Stability of the Monoelectronic Reduction Product from 1,2,5‐Thiadiazole S,S‐Dioxides. Electrochemical, Chemical, and Photoinduced Doping

Abstract

The accumulation of radical anions from 3,4-disubstituted 1,2,5thiadiazole S,S-dioxides (1) in solution of organic solvents is achieved by electrochemical, chemical and photoinduced pathways. Electrolytic and chemical reduction of 1 produces the radical anions (1 *-) in high molar yields (> ca. 80%). The survival of 1 *is measured for ca. three months. The stability of the reduced state depends on its structure and the composition (solvent, reducing agent, and supporting electrolyte) of the medium in which it is generated by the chemical and electrochemical routes. In solution of polar aprotic solvents, 1 *persists even in the presence of water or oxygen. Irradiation with 254 nm light generates 1 *but only in DMF solution; the proposed reaction mechanism involves the participation of the solvent. Radical anions are characterized by cyclic voltammetry, UV-Vis spectrophotometry and electron spin resonance spectroscopy. The chemical stability and the properties of 1/1

1,2,5-Thiadiazole 1,1-dioxides and Their Radical Anions: Structure, Properties, Reactivity, and Potential Use in the Construction of Functional Molecular Materials

“…Table 1 supplements Figure 1 b by providing references to the CSD database and the relevant publications, where the respective compounds are reported [ 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ]. Sulfonyl (>SO 2 ) is a very strong electron-withdrawing group; therefore, compared to thiadiazoles, their dioxidized analogues containing this sulfonyl group are superior when it comes to negative charge accommodation within the heterocyclic ring and the resulting radical anion formation and their stability.…”

“…In aprotic solvents (DMF, DMSO, MeCN, or DCM) some of the radicals are stable for prolonged periods of time (up to several days), even in the presence of water and oxygen, in dry solvents, they are stable almost indefinitely. The stability of the radical anions depends on the solvent used and decreases in the following series: DMF > DMSO > MeCN >> DCM [ 48 ]. It is noteworthy that the use of highly nucleophilic reductors, such as cyanide, may result in the formation of side products by addition to the C=N double bond of the heterocycle.…”

“…Therefore, sterically hindered reducing agents are preferred, as they allow for better selectivity of anion radical formation; however, even the change of the cation in the system (e.g., KCN vs. LiCN) may play an important role in avoiding the addition reaction [ 42 ]. Finally, radical anions of dioxothiadiazoles can be generated with moderate yields in the photochemical approach using the wavelength of around 254 nm and DMF as a solvent [ 48 ]. However, reduction is taking place because of the initial radical formation involving DMF molecules that further reduce dioxothiadiazole species.…”

“…Typically, two distinct, reversible reduction events can be observed for any dioxothiadiazole within the MeCN electrochemical window. Reduction to radical anions usually takes place at around −0.5 V vs. Fc/Fc + and is followed by the second reduction to a diamagnetic dianion at around −1.3 V [ 14 , 21 , 43 , 44 , 48 , 51 , 60 , 94 ]. It is noteworthy that most thiadiazoles cannot be reduced twice at all and are hardly even reduced to monoanions [ 21 , 43 , 44 , 94 , 95 ].…”

“…It is noteworthy that most thiadiazoles cannot be reduced twice at all and are hardly even reduced to monoanions [ 21 , 43 , 44 , 94 , 95 ]. Electrochemistry, HOMO, and LUMO levels can be easily manipulated and tuned by the introduction of electron-withdrawing substituents such as bromine atoms [ 45 , 48 ].…”