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).

在热水解条件下，4-羟基吡啶-2,6-二羧酸（H3CAM）与吡啶-2,6-二羧酸（H2PDA）与Zn(II)盐的自身组装产生了两种新型配位聚合物{[Zn(HCAM)]·H2O}n（1）和{[Zn(PDA)(H2O)1.5]}n（1a）。其中，1由二维（4,4）网状结构和一维之字形链组成，而1a则发现了一种新的PDA配位模式。H3CAM和H2PDA与Nd2O3在M/L比率为2:3的反应中，生成了{[Nd2(HCAM)3(H2O)4]·2H2O}n（2）和{[Nd2(PDA)3(H2O)3]·0.5H2O}n（2a）。在2中，由四个Nd(III)离子构成的方形基元进一步组装成高度有序的二维（4,4）网格。2a是一种三维微孔配位聚合物。值得注意的是，当使用镧系金属盐而非氧化物时，与H3CAM的反应产生了{[Ln(CAM)(H2O)3]·H2O}n（Ln = Gd, 3; Dy, 4; Er, 5）和对于H2PDA，生成了{[Gd2(PDA)3(H2O)3]·H2O}n（3a）。3-5为具有3342均匀网状的二维配位聚合物，其中H3CAM的羟基与金属离子配位。H3CAM与Er2O3而非Er(ClO4)3的反应生成了{[Er2(HCAM)3(H2O)4]·2H2O}n（6）。化合物2a和3a，2和6为同质异构体。羟基的立体化学和超分子效应导致了从一维（1a）到二维（1）以及从二维（2）到三维（2a）的显著结构变化。当在热水解反应中使用镧系金属盐而非镧氧化物与H3CAM反应时，不同的自组装过程产生了与反应物（3-5）不同的金属/配体比率。