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

•The rare geometric isomerism between two dinuclear complexes is reported.•The computed Gibbs free energy difference is 4.52 kcal/mol.•Both isomeric compounds show the same transition behavior(dp→π*) in DMA solution corresponding to the calculated results.

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.

Graphical abstractThe rare geometric isomerism in a pair of dinuclear complexes is reported. The computed Gibbs free energy difference is 4.52 kcal/mol. The high energy isomer 2 can transform into more stable 1 in DMA solution.Figure optionsDownload full-size imageDownload as PowerPoint slide