Exchange coupled Co(ii) based layered and porous metal–organic frameworks: structural diversity, gas adsorption, and magnetic properties

Abstract

Four new Co(II) based metal–organic frameworks (MOFs) ({[Co₃(L)(TDCA)₃(DMF)₂]_n·2nCH₃CN}) (1), ({[Co₃(L)₂(BDCA)₃]_n·2nCH₃CN}) (2), {[Co₂(L)₂(CA)₂]_n·4nCH₃CN} (3) and {[Co₂(L)(OBBA)₂]_n·3nCH₃CN} (4) are synthesized, where L is [4′-(4-methoxyphenyl)-4,2′:6′,4′′-terpyridine], a V-shaped flexible neutral spacer, and the four dicarboxylates are TDCA = thiophene 2,5-dicarboxylic acid, BDCA = benzene 1,4-dicarboxylic acid, CA = (1R,3S)-(+)-camphoric acid and OBBA = 4,4′-oxybisbenzoic acid. Structural analysis reveals that 1 and 2 are two dimensional (2D) layered structures having interesting sql and hxl topologies respectively with trinuclear SBUs (secondary building units). Compound 3 has a 3D structure, whereas 4 has a 2-fold interpenetrated 3D packing structure with a paddlewheel dinuclear SBU and both have pcu topology. Magnetic investigation revealed that 1, 3 and 4 show dominant antiferromagnetic behavior, while 2 shows ferromagnetic interaction at very low temperature. Interestingly 4 shows a sharp decrease in the χ_MT value from room temperature and this may be because of the direct Co(II)⋯Co(II) interaction. Gas sorption studies reveal that 1, 2 and 3 show surface areas of 11.8 m² g⁻¹, 8.3 m² g⁻¹ and 28.5 m² g⁻¹ respectively and better adsorption behavior for CO₂ over CH₄, whereas 4 is nonporous in nature due to its 2-fold interpenetrated structure.