Isolation of a Triplet Benzene Dianion

Published February 09, 2022

The six-carbon benzene ring (highlighted in green) is stabilized via the surrounding rigid ligand scaffold, with the pair of metal ions (M = Y or Gd) above and below, further promoting the magnetic (triplet) ground state. The delocalized nature (aromaticity) of the unpaired electrons manifests as an equalization of the carbon-carbon bond lengths (right), resulting in an undistorted ring structure

High-magnetic-field, high-frequency electron paramagnetic resonance demonstrates how coordination chemistry can be leveraged to stabilize a desired electronic/magnetic state in an organic molecule. In this experiment, the long-sought magnetic (triplet) ground state in a benzene ring is stabilized by a pair of metal ions above and below the six-carbon ring.

What did scientists discover?

This study reports stabilization of the elusive benzene radical dianion with a magnetic ground state, that is, a doubly reduced form of the benzene molecule in which the two unpaired electrons align their magnetic moments, giving rise to a total spin S = ½ + ½ = 1 (triplet) ground state. High-field electron paramagnetic resonance (EPR) studies were employed in order to confirm the magnetic state of this unusual molecule.

Why is this important?

This study demonstrates how coordination chemistry can be leveraged to stabilize a desired electronic/ magnetic state in an organic molecule. Specifically, this approach enables isolation of a negatively charged (2-) benzene dianion in which the magnetic S = 1 (triplet) state—typically a high-energy excited state in such ring-like organic molecules—instead exists as the well-isolated molecular ground state. In turn, this enabled verification of a decades-old theoretical model predicting a delocalized nature of the unpaired electrons on the benzene ring.

Who did the research?

C. A. Gould¹, J. Marbey^2,3, V. Vieru⁴, D. A. Marchiori⁵, R. D. Britt⁵, L. F. Chibotaru⁴, S. Hill^2,3 and J. R. Long^1,6

¹UC Berkeley; ²National MagLab; ³Florida State University; ⁴KU Leuven; ⁵UC Davis; ⁶LBNL

Why did they need the MagLab?

Measurements at multiple high magnetic fields and frequencies (a factor of four above commercial spectrometers) available at the MagLab were essential in order to deconvolute several contributions to the EPR spectra. In particular, separating the contributions from intra-molecular and inter-molecular interactions is impossible at low magnetic fields. In this way, analysis of spectra recorded at 13T and 371GHz provides information on the isolation of the S=1 state, without dependence on other unknown interactions.

Details for scientists

View or download the expert-level Science Highlight, Isolation of a Triplet Benzene Dianion
Read the full-length publication, Isolation of a Triplet Benzene Dianion, in Nature Chemistry

Funding

This research was funded by the following grants: G.S. Boebinger (NSF DMR-1644779); J. R. Long (NSF CHE-1610226); S. Hill (NSF DMR-1610226); R. D. Britt (NIH 1R35GM126961); V. Vieru (Flemish Science Foundation)

For more information, contact Stephen Hill.