Helical Co-based coordination polymer based on asymmetric ligands: synthesis, structure and properties

In the presence of 4-[4-(1H-imidazol-1-yl)phenyl]pyridine (ipp), the self-assembly reaction of 2-methoxyterephthalic acid (H₂mta) with Co^II ions produced the complex poly[[{μ-4-[4-(1H-imidazol-1-yl)phenyl]pyridine}(μ-2-methoxyterephthalato)cobalt(II)] monohydrate], {[Co(C₉H₆O₅)(C₁₄H₁₁N₃)]·H₂O}_n or {[Co(mta)(ipp)]·H₂O}_n (denoted as HU29). The complex was structurally characterized by thermogravimetric analysis, elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. Single-crystal structure analysis revealed that the mta^2- anions and ipp ligands are connected according to AABB and ABAB modes to form four distinct helical chains. These helical chains further construct a three-dimensional (3D) framework. Owing to the similar geometries of mta^2- and ipp, combined with the low coordination numbers of the Co^II centres, the resulting 3D framework exhibits a fivefold interpenetrating structure. Topological analysis indicates that HU29 can be simplified as a four-connected dia net. UV-Vis absorption spectroscopy shows a broad absorption band in the UV-Vis region and a band gap of 2.72 eV based on the Kubelka-Munk method, indicating semiconductor behaviour. Moreover, a magnetic study reveals that antiferromagnetic interactions occur in HU29 between 300 and 2 K.