Fig. 1. ORTEP drawing of the asymmetric unit of 12·Li8, as degewrmibed at the ChemMatCARS Advanced Crystallography facility, drawn with thermal ellipsoids at the 40% probability level. The interstitial THF molecules and minor disorder components are omitted for clarity. Color key: C grey, O red, and Li sky-blue.
Molecular orbit theory has stood unchallenged for decades ever since Sondheimer’s 1960 study of annulene, (CH)18, one of the iconic molecules of organic chemistry; and Oth, Woo, and Sondheimer’s subsequent 1973 study, which held that annulene can be reduced to an anti-aromatic dianion (20 π electrons). Now, a new study carried out via a variety of contemporary research techniques has brought about a re-evaluation of the structure of the [18]annulene dianion.
The researchers in this study, from the University at Albany, State University of New York; and Oxford University synthesized annulene. Their studies included x-ray crystallography and nuclear magnetic resonance spectroscopy carried out at Oxford University; and x-ray crystallography performed at the Advanced Crystallography facility at NSF’s ChemMatCARS, beamline 15-ID-D of the U.S. Department of Energy’s Advanced Photon Source Argonne National Laboratory.
Their results show that the widely accepted geometry of dianionic [18]annulene was incorrect. In fact, [18]annulene undergoes a dramatic switch in conformation on reduction. These results, unlike previous reports, show that this dianion, when shielded from oxygen and moisture, is stable even at 40°C.
They further reveal that the dianion can be reduced to a tetraanion with a substantial aromatic ring current, while forming a lithium-intercalated sandwich. Importantly, this redox activity, and the ability to intercalate lithium cations, point to potential applications of annulenes as energy storage materials.
In 1973, Oth et al., using the research tools available to them at the time to maximum effect, correctly postulated that the dianion of [18]annulene is anti-aromatic, but the availability of high-field 1H NMR spectroscopy and modern x-ray crystallography changes our view of these anions.
See: Wojciech Stawski1,2, Yikun Zhu1, Igor Roncevic2, Zheng Wei1, Marina A. Petrukhina1*, and Harry L. Anderson2**, “The anti-aromatic dianion and aromatic tetraanion of [18]annulene,” Nat. Chem.16, 998–1002 (June 2024).
Author affiliations: 1University at Albany, State University of New York; 1Oxford University
Correspondence: *mpetrukhina@albany.edu; **harry.anderson@chem.ox.ac.uk
The authors thank the European Research Council (grant no. 885606, ARO-MAT), European Commission (MSCA project no. 101064401 ElDelPath) and U.S. National Science Foundation (NSF) (grant no. CHE-2003411) for funding. European Community Computational resources were provided by the Cirrus UK National Tier-2 HPC Service at the EPCC (http://www.cirrus.ac.uk) funded by the University of Edinburgh and Engineering and Physical Sciences Research Council (grant no. EP/P020267/1), as well as the Ministry of Education, Youth and Sports of the Czech Republic through the e-INFRA CZ (grant no. ID 90140). This research used resources of the NSF’s ChemMatCARS, Sector 15 at the Advanced Photon Source (APS), Argonne National Laboratory (ANL) supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), NSF, under grant number NSF/CHE-1834750. Use of APS, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by ANL, was supported by the U.S. DOE under contract no. DE-AC02-06CH11357. We thank N. H. Rees for assistance with exchange spectroscopy NMR measurements and Ke
For information on advanced crystallography at NSF’s ChemMatCARS contact:
Yu-Sheng Chen
(630) 252-0471
yushengchen@uchicago.edu