Anton V. Lonchakov

Interaction of 1,2,5-Chalcogenadiazole Derivatives with Thiophenolate: Hypercoordination with Formation of Interchalcogen Bond versus Reduction to Radical Anion

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.

Heterospin π-Heterocyclic Radical-Anion Salt: Synthesis, Structure, and Magnetic Properties of Decamethylchromocenium [1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazolidyl

Decamethylchromocene, Cr(II)(eta(5)-C(5)(CH(3))(5))(2) (2), readily reduced [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (1) in a tetrahydrofuran solvent at ambient temperature with the formation of radical-anion salt [2](+)[1](-) (3) isolated in 97% yield. The heterospin salt 3 ([2](+), S = 3/2; [1](-), S = 1/2) was characterized by single-crystal X-ray diffraction as well as magnetic susceptibility measurements in the temperature range 2-300 K. The experimental data together with theoretical analysis of the salts magnetic structure within the CASSCF and spin-unrestricted broken-symmetry (BS) density functional theory (DFT) approaches revealed antiferromagnetic (AF) interactions in the crystalline 3: significant between anions [1](-), weak between cations [2](+), and very weak between [1](-) and [2](+). Experimental temperature dependences of the magnetic susceptibility and the effective magnetic moment of 3 were very well reproduced in the assumption of the AF-coupled [1](-)...[1](-) (J(1) = -40 +/- 9 cm(-1)) and [2](+)...[2](+) (J(2) = -0.58 +/- 0.03 cm(-1)) pairs. The experimental J(1) value is in reasonable agreement with the value calculated using BS UB3LYP/6-31+G(d) (-61 cm(-1)) and CASSCF(10,10)/6-31+G(d) (-15.3 cm(-1)) approaches. The experimental J(2) value is also in agreement with that calculated using the BS DFT approach (-0.33 cm(-1)).

Donor-acceptor coordination of anions by chalcogen atoms of 1,2,5-chalcogenadiazoles

Synthetic, structural, and thermodynamic aspects of the recently discovered new reaction, donor-acceptor coordination of anions (A–) by chalcogen atoms (E) of 1,2,5-chalcogenadiazoles, are considered. According to the quantum chemical calculations, the charge transfer from A– to the heterocycle via the mechanism of negative hyperconjugation (i.e., from the MO of the lone pair of A– to the virtual σ* orbital of the E—N bond of chalcogenadiazole) depends on the nature of E and A–, being 0.42—0.52 and 0.30—0.44 e in terms of the Mulliken and NBO methods, respectively. According to the X-ray diffraction data, the E—A– coordinate bond is always longer than the sum of the covalent radii but shorter than the sum of the van der Waals radii of the atoms forming the bond. The E—A– bond energy varies in a wide range, from ~25 kcal mol–1 comparable to the energy of weak covalent bonds (e.g., internal N—N bond in organic azides) to ~86 kcal mol–1 comparable to the C—C bond energy in organic compounds. The quantum chemical estimations of the thermodynamics of the donor-acceptor coordination of the anions by the chalcogen atoms of 1,2,5-chalcogenadiazoles indicate that for E = Te and Se this reaction may be of general character also covering E = S in some cases.