Pushing the Boundaries of Supraicosahedral Heteroborane Chemistry: New Supraicosahedral Carboranes and Metallacarboranes

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science

Abstract

This research is concerned with the preparation of large cluster molecules, mainly of boron atoms but also carbon and metal atoms as well. Boron cluster chemistry is dominated by molecules with only 12 atoms in the cluster / the icosahedron / of which there are thousands of known examples. In contrast there are probably less than 200 such clusters with 13 atoms, only a handful with 14 atoms, and, until very recently, no such clusters with 15 atoms, and nothing larger. This is unfortunate as several of the actual and potential applications of such clusters, including their use in catalysis and in tumour therapy, would be enhanced if larger compounds could be made, particularly if more boron atoms could be incorporated.The biggest obstacle to the preparation of these large supraicosahedral boron clusters is that once you get beyond the icosahedron the atoms in the cluster have to make more bonds to their neighbours than they can comfortably do. However, previous theoretical work has suggested that this may only be an issue for 13, 14 and 15-vertex compounds, and that for those clusters with 16 or more vertices, shapes can be adopted which avoid this problem. One of the major aims of the proposed work is to try to get to 16 vertices and beyond (we are currently only one vertex away from this) to see what actually happens.The way we will make the supraicosahedral molecules is by a technique know as polyhedral expansion , a two step process in which we first open up the cluster by adding 2 electrons then add the additional atom to recluse the cage. Even though boron is uncomfortable in making many bonds in supraicosahedral clusters we can still prepare and study such compounds by adding transition metal atoms instead of boron, because metals are significantly larger than boron. Part of our intended strategy, then, is deliberately to use transition metal vertices to learn more about these unusual molecules, and to help us discover ways to ultimately make the same compounds with boron. A halfway-house would be to use main group metals, which have characteristics between those of boron and transition metals, and we also propose to research in this area.Most of the planned work is concerned with preparing new clusters and determining their structures. But in addition we are also planning to study aspects of this area computationally, with the ultimate hope of being able to establish a set of working rules, both to help us understand the experimental results retrospectively and to help us establish useful protocols that will guide future experimental work.

Publications

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Abram PD (2009) Exopolyhedral ligand flipping on isomerisation of novel supraicosahedral stannacarboranes. in Chemical communications (Cambridge, England)

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Ellis D (2010) The first supraicosahedral bis(heteroborane). in Chemical communications (Cambridge, England)

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Ellis D (2010) Room-temperature C-C bond cleavage of an arene by a metallacarborane. in Angewandte Chemie (International ed. in English)

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Hutton BW (2008) Unprecedented steric deformation of ortho-carborane. in Chemical communications (Cambridge, England)

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McLellan R (2011) Diphosphaborane and metalladiphosphaborane: ligands for transition-metal chemistry. in Angewandte Chemie (International ed. in English)

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McLellan R (2013) Synthesis and Characterisation of Sigma- and Pi-Bonded Metallaphosphacarboranes in Zeitschrift für anorganische und allgemeine Chemie

 
Description Through this award we have significantly expanded and developed the chemistry of supraicosahedral heteroboranes. We have explored their synthesis, established their structures (spectroscopically and crystallographically) and in many cases understood these structures and the reactivity of these species through DFT calculations. In several cases completely unexpected behaviour was observed, e.g. decarbonylation of a 13-vertex species and room-temperature cleavage of an aromatic C-C bond. A consistent theme which arose from the work was the flexibility of supraicosahedral heteroboranes and their ability to access several isomeric forms. A key output was a computational paper which brought many of these findings together and identified new structural patterns in heteroborane chemistry.
Exploitation Route Our findings have been widely reported and the outputs are available to others working in the field. We ourselves have taken on and are further developing several of the key findings from this project in a subsequent one.
Sectors Chemicals,Energy