Radical Anions, Radical-Anion Salts, and Anionic Complexes of 2,1,3-Benzochalcogenadiazoles

By means of cyclic voltammetry (CV) and DFT calculations, it was found that the electron-acceptor ability of 2,1,3-benzochalcogenadiazoles 1-3 (chalcogen: S, Se, and Te, respectively) increases with increasing atomic number of the chalcogen. This trend is nontrivial, since it contradicts the electronegativity and atomic electron affinity of the chalcogens. In contrast to radical anions (RAs) [1]^.- and [2]^.- , RA [3]^.- was not detected by EPR spectroscopy under CV conditions. Chemical reduction of 1-3 was performed and new thermally stable RA salts [K(THF)]⁺ [2]^.- (8) and [K(18-crown-6)]⁺ [2]^.- (9) were isolated in addition to known salt [K(THF)]⁺ [1]^.- (7). On contact with air, RAs [1]^.- and [2]^.- underwent fast decomposition in solution with the formation of anions [ECN]^- , which were isolated in the form of salts [K(18-crown-6)]⁺ [ECN]^- (10, E=S; 11, E=Se). In the case of 3, RA [3]^.- was detected by EPR spectroscopy as the first representative of tellurium-nitrogen π-heterocyclic RAs but not isolated. Instead, salt [K(18-crown-6)]⁺ ₂ [3-Te₂ ]^2- (12) featuring a new anionic complex with coordinate Te-Te bond was obtained. On contact with air, salt 12 transformed into salt [K(18-crown-6)]⁺ ₂ [3-Te₄ -3]^2- (13) containing an anionic complex with two coordinate Te-Te bonds. The structures of 8-13 were confirmed by XRD, and the nature of the Te-Te coordinate bond in [3-Te₂ ]^2- and [3-Te₄ -3]^2- was studied by DFT calculations and QTAIM analysis.