Synthesis, Structure, and Magnetic Properties of Cobalt(II) Coordination Polymers from a New Tripodal Carboxylate Ligand: Weak Ferromagnetism and Metamagnetism

A new tripodal carboxylic ligand, 5-(4-carboxy-phenoxy)-isophthalic acid, readily (cpiaH₃) reacts with Co(II) salts in the presence of pyridine-based coligands under solvothermal conditions to afford four different coordination polymers, {Co₂(cpia)(OH)(bipy)_0.5}_n (1), {[Co₂(cpia)(OH)(H₂O)₂]·H₂O}_n (2), {Co_1.5(cpia)(bipyethane)}_n (3), and {Co_1.5(cpia)(pyridine)₂}_n (4). All these coordination polymers have been characterized by single crystal X-ray diffraction, IR spectroscopy, thermogravimetry and elemental analysis. The structures of 1 and 2 can be described as a repetition of a tetranuclear core with subtle differences  two tetranuclear cores are shared by an O atom in 2, while no such bridging is present in 1. The structures of 3 and 4, on the other hand, consist of a trinuclear core extending to three-dimensional networks. Variable temperature magnetic susceptibility measurements indicate that 1 exhibits ferromagnetic behavior below 13 K, while 2 shows metamagnetic behavior. These interesting magnetic features are associated with the mixed hydroxyl/carboxylate-bridged polynuclear Co(II) clusters as subunits.