Mixed CoN4Cl2 and CoCl4 units versus two CoN2Cl3 units in [Co2(Htbi)2Cl4] isomers

The solvothermal reaction of CoCl2 and 2-(4-thiazolyl)-benzimidazole (Htbi) in ethanol solution at 160 °C and 72 h gave rise to a majority of green crystals of [Co2(Htbi)2Cl4] 1 and minor isomeric blue crystals of 2. Two isomers show quite different isolated dinuclear structures bridged by two chlorides. Complex 1 consists of edge-sharing CoN4Cl2 octahedron and CoCl4 tetrahedron, while 2 is composed of two edge-sharing CoN2Cl3 square pyramids. The intra- and inter-molecular halogen hydrogen bonds play an important role at the stabilization in the solid state. Density functional theory calculations manifest that the optimized geometric parameters match very well with the experimental ones for both isomers. The difference of calculated Gibbs free energies demonstrates that intrinsic stability of 1 is superior to 2, in agreement with their relative yields under experimental conditions. The experimental UV–vis absorption spectra in N, N-dimethyl acetamide for 1 and 2 show the same absorption bands and match the computed results. Magnetic studies of 1 reveal that strong antiferromagnetic interactions were detected between octahedral and tetrahedral Co(II) atoms.