Two- and Three-Dimensional Lanthanide Complexes:  Synthesis, Crystal Structures, and Properties

3,5-Pyrazoledicarboxylic acid (H<sub>3</sub>L) reacts with nitrate salts of lanthanide(III) (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) under hydrothermal conditions to form a series of lanthanide polymers <b>1</b>−<b>9</b>. These nine polymers have the same crystal system of monoclinic, but they exhibit three different kinds of metal−organic framework structures. The complexes {[Ln<sub>2</sub>(HL)<sub>3</sub>(H<sub>2</sub>O)<sub>4</sub>]·2H<sub>2</sub>O}<i><sub>n</sub></i> (Ln = Pr (<b>1</b>), Nd (<b>2</b>), and Sm (<b>3</b>)) were isostructural and exhibited porous 3D frameworks with a <i>Cc</i> space group. The complexes {[Ln<sub>2</sub>(HL)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>]·3H<sub>2</sub>O}<i><sub>n</sub></i> (Ln = Eu (<b>4</b>), Gd (<b>5</b>), and Tb (<b>6</b>)) were isostructural and built 2D double-decker (2DD) frameworks with a <i>P</i>2<sub>1</sub>/<i>c</i> space group. The complexes {[Ln(HL)(H<sub>2</sub>L)(H<sub>2</sub>O)<sub>2</sub>]}<i><sub>n</sub></i> ((Ln = Dy (<b>7</b>), Ho (<b>8</b>), and Er (<b>9</b>)) were also isostructural and formed 2D monolayer (2DM) frameworks with a <i>P</i>2<sub>1</sub>/<i>n</i> space group. The structure variation from the 3D porous framework to the 2D double-decker to the 2D monolayer is attributed to the lanthanide contraction effect. Notably, six new coordination modes of 3,5-pyrazoledicarboxylic acid were observed, which proved that 3,5-pyrazoledicarboxylic acid may be used as an effective bridging ligand to assemble lanthanide-based coordination polymers. The photophysical and magnetic properties have also been investigated.