Hydrothermal Syntheses and Structural Diversity of Cobalt Complexes with 2,2′-Bibenzimidazole Ligand by Temperature Tuning Strategy

Five different cobalt complexes were synthesized by the combination of the multidentate ligand 2,2′-bibenzimidazole (H2bbim) and the coordination plasticity of cobalt ion in the presence of 4-cyanopyridine under hydrothermal conditions using temperature as the only independent variable. The structural features of theses complexes, which ranged from zero-dimensional (0D) in [Co(Hbbim)3]·3H2O (1) and [Co(ina)2(H2O)4] (5) to two-dimensional (2D) layer in {[Co4(Hbbim)4(bbim)2]·2H2O}n (2) or three-dimensional (3D) networks in [Co2(Hbbim)(bbim)2(H2O)]n (3) and [Co5(Hbbim)(bbim)3(ina)2(μ3-OH)]n (4), were extremely dependent on the reaction temperature. At 130 °C, discrete 1 was the unique product. However, 4 was the main product when the reaction temperatures were set in the range of 160−200 °C. 2 and 3 yielded at 140 °C as main products. In spite of the structural diversity of this system, crystallographic studies reveal that the H2bbim ligand typically displays tridentate (Hbbim−) and/or tetradentate (bbim2−) coordination modes at high temperature, however, a chelating mode at low temperature. The ligands Hbbim− and bbim2− offering donor atoms and charge balance may have both advantages of bipyrimidine and oxalate in the assembly of high-dimensional structures. Indeed, 2 is the first instance of 2D honeycomb (6,3), neutral coordination polymer constructed by only one type of ligand and one type of metal ion. Magnetic properties of 3 and 4 have been studied in the temperature range of 2−300 K. Both compounds exhibit antiferromagnetic interactions between metal ions through the bridging ligand.