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 (H<sub>3</sub>CAM) and pyridine-2,6-dicarboxylic acid (H<sub>2</sub>PDA) with Zn(II) salts under hydrothermal conditions gave two novel coordination polymers {[Zn(HCAM)]·H<sub>2</sub>O}<i><sub>n</sub></i> (<b>1</b>) and {[Zn(PDA)(H<sub>2</sub>O)<sub>1.5</sub>]}<i><sub>n</sub></i> (<b>1a</b>). <b>1</b> and <b>1a</b> 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 H<sub>3</sub>CAM and H<sub>2</sub>PDA with Nd<sub>2</sub>O<sub>3</sub> in the M/L ratio 2:3 gave {[Nd<sub>2</sub>(HCAM)<sub>3</sub>(H<sub>2</sub>O)<sub>4</sub>]·2H<sub>2</sub>O}<i><sub>n</sub></i> (<b>2</b>) and {[Nd<sub>2</sub>(PDA)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>]·0.5H<sub>2</sub>O}<i><sub>n</sub></i> (<b>2a</b>). In <b>2</b>, a square motif as a building block constructed by four Nd(III) ions was further assembled into a highly ordered 2D (4,4) grid. <b>2a </b>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)(H<sub>2</sub>O)<sub>3</sub>]·H<sub>2</sub>O}<i><sub>n</sub></i> (Ln = Gd, <b>3</b>; Dy, <b>4</b>; Er, <b>5</b>) for H<sub>3</sub>CAM and {[Gd<sub>2</sub>(PDA)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>]·H<sub>2</sub>O}<i><sub>n</sub></i> (<b>3a</b>) for H<sub>2</sub>PDA. <b>3</b>−<b>5</b> are 2D coordination polymers with a 3<sup>3</sup>4<sup>2</sup> uniform net, where hydroxyl groups of H<sub>3</sub>CAM coordinate with metal ions. The reaction of H<sub>3</sub>CAM and Er<sub>2</sub>O<sub>3</sub> instead of Er(ClO<sub>4</sub>)<sub>3</sub> produced {[Er<sub>2</sub>(HCAM)<sub>3</sub>(H<sub>2</sub>O)<sub>4</sub>]·2H<sub>2</sub>O}<i><sub>n</sub></i> (<b>6</b>). The compounds <b>2a</b> and <b>3a</b>, <b>2</b> and <b>6</b> are isomorphous. The stereochemical and supramolecular effects of hydroxyl groups result in the dramatic structural changes from 1D (<b>1a</b>) to 2D (<b>1</b>) and from 2D (<b>2</b>) to 3D (<b>2a</b>). When Ln(III) salts instead of Ln<sub>2</sub>O<sub>3</sub> were employed in the hydrothermal reactions with H<sub>3</sub>CAM, different self-assembly processes gave the products of different metal/ligand ratio with reactants (<b>3</b>−<b>5</b>).