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.