Halide Complexes of 5,6-Dicyano-2,1,3-Benzoselenadiazole with 1:4 Stoichiometry: Cooperativity between Chalcogen and Hydrogen Bonding.

The [M4-Hal]- (M = the title compound; Hal = Cl, Br, and I) complexes were isolated in the form of salts of [Et4N]+ cation and characterized by XRD, NMR, UV-Vis, DFT, QTAIM, EDD, and EDA. Their stoichiometry is caused by a cooperative interplay of σ-hole-driven chalcogen (ChB) and hydrogen (HB) bondings. In the crystal, [M4-Hal]- are connected by the π-hole-driven ChB; overall, each [Hal]- is six-coordinated. In the ChB, the electrostatic interaction dominates over orbital and dispersion interactions. In UV-Vis spectra of the M + [Hal]- solutions, ChB-typical and [Hal]--dependent charge-transfer bands are present; they reflect orbital interactions and allow identification of the individual [Hal]-. However, the structural situation in the solutions is not entirely clear. Particularly, the UV-Vis spectra of the solutions are different from the solid-state spectra of the [Et4N]+[M4-Hal]-; very tentatively, species in the solutions are assigned [M-Hal]-. It is supposed that the formation of the [M4-Hal]- proceeds during the crystallization of the [Et4N]+[M4-Hal]-. Overall, M can be considered as a chromogenic receptor and prototype sensor of [Hal]-. The findings are also useful for crystal engineering and supramolecular chemistry.