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Issue 16, 2020
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Aromaticity and sterics control whether a cationic olefin radical is resistant to disproportionation

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Abstract

We elucidate why some electron rich-olefins such as tetrathiafulvalene (TTF) or paraquat (1,1′-dimethyl-4,4′-bipyridinylidene) form persistent radical cations, whereas others such as the dimer of N,N′-dimethyl benzimidazolin-2-ylidene (benzNHC) do not. Specifically, three heterodimers derived from cyclic (alkyl) (amino) carbenes (CAAC) with N,N′-dimethyl imidazolin-2-ylidene (NHC), N,N′-dimethyl imidazolidin-2-ylidene (saNHC) and N-methyl benzothiazolin-2-ylidene (btNHC) are reported. Whereas the olefin radical cations with the NHC and btNHC are isolable, the NHC compound with a saturated backbone (saNHC) disproportionates instead to the biscation and olefin. Furthermore, the electrochemical properties of the electron-rich olefins derived from the dimerization of the saNHC and btNHC were assessed. Based on the experiments, we propose a general computational method to model the electrochemical potentials and disproportionation equilibrium. This method, which achieves an accuracy of 0.07 V (0.06 V with calibration) in reference to the experimental values, allows for the first time to rationalize and predict the (in)stability of olefin radical cations towards disproportionation. The combined results reveal that the stability of heterodimeric olefin radical cations towards disproportionation is mostly due to aromaticity. In contrast, homodimeric radical cations are in principle isolable, if lacking steric bulk in the 2,2′ positions of the heterocyclic monomers. Rigid tethers increase accordingly the stability of homodimeric radical cations, whereas the electronic effects of substituents seem much less important for the disproportionation equilibrium.

Graphical abstract: Aromaticity and sterics control whether a cationic olefin radical is resistant to disproportionation

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Supplementary files

Article information


Submitted
06 Feb 2020
Accepted
28 Mar 2020
First published
30 Mar 2020

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2020,11, 4138-4149
Article type
Edge Article

Aromaticity and sterics control whether a cationic olefin radical is resistant to disproportionation

J. Messelberger, A. Grünwald, S. J. Goodner, F. Zeilinger, P. Pinter, M. E. Miehlich, F. W. Heinemann, M. M. Hansmann and D. Munz, Chem. Sci., 2020, 11, 4138
DOI: 10.1039/D0SC00699H

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