New products of coordination of anions X– (X = F, I, PhS) to the Te atom of 3,4-dicyano-1,2,5-telluradiazole (1) were synthesized in high yields and characterized by X-ray diffraction (XRD) as the salts [(Me2N)3S]+[1-F]− (9), [K(18-crown-6)]+[1-I]− (10), and [K(18-crown-6)]+[1-SPh]− ·THF (11), respectively. In the crystal lattice of 10, I atoms are bridging between two Te atoms. The bonding situation in anions of the salts 9–11 and some other adducts of 1,2,5-chalcogenadiazoles (chalcogen = S, Se, Te) and anions X– (X = F, Cl, Br, I, PhS) was studied using DFT, QTAIM, and NBO calculations, for 9–11 in combination with UV–vis, IR/Raman, and MS-ESI techniques. In all cases, the nature of the coordinate bond is negative hyperconjugation involving the transfer of electron density from X– to the heterocycles. The energy of the bonding interaction varies in a range from ∼30 kcal mol–1 comparable with energies of weak chemical bonds (e.g., internal N–N bond in organic azides) to ∼86 kcal mol –1 comparable with an energy of the C–C covalent bonds. The thermodynamics of the anions’ coordination to 1 and their Se and S congeners was also studied by quantum chemical calculations. The general character of this reaction and favorable thermodynamics in the case of heavier chalcogens (Se, Te) were established. Comparison with available data on acyclic analogues, i.e. the chalcogen diimines RNXNR, reveals that they also coordinate various anions but in addition reactions across XN (X = S, Se, Te) double bonds. Attempts to prepare the anion [1-TePh]− led to disintegration of 1. The only unambiguously identified product was a rather rare tellurocyanate that was characterized by XRD and elemental analysis as the salt [K(18-crown-6)]+[TeCN]− (13).

According to the DFT calculations, [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (4), [1,2,5]selenadiazolo[3,4-c][1,2,5]thiadiazole (5), 3,4-dicyano-1,2,5-thiadiazole (6), and 3,4-dicyano-1,2,5-selenadiazole (7) have nearly the same positive electron affinity (EA). Under the CV conditions they readily produce long-lived π-delocalized radical anions (π-RAs) characterized by EPR. Whereas 4 and 5 were chemically reduced into the π-RAs with thiophenolate (PhS(-)), 6 did not react and 7 formed a product of hypercoordination at the Se center (9) isolated in the form of the thermally stable salt [K(18-crown-6)][9] (10). The latter type of reactivity has never been observed previously for any 1,2,5-chalcogenadiazole derivatives. The X-ray structure of salt 10 revealed that the Se-S distance in the anion 9 (2.722 Å) is ca. 0.5 Å longer than the sum of the covalent radii of these atoms but ca. 1 Å shorter than the sum of their van der Waals radii. According to the QTAIM and NBO analysis, the Se-S bond in 9 can be considered a donor-acceptor bond whose formation leads to transfer of ca. 40% of negative charge from PhS(-) onto the heterocycle. For various PhS(-)/1,2,5-chalcogenadiazole reaction systems, thermodynamics and kinetics were theoretically studied to rationalize the interchalcogen hypercoordination vs reduction to π-RA dichotomy. It is predicted that interaction between PhS(-) and 3,4-dicyano-1,2,5-telluradiazole (12), whose EA slightly exceeds that of 6 and 7, will lead to hypercoordinate anion (17) with the interchalcogen Te-S bond being stronger than the Se-S bond observed in anion 9.

Recent progress in the design, synthesis and characterization of chalcogennitrogen π-heterocycles, mostly 1,2,5-chalcogenadiazoles (chalcogen: S, Se and Te) and their fused derivatives, possessing positive electron affinity is discussed together with their use in preparation of charge-transfer complexes and radical-anion salts-candidate building blocks of molecule-based electrical and magnetic functional materials.