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).

在水热条件下，4-羟基吡啶-2,6-二羧酸（4-hydroxypyridine-2,6-dicarboxylic acid，H3CAM）与吡啶-2,6-二羧酸（pyridine-2,6-dicarboxylic acid，H2PDA）分别与锌(II)盐进行自组装反应，得到两种新型配位聚合物{[Zn(HCAM)]·H2O}n（1）与{[Zn(PDA)(H2O)1.5]}n（1a）。1与1a分别具有二维(4,4)网格与一维锯齿链结构，研究中发现了PDA的一种新型配位模式。将H3CAM与H2PDA按金属/配体（M/L）摩尔比2:3与氧化钕（Nd2O3）进行反应，分别得到{[Nd2(HCAM)3(H2O)4]·2H2O}n（2）与{[Nd2(PDA)3(H2O)3]·0.5H2O}n（2a）。在化合物2中，由四个Nd(III)离子构成的方形基元作为结构基元，进一步组装为高度有序的二维(4,4)网格。2a则为三维微孔配位聚合物。值得注意的是，当采用镧系(III)盐（Ln(III) salts）而非氧化物作为反应物时，针对H3CAM的反应产物为{[Ln(CAM)(H2O)3]·H2O}n（Ln=Gd，3；Dy，4；Er，5），针对H2PDA的反应产物为{[Gd2(PDA)3(H2O)3]·H2O}n（3a）。3~5均为具有3342均匀网格的二维配位聚合物，其中H3CAM的羟基与金属离子发生配位作用。若将高氯酸铒（Er(ClO4)3）替换为氧化铒（Er2O3）与H3CAM进行反应，则得到{[Er2(HCAM)3(H2O)4]·2H2O}n（6）。化合物2a与3a、2与6分别互为同构体。羟基的立体化学效应与超分子效应引发了显著的结构演变：从一维结构（1a）转变为二维结构（1），以及从二维结构（2）转变为三维结构（2a）。当在H3CAM的水热反应中使用镧系(III)盐（Ln(III) salts）而非Ln2O3作为反应物时，不同的自组装过程会得到金属/配体比例与反应物不同的产物（3~5）。