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Physical Chemistry

Boron Runs Rings Around Metals

Study leads to 10 boron atoms around a metal ion, setting a coordination record for a planar species

by Stephen K. Ritter
February 13, 2012 | APPEARED IN VOLUME 90, ISSUE 7

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Credit: Alexander Boldyrev
The computed structure and aromatic molecular orbitals for TaB10.
09007-scicon-aromvertcxd.jpg
Credit: Alexander Boldyrev
The computed structure and aromatic molecular orbitals for TaB10.
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Credit: Angew. Chem. Int. Ed.
This crystal structure image shows coordination of a planar B6 ring to titanium atoms in the solid Ti7Rh4Ir2B8. (Rh and Ir are omitted for clarity.)
09007-scicon-figure2colorcxd.jpg
Credit: Angew. Chem. Int. Ed.
This crystal structure image shows coordination of a planar B6 ring to titanium atoms in the solid Ti7Rh4Ir2B8. (Rh and Ir are omitted for clarity.)

A multi-institution research team has created wheel-like TaB10 and NbB10 clusters that set a record for the highest coordination number in any planar molecule (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201107880). Alexander I. Boldyrev of Utah State University and Lai-Sheng Wang of Brown University and coworkers have often teamed up to engineer seemingly impossible molecular combinations, including the previous record-holding hypercoordinated planar species RuB9. For three-dimensional molecules, the highest coordination so far is 14. Boldyrev’s group focuses on computational chemistry to determine whether the imagined molecules are physically plausible, and Wang’s group generates the species by laser vaporization of a target disk containing the elements of interest and confirms their existence in the gas phase by using photoelectron spectroscopy. They found that the TaB10 and NbB10 wheels are perfect decagons (shown) and are aromatic by virtue of their delocalized σ and π bonds. Chemists are interested in making these materials for their superconducting or semiconducting properties. In a separate result, Boniface P. T. Fokwa and Martin Hermus of Germany’s RWTH Aachen University succeeded in using solid-state synthesis methods to make Ti7Rh4Ir2B8, which contains a planar B6 ring as a building block. This is the first example of an isolable compound containing a planar all-boron ring (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201106798).

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