Chalcogen-bonded donor-acceptor complexes of 5,6-dicyano[1,2,5]selenadiazolo[3,4-b]pyrazine with halide ions

New complexes of 5,6-dicyano[1,2,5]selenadiazolo[3,4-b]pyrazine 1 with halide anions X- (X = Cl, Br, I) are prepared. With XRD, DFT, QTAIM and NBO, it is found that the complexes are formed via Se-X- chalcogen bonding. Diffuse reflectance spectroscopy of polycrystalline samples and UV/Vis spectroscopy of solutions of [1-X](-) reveal significant spectral changes upon complexation. According to time-dependent DFT, these changes are caused by the appearance of charge transfer bands (p-AO(X) -> LUMO(1)) in the visible region. The equilibrium constants for complex formation were measured with UV/Vis spectrophotometric titration and found to be significantly lower than those reported for structurally similar complexes of 3,4-dicyano-1,2,5-selena(tellura)diazoles, which indicates relatively weak chalcogen bonding in [1-X](-). Dispersion-corrected DFT reproduces well the Gibbs free energies of complexation. The QTAIM descriptors and NBO analyses using second-order perturbation theory suggest that the Se-X- chalcogen bonding in the studied complexes is predominantly electrostatic and dispersive in nature with a small but significant contribution from donor-acceptor (or orbital) interactions. The strongest contribution to the latter comes from negative hyperconjugation, i.e. donation of the electron density of the lone-pair of X- onto the sigma*-MO of the Se-N bond of 1.