• CORRECTION: This story was updated on Dec. 3, 2015, to correct the statement that the new B3 ring is the smallest and lightest aromatic ring possible. It is only the lightest aromatic ring possible.
Volume 93 Issue 47 | p. 4 | News of The Week
Issue Date: November 30, 2015 | Web Date: November 24, 2015

Boron Ring Takes Aromaticity To A New Low

Chemical Bonding: A new sandwich molecule contains the smallest and lightest aromatic ring possible
Department: Science & Technology
News Channels: Organic SCENE
Keywords: aromaticity, chemical bonding, structure, planar boron

By virtue of synthesizing a stable B3 ring, a team of inorganic chemists has prepared the lightest aromatic species that is experimentally possible. In addition to helping researchers better understand chemical structure and bonding, the sandwich molecule the team made containing two B3 rings connected by sodium ions could serve a practical purpose as the first of a family of precursor compounds for preparing semiconducting, superconducting, and magnetic materials.

Thomas Kupfer, Holger Braunschweig, and Krzysztof Radacki of Julius Maximilian University Würzburg made the triboracyclopropenyl dianion by treating cyclohexyl-substituted dichloroaminoborane with sodium metal in dimethoxyethane solvent (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201508670). The Würzburg team has shown through computational, spectroscopic, and electrochemical studies that the B3 ring has an electronic structure consistent with classical aromatic carbon compounds such as the cyclopropenyl cation and benzene.

The announcement “is certainly a great breakthrough and opens a new direction in boron chemistry,” comments Alexander I. Boldyrev of Utah State University, whose group has done much of the computational work on planar all-boron rings during the past 15 years.

Aromaticity as defined by the Hückel Rule has typically been the domain of carbon compounds, with only a few noncarbon analogs made from elements heavier than carbon. Chemists have long predicted that planar boron rings from B3 up to B15 or greater might also be aromatic. Researchers have been hot on the trail of making the boron rings, but the rings are challenging to stabilize in isolable compounds. The greatest success has come in generating the molecules with laser beams and studying them in the gas phase.

“This discovery may help settle the discussion on the validity, or not, of applying the concept of aromaticity to noncarbon-based systems,” says Boniface Fokwa of the University of California, Riverside. In 2012, Fokwa was part of a team that prepared the solid-state material Ti7Rh4Ir2B8, which was the first example of an isolable compound containing a planar all-boron ring. “The discovery of the new planar B3 entity strongly hints at its possible stabilization in a solid-state compound as well,” Fokwa says. “Hopefully, exciting new functional materials based on it will be achieved.”

This article has been translated into Spanish  by Divulgame.org  and can be found here.

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Steve Ritter (Tue Dec 01 11:50:11 EST 2015)
Please note the triboracyclopropenyl dianion is not the smallest possible aromatic ring--the cyclopropenyl cation is still smallest because carbon's atomic radius is slightly smaller than boron's. The B3 ring is the lightest aromatic ring.
George Kenney (Sat Dec 05 20:44:11 EST 2015)
Cool article.

what is the structure of the Nitrogen - sp3 or flat??
Verno G. S. Box (Fri Dec 11 17:12:08 EST 2015)
I was as excited as anyone else reading this news about the the triboracyclopropenyl dianion, and so I downloaded the crystal structure data from the Cambridge Database, the CCDC.

Alas the structure is best described as a boride, since the B-B bonds are clearly single bonds.

Best wishes.

Vernon G. S. Box
City College of CUNY
NY, NY 10031
Steve Ritter (Thu Dec 17 12:40:48 EST 2015)
The Braunchsweig group replies:
Thank you for your interest in our research. Please let me provide two remarks to your comment: (i) The B-B bond lengths within the dianionic B3 rings (162-163 pm, formal bond order 1.33) lie in-between those observed in diborenes (156-160 pm, formal bond order 2) and diborene radical cations (164 pm, formal bond order 1.5). Moreover, a typical B-B single bonds is approx. 172 pm or even longer (Pt-B-B-Pt: 178 pm; Braunschweig et al., Chem. Commun. 2013, 49, 2439). Thus, the B-B bond lengths clearly suggest the presence of multiple bonding in the triboracyclopropenyl dianion (further support is derived from detailed DFT calculations). (ii) The term 'boride' usually accounts for anionic boron materials without any sigma-bonded organic substituents. However, all boron atoms in the triboracyclopropenyl dianion feature sigma-bonded amino substituents, for which reason a description as boride does not apply.

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