With cyclic voltammetry (CV) and DFT calculations, it was found that electron acceptor ability of the 2,1,3-benzochalcogenadiazoles 1-3 (chalcogen: S, Se and Te, respectively) increases with the atomic number of 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 under the 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). Upon contact with air, RAs [1]*- and [2]*- underwent fast decomposition in solution with the formation of anions [ECN]- 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 to be 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 the coordinate bond Te-Te was obtained. Upon contact with air, salt 12 transformed into salt [K(18-crown-6)]+2[3-Te4-3]2- (13) revealing an anionic complex with two coordinate bonds Te-Te. The structures of 8-13 were confirmed by X-ray diffraction and the nature of the coordinate bonds Te-Te in [3-Te2]2- and [3-Te4-3]2- was studied with DFT calculations and QTAIM analysis.

The cocrystallization of 1,2,5-chalcogenadiazoles (chalcogen E = S, Se, and Te) with cyclic polyethers 18-crown-6 (18-c-6) and dibenzo-18-crown-6 (db-18c-6) yielded the molecular complexes characterized by X-ray diffraction and thermogravimetry/differential scanning calorimetry techniques together with density functional theory (DFT) calculations and quantum theory of atoms in molecule and natural bond orbitals analysis. The complexes are bound by multiple secondary bonding interactions, the most important of which reflects the Lewis ambiphilicity of 1,2,5-chalcogenadiazoles and includes charge transfer from O atoms of the ethers onto E atoms of the chalcogenadiazoles (i.e., chalcogen bonding), and the back-donation from E to O. For complexes of 18-c-6, the DFT-calculated energies of bonding interactions correlate with the melting temperatures of the complexes, as well as with the atomic number of E and the size of the E-associated σ-holes but not with the maximum of molecular electrostatic potential at the σ-holes. Taking into account the previous results, the Lewis ambiphilicity of 1,2,5-chalcogenadiazoles may be used for applications in crystal engineering.

Two novel applications of functionalized 2,1,3-benzothiadiazoles for metal coordination chemistry and crystal engineering of organic solids are presented. 4-Amino-2,1,3-benzothiadiazole (1) forms a 2 : 1 complex with ZnCl 2 (complex 2) and a 1 : 1 complex with 4-nitro-2,1,3-benzothiadiazole (3) (complex 4). The structures of compounds 1-4 were confirmed by single-crystal X-ray diffraction and studied by UV-Vis and IR spectroscopy, and DFT and QTAIM calculations. Complex 2 is the first structurally defined Zn complex with 2,1,3-benzothiadiazole ligands. In this complex, both molecules 1 are coordinated to Zn only by amino groups, thus revealing a novel type of coordination of this ligand to the metal center.According to 1 H NMR data, complex 2 dissociates in CHCl 3 , THF or DMSO solutions. There are only a few examples of similar complexes, which are also considered to dissociate in solutions. In crystalline complex 4, molecules 1 and 3 form infinite alternating p-stacks connected by lateral S/N interactions between the neighboring stacks. Intermolecular S/N interactions are also observed in the crystals of individual 1 and 3 but the packing motifs are different from those in 4. DFT calculations predict a small charge transfer (CT, $0.02e at B97-D3 level) from 1 to 3 upon the formation of 4, which therefore is an unprecedented CT complex where both donor and acceptor moieties are the derivatives of the 2,1,3benzothiadiazole ring system. This finding creates some new prospects for the crystal engineering of organic solids. Crystalline complex 4 is characterized by an intense CT absorption band with a maximum at $550 nm. However, according to DFT and QTAIM calculations the complex is weakly bonded and its formation is not observed in CH 2 Cl 2 solution with 1 H NMR and UV-Vis techniques.