Kateryna GOLOVIZNINA1,2, Matheiu SALANNE1,2,3
1Sorbonne Université, CNRS, Physico-chimie des Électrolytes et Nanosystèmes Interfaciaux, PHENIX, Paris, France
2Réseau sur le Stockage Electrochimique de l'Energie (RS2E), Amiens, France
3Institut Universitaire de France (IUF), Paris, France
Redox-active organic molecules are known to play a significant role in energy storage devices and catalytic reactions. One of the representatives is 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical, used as a mediator in organic synthesis and regarded as a safe alternative to heavy metals. Considering solvation media with adjustable properties can provide solid control over the electrochemical and catalytic characteristics of the TEMPO/TEMPO+ redox system. Being highly conductive, stable, and low flammable fluids, ionic liquids (ILs) are promising candidates with easily tunable physical and solvation properties. Unfortunately, the literature data on the TEMPO/IL systems are rather scarce, with only a few general studies present. [1,2]
In the present work, we propose a systematic investigation of the effect of the nature of IL ions on the properties of the TEMPO/TEMPO+ redox pair, which includes imidazolium, pyrrolidinium, and phosphonium-based families of cations combined with dicyanamide, tetrafluoroborate, methylsulfonate, and bis(trifluoromethane)sulfonimide anions. Classical molecular dynamics with state-of-art polarizable models [3,4] provides a complete and reliable insight into structure-property relationships on the molecular level.
Local structure analysis shows the unlike behavior of TEMPO and TEMPO+ in an IL solution. Playing a role of a hydrogen bond acceptor, TEMPO preferably interacts with hydrogen atoms of the head group of the IL cations via its nitroxyl group. On contrary, the IL cations are repelled from the TEMPO+ cation and the latter acts as a donor forming doubly ionic hydrogen bonds with the IL anions. The nature of the IL ions then becomes a determining factor of the electrochemical properties of the redox system. By combining bulky cations with basic anions, the oxidation potential can be lowered, while the use of small acid cations and hydrophobic anions results in higher values, favoring the reaction from the thermodynamic point of view.  The obtained results will allow performing rational design of TEMPO-based redox systems to match the properties of interest for their future applications.
 Delorme et al., ACS Sustainable Chem. Eng. 2019, 7, 11691;
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 Goloviznina et al., J. Chem. Theory Comput. 2019, 15, 5858;
 Berthin et al., J. Chem. Phys. 2021, 155, 074504;
 Goloviznina et al., ChemRxiv, 2022, DOI: 10.26434/chemrxiv-2022-cd8xm.