Novel applications of functionalized 2,1,3-benzothiadiazoles for coordination chemistry and 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 ZnCl2 (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 H-1 NMR data, complex 2 dissociates in CHCl3, 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 pi-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, similar to 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 2,1,3-benzothiadiazole 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 similar to 550 nm. However, according to DFT and QTAIM calculations the complex is weakly bonded and its formation is not observed in CH2Cl2 solution with H-1 NMR and UV-Vis techniques.