Four new 3D metal–organic frameworks constructed by the asymmetrical pentacarboxylate: gas sorption behaviour and magnetic properties

Abstract

By using an asymmetrical rigid pentacarboxylic acid ligand, 2,4-di(3′,5′-dicarboxylphenyl)benzoic acid (H₅L), four new three-dimensional (3D) metal–organic frameworks (MOFs), namely {[Cu₂(HL)(H₂O)₂]·2DMF·2H₂O}_n (1), {[Co₂(L)(DMA)]·H₂N(Me)₂}_n (2), {[Co₂(L)(H₂O)]·H₂N(Me)₂}_n (3), {[Mn₂(L)(DMF)(H₂O)]·H₂N(Me)₂}_n (4), were solvothermally synthesized. H₅L in 1–4 shows different coordination modes and can easily form various metal clusters (secondary building units, SBUs) in the final structures. 1 is a 3D porous framework with a (4,4)-connected pts topology based on the [Cu₂(COO)₄] paddlewheel SBU, wherein six SBUs are connected by twelve HL⁴⁻ to get an unprecedented Cu₁₂ hendecahedron nanocage. 2–4 possess similar dinuclear [M₂(COO)₅] SBUs (M = Co, Mn), which are further extended by L⁵⁻ to give rise to 3D frameworks with the uncommon (5,5)-connected nia-5,5-P2₁/c and bnn topologies. In addition, the desolvated framework of 1 contains polar channels decorated with uncoordinated carboxylate groups, leading to selective adsorption for CO₂ over CH₄ at 195, 273 and 298 K. Moreover, the magnetic properties of 1–4 show that there exist antiferromagnetic interactions between metal ions.