A Boron Cluster‑Go-Round

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boron, cluster compounds, double aromaticity

Courtesy of Gabriel Merino

Magic Roundabout Rapid bond breaking and re-forming leads concentric B₆ and B₁₃ units in this B₁₉^– cluster to rotate in opposite directions. The cluster behaves like a molecular Wankel engine, a type of rotary motor used in automobiles in which a rotor turns inside a housing. During the rotation, the cluster remains planar, so the energy barrier is minimal.

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Angew. Chem. Int. Ed.

In theory, fluxional bonding permits the inner B₆ pentagon and outer B₁₃ ring of B₁₉^– to rotate in opposite directions.

A theoretical study of a recently observed boron cluster compound is prompting chemists to stretch their imaginations: The study proposes that bonds between the B₁₉ ^– cluster’s inner B₆ ring and outer B₁₃ ring constantly break and re-form via a low-energy transition state in such a way that the pentagon-shaped “hub” and the B₁₃ “wheel” rotate in opposite directions.

The study, led by Gabriel Merino of Mexico’s University of Guanajuato and Thomas Heine of Germany’s Jacobs University, highlights the peculiar nature of aromatic bonding in inorganic molecules (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201001275).

The B₁₉ ^– cluster has a nearly circular planar structure and is doubly π-aromatic, with two π electrons delocalized over the B₆ ring and another 10 π electrons shared between the B₆ and B₁₃ rings. A team led by Alexander I. Boldyrev of Utah State University and Lai-Sheng Wang of Brown University detected it earlier by mass spectrometry and gas-phase photoelectron spectroscopy experiments and explained its bonding system (Nat. Chem. 2010, 2, 202).

Boldyrev, Wang, and coworkers noted that the dual aromatic system can give rise to electron ring currents in opposite directions, but they didn’t recognize the potential for structural rotation. Discovery of the proposed rotation is “extraordinary,” Boldyrev says, and is prompting the team to consider how to verify it experimentally.

The B₁₉ ^– cluster is structurally similar to an aromatic C₅B₁₁ ⁺ cluster with concentric C₅ and B₁₁ rings predicted by Gernot Frenking of Philipps University, in Marburg, Germany; Paul von Ragué Schleyer of the University of Georgia; and coworkers (Angew. Chem. Int. Ed. 2005, 44, 1078). The rotation in the B₁₉ ^– cluster “is unique and apparently has not been recognized before,” Frenking notes, although it’s similar to the C₅B₁₁ ⁺ cluster, where counterrotating rings might also exist. The difference is that B₁₉ ^– has been observed, whereas the modeled C₅B₁₁ ⁺ cluster is not energetically possible.

Frenking and Schleyer say they doubt “bottleable” amounts of the compound will come to pass. Nevertheless, “the topic does challenge one’s design imagination to next devise a system of three counterrotating concentric rings,” Schleyer adds.

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