Cluster increments for macropolyhedral boranes

Kiani, Farooq Ahmad; Hofmann, Matthias

doi:10.1039/b610138k

Computational Studies: Boranes

Shameema

¹

,

Jemmis

²

2005

Encyclopedia of Inorganic Chemistry

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Computational studies on boranes, in recent years, are summarized with the special emphasis on the macropolyhedral boranes. Computational studies to understand the energetics, geometry, aromaticity, and reactivity of boranes and substituted boranes are reviewed. There is a recent interest in macropolyhedral boranes because of their uses in boron neutron capture therapy (BNCT) of cancer, ionic liquids, weakly coordinating ions, and materials chemistry. The mno rule, which is the electronic counting rule for the macropolyhedral boranes, is explained. Even though the electron counting rule explains the electronic structure of macropolyhedra, it cannot explain the relative stability of isomers since different structures are possible with the same electron count. There have been many computational studies to explain the relative stability of various structural patterns in macropolyhedral boranes. We have used the concept of orbital compatibility to explain the relative energies of different macropolyhedral structural patterns. Computations on the closo ‐ closo macropolyhedral boranes show that a large polyhedral borane prefers to condense with a smaller polyhedron owing orbital compatibility. Even though only a few closo ‐ closo macropolyhedral structures have been synthesized experimentally, calculations suggest optimism for the synthesis of many condensation products. A brief discussion about the use of the computational methods for the rearrangement of the B 20 H 16 skeleton reacting with ligands is also provided.

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Computational Studies: Boranes

Shameema

¹

,

Jemmis

²

2011

Encyclopedia of Inorganic and Bioinorganic Chemistry

0

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Computational studies on boranes, in recent years, are summarized with the special emphasis on the macropolyhedral boranes. Computational studies to understand the energetics, geometry, aromaticity, and reactivity of boranes and substituted boranes are reviewed. There is a recent interest in macropolyhedral boranes because of their uses in boron neutron capture therapy (BNCT) of cancer, ionic liquids, weakly coordinating ions, and materials chemistry. The mno rule, which is the electronic counting rule for the macropolyhedral boranes, is explained. Even though the electron counting rule explains the electronic structure of macropolyhedra, it cannot explain the relative stability of isomers since different structures are possible with the same electron count. There have been many computational studies to explain the relative stability of various structural patterns in macropolyhedral boranes. We have used the concept of orbital compatibility to explain the relative energies of different macropolyhedral structural patterns. Computations on the closo ‐ closo macropolyhedral boranes show that a large polyhedral borane prefers to condense with a smaller polyhedron owing orbital compatibility. Even though only a few closo ‐ closo macropolyhedral structures have been synthesized experimentally, calculations suggest optimism for the synthesis of many condensation products. A brief discussion about the use of the computational methods for the rearrangement of the B 20 H 16 skeleton reacting with ligands is also provided.

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Quantifying Building Principles of Borane Clusters

Hofmann

¹

2011

Modeling of Molecular Properties

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Boranes show an intriguing structural variety; notably, in order to reduce their electron deficiency they form nonclassical multicenter bonds and adopt cluster structures. Structural relationships among borane clusters were realized after an increasing number of structures had became known. Wades rule [1] allows (hetero) boranes to be classified according to their number of skeletal electrons, and the shape of the corresponding cluster framework to be determined. Closo, nido, arachno, and hypho are used to label compounds with 2n þ 2, 2n þ 4, 2n þ 6, and 2n þ 8 skeletal electrons, respectively, where n equals the number of cluster atoms. Closo compounds adopt the most spherical deltahedral structures -that is, polyhedra with only trigonal faces. Nido, arachno, and hypho compounds have polyhedral structures in which one, two or three vertices of the corresponding closo deltahedra remain unoccupied. Examples of these are shown in Scheme 25.1.In heteroboranes, one or more boron atoms are replaced by heteroatoms such as C, N, and S. While the cluster type may be derived from the number of skeletal electrons, the placement of heteroatoms allows for various possible isomers. The preferred sites for heteroatom placement were derived from the arrangements found for structurally characterized compounds. For example, the most stable carborane isomer has carbon atoms at low coordinate sites and in nonadjacent positions [2,3]. Other disfavored arrangements, for example, endo terminal rather than bridging hydrogen atoms, were also identified. However, in general, not all of these may be avoided at the same time for a given formula. So, the question arises: Which structural conflict(s) may be tolerated best? Clearly, values are needed to specify the severity of the violation of a particular structural rule, and only if such quantitative rules are available can the best -that is, the thermodynamically most stable -structure be predicted, both easily and reliably. The derivation of meaningful energy penalties for important unfavorable structural features in heteroboranes, and how they may be applied, are described in the following subsections.Modeling of Molecular Properties, First Edition. Edited by Peter Comba.

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Cluster increments for macropolyhedral boranes

Cited by 13 publications

References 37 publications

Computational Studies: Boranes

Computational Studies: Boranes

Computational Studies: Boranes

Quantifying Building Principles of Borane Clusters

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