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

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. 3−5 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 (3−5).