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Synthesis

A Boron Cluster‑Go-Round

Chemical Bonding: Fluxional behavior in B19 cluster suggests rotating concentric rings

by Stephen K. Ritter
July 12, 2010 | APPEARED IN VOLUME 88, ISSUE 28

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Credit: Angew. Chem. Int. Ed.
In theory, fluxional bonding permits the inner B6 pentagon and outer B13 ring of B19 to rotate in opposite directions.
Credit: 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.

Magic Roundabout
Credit: Courtesy of Gabriel Merino
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.

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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