Three Co(II) Metal-Organic Frameworks with Diverse Architectures for Selective Gas Sorption and Magnetic Studies

Three Co(II) metal-organic frameworks, namely, {[Co₂(L)₂(OBA)₂(H₂O)₄]· xG} _n (1), {[Co(L)_0.5(OBA)]· xG} _n (2), and {[Co₂(L)₂(OBA)₂(H₂O)]·DMA· xG} _n (3) [where L = 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene, H₂OBA = 4,4'-oxybisbenzoic acid, DMF = dimethylformamide, DMA = dimethylacetamide, and G denotes disordered guest molecules], have been synthesized under diverse reaction conditions through self-assembly of a bent dicarboxylate and a linear spacer with a Co(II) ion. While 1 is crystallized at room temperature in DMF to form a 2D layer structure, 2 is formed by the assembly of similar components under solvothermal conditions with a 3D network structure. On the other hand, changing the solvent to DMA, 3 could be crystallized at room temperature with a 3D architecture. Out of the three, activated sample 2 was found to be permanently microporous in nature, with a BET surface area of 385 m²/g, and exhibited moderately high uptake capacity for C₂H₂ and CO₂ while taking up much less CH₄ and N₂ at ambient conditions. As a result, high ideal adsorbed solution theory (IAST) separation selectivities are obtained for CO₂/N₂ (15:85), CO₂/CH₄ (50:50), and C₂H₂/CH₄ (50:50) gas mixtures, making 2 a potential candidate for those important gas separations at ambient conditions. Moreover, the magnetic properties of 1-3 were studied. 1 and 2 show antiferromagnetic interaction between the Co(II) centers, whereas 3 displays ferromagnetic behavior arising from a counter-complementary effect between two types of links among Co(II) centers in 3.