Four MOFs with 2,2′-dimethoxy-4,4′-biphenyldicarboxylic acid: Syntheses, structures, topologies and properties

Four novel metal-organic frameworks, [MnL(DMF)]_n (1), [NiL(H₂O)₂(DMF)]_n (2), [Cu₂L ₂(H₂O)₂]·6H₂O·2DMF (3), and Na₂(Me₂NH₂)₄[Cd ₇L₁₀]·9H₂O·17Me ₂NH·2DMF (4), were successfully synthesized in solvothermal conditions using 2,2′-dimethoxy-4,4′-biphenyldicarboxylic acid (H₂L) as the ligand. The X-ray crystallography analysis reveals that MOF 1 exhibits a 3D framework with sra topology, while MOF 2 shows a 2D network with sql topology though they possess a similar secondary building unit (SBU) [MO₆] (M = Mn for 1 and Ni for 2). MOF 3 reveals an unprecedented 4-fold interpenetrated 3D framework with lvt topology constructed from only planar 4-connected nodes ([Cu₂(CO₂)₄] SBUs) with the rigid dicarboxylic acids. MOF 4 features an unique acentric 3D/3D hetero-interpenetrated framework built from two different nodes (4- and 8-connected), different chemical compositions [Cd(CO₂)₄ and Cd₃(CO₂)₈], and different topology of networks (6⁶ and 4²⁴·6⁴) and contains an inorganic 1D chain consisting of a Na⁺ and three Cd²⁺ ions. The solid state ¹¹³Cd NMR spectrum indicates that 4 has three 6-, 7-, and 8-coordinated Cd²⁺ ions with chemical shifts at δ -25.3, -41.2 and -83 ppm, respectively. MOF 3 has two kinds of 1D channels with diameters of 11 Å and 7 Å and exhibits a selective gas adsorption for CO₂-N₂ and CH₄-N₂. MOF 4 shows a selective adsorption for cationic dyes and a nonlinear SHG activity (ca. 0.8 times that of urea).