One-, Two-, and Three-Dimensional Lanthanide Complexes Constructed from Pyridine-2,6-dicarboxylic Acid and Oxalic Acid Ligands

The reaction of pyridine-2,6-dicarboxylic acid (H2PDA) and oxalic acid (H2ox) with Ln2O3/Ce(NO3)3 under hydrothermal conditions generated a series of new one-, two-, and three-dimensional (1D, 2D, and 3D) coordination polymers, namely, {[Ln(PDA)(ox)0.5(H2O)3]·H2O} n (Ln = La(1) and Ce(2)), {[Ln2(PDA)(ox)2·4(H2O)]} n (Ln = Nd(3) and Sm(4)), {[Ln2(PDA)(ox)2·7(H2O)]} n (Ln = Eu(5), Tb(6), and Er(7)) (PDA2− = pyridine-2,6-dicarboxylate anion, ox2− = oxalate anion). The complexes were characterized by X-ray single-crystal determination, spectroscopy and fluorescent analyses. Compounds 1 and 2 (type I structure) are 1D zigzag chain structures bridged by bis-bidentate oxalate. The 1D chains are further linked by the significant intermolecular π···π interactions between pyridine rings of coordinating PDA2− groups to form a 2D supramolecular framework network. In compounds 3 and 4 (type II structure), the square motifs formed by four Ln(III) ions, serving as the building blocks, are assembled into a highly ordered 2D (4,4) grid. Similarly, via weak π···π interactions between two central pyridine rings from two adjacent sheets, the 2D layers are further stacked up into the final 3D structures. Complexes 5, 6, and 7 (type III structure) are isomorphous and have the same 3D network structures fabricated through two alternately arranging hexanuclear Ln3+ (Ln = Eu(5), Tb(6) and Er(7)) units, and the guest water molecules are trapped in the cavity. The progressive structural variation from the 1D zigzag chains (1−2) to 2D networks (3−4) and to 3D frameworks (5, 6, and 7) is attributed to the lanthanide contraction effect and the different coordination modes of PDA2−. Two new coordination modes of pyridine-2,6-dicarboxylic acid are observed, which proved that pyridine-2,6-dicarboxylic acid could be used as an effective bridging ligand to construct lanthanide-based coordination polymers. The fluorescent properties of compounds 3, 5, and 6 have also been investigated.