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Synthesis and Characterization of Metal−Organic Frameworks Based on 4-Hydroxypyridine-2,6-dicarboxylic Acid and Pyridine-2,6-dicarboxylic Acid Ligands

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posted on 24.07.2006, 00:00 by Hong-Ling Gao, Long Yi, Bin Zhao, Xiao-Qing Zhao, Peng Cheng, Dai-Zheng Liao, Shi-Ping Yan
The self-assembly of 4-hydroxypyridine-2,6-dicarboxylic acid (H3CAM) and pyridine-2,6-dicarboxylic acid (H2PDA) with Zn(II) salts under hydrothermal conditions gave two novel coordination polymers {[Zn(HCAM)]·H2O}n (1) and {[Zn(PDA)(H2O)1.5]}n (1a). 1 and 1a comprise of a 2D (4,4) net and a 1D zigzag chain, respectively, in which a new coordination mode of PDA is found. The reactions of H3CAM and H2PDA with Nd2O3 in the M/L ratio 2:3 gave {[Nd2(HCAM)3(H2O)4]·2H2O}n (2) and {[Nd2(PDA)3(H2O)3]·0.5H2O}n (2a). In 2, a square motif as a building block constructed by four Nd(III) ions was further assembled into a highly ordered 2D (4,4) grid. 2a is a 3D microporous coordination polymer. It is interesting to note that, when Ln(III) salts rather than oxides were employed, the reaction produced {[Ln(CAM)(H2O)3]·H2O}n (Ln = Gd, 3; Dy, 4; Er, 5) for H3CAM and {[Gd2(PDA)3(H2O)3]·H2O}n (3a) for H2PDA. 35 are 2D coordination polymers with a 3342 uniform net, where hydroxyl groups of H3CAM coordinate with metal ions. The reaction of H3CAM and Er2O3 instead of Er(ClO4)3 produced {[Er2(HCAM)3(H2O)4]·2H2O}n (6). The compounds 2a and 3a, 2 and 6 are isomorphous. The stereochemical and supramolecular effects of hydroxyl groups result in the dramatic structural changes from 1D (1a) to 2D (1) and from 2D (2) to 3D (2a). When Ln(III) salts instead of Ln2O3 were employed in the hydrothermal reactions with H3CAM, different self-assembly processes gave the products of different metal/ligand ratio with reactants (35).