic activity above 150–250 °C, and the activity is retained up to 350–450 °C by taking advantage of their thermal stability, low vapor pressure, and high melting point. Two types of active site function: the solid Brønsted 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 °C. At high temperatures, apparent activation energy decreases, and hence weak acid can be a catalyst without decomposing reactants. Full article
(This article belongs to the Special Issue Metal and Molecular Clusters)
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1621 KiB  
Review
Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes
by Satoshi Kamiguchi, Sayoko Nagashima and Teiji Chihara
Metals 2014, 4(2), 84-107; https://doi.org/10.3390/met4020084 - 1 Apr 2014
Cited by 20 | Viewed by 11312
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Metal and Molecular Clusters)
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