Construction, structures, and magnetic studies of two cobalt(ii) and nickel(ii) coordination polymers displaying bex topology†

The synthesis, structures, adsorption, and magnetic properties of two two-dimensional (2D) coordination polymers, [M(btca)_1/2(bpe)_1/2(H₂O)₂]_n (M = Ni²⁺ (1) and Co²⁺ (2); H₄btca = 1,2,4,5-benzenetetracarboxylic acid; bpe = 1,2-di(4-pyridyl)ethane) constructed by mixed bipyridyl–tetracarboxylate ligands were reported. Single-crystal X-ray diffraction studies reveal the 3D hydrogen-bonded networks sustained by O–H⋯O H bonds between the 2D layers. The 2D structure topology of the compounds is a new {4.6·2}2{4·2.6·2.8·2} bex topology while the 3D framework is different because of the distinct H bond interactions. Both compounds exhibit high thermal stability above 150 °C. N₂ adsorption measurements reveal that both compounds are non-porous structures, while compound 1 exhibits CO₂ adsorption, possibly due to structural expansion after CO₂ accommodation. Magnetic investigations indicated that framework 2 exhibits field-induced slow magnetic relaxation due to the large magnetic anisotropy of Co²⁺ ions (zero-field splitting parameter D = 42.1 cm⁻¹), while framework 1 shows simple paramagnetism with a small D of −11.3 cm⁻¹. This work not only provides two new coordination polymers but also a potential bipyridyl–tetracarboxylate approach for the design and construction of single-ion magnet (SIM) frameworks.