Journal of the American Chemical Society, Vol.141, No.24, 9701-9711, 2019
Organic Redox Systems Based on Pyridinium-Carbene Hybrids
New redox systems with three oxidation states are highly sought-after, for example, for redox-flow battery applications, selective reducing agents, or organic electronics. Herein, we describe a straightforward and modular synthesis of a new class of such a three-state redox system based on the intermolecular reaction of a large variety of pyridinium salts with carbenes. These hybrids represent organic (super) electron donors with tailored electrochemical properties and feature three stable oxidation states, which could be fully characterized including by X-ray diffraction. We elaborate which electronic factors either promote attainment of stable radicals through one electron transfer or instead favor 2e(-) processes. Indeed, based on X-ray data, a verification for a potential compression mechanism is given that originates through a large structural distortion in the first oxidation event. By geometrically locking this hybridization change, a potential expansion can be realized. The new class of stable organic radicals are highly persistent and even moderately stable toward air. Additionally, we demonstrate that our modular synthesis approach is also applicable to remarkably strong multielectron (4e(-)) donors by utilizing bridged pyridinium salts. Based on the stability and reversibility of the new redox system, we could demonstrate by charge-discharge experiments the use of the hybrid molecules as novel anolyte materials for nonaqueous redox-flow batteries.