Sulfate-incorporated Co(II) coordination frameworks with bis-imidazole bridging ligands constructed by covalent and noncovalent interactions

Three 3D supramolecular networks, [Co(H ₂L1) ₂] •(SO ₄)•2H ₂O (1), [Co(H ₂L1)(SO ₄)(H ₂O)(DMF)] (2), and [Co(H ₂L2)(SO ₄)(H ₂O)] (3), were prepared by reacting Co(II) sulfate with the rigid bis-imidazoles of 1,4-di(1H-imidazol-4-yl)benzene (H ₂L1) and 1,3-di(1H-imidazol-4-yl)benzene (H ₂L2) in solvothermal conditions. Compounds 1 and 2 containing the H ₂L1 ligand were isolated under different solvent-pair ratios. The structure of 1 can be described as a 6-fold interpenetrating 3D dia net in which the sulfate anions are positioned in the void spaces to balance the overall charge of the framework. In comparison, complex 2 shows a rectangular 2D grid consisting of 1D sulfate-bridged chains linked by H ₂L1. When H ₂L2 is used in the reaction, the complex 3 having a 3D interdigitaed network with helical chains is formed, which is the first example of an H ₂L2-connected coordination polymer. The sulfate ions essentially contribute to the entanglement of the structures through extensive hydrogen bonding. Magnetic measurements for 2 indicate that very weak ferromagnetic interactions are operative between anisotropic Co(II) centers via sulfate bridges.