Structures, Luminescence, and Magnetic Properties of Several Three-Dimensional Lanthanide–Organic Frameworks Comprising 4‑Carboxyphenoxy Acetic Acid

Ten new 3D lanthanide-based compounds, {[Ln₄(CPOA)₆(H₂O)₄]·H₂O}<i>_n</i> (Ln = Eu (<b>1</b>), Gd (<b>2</b>), Tb (<b>3</b>), Dy (<b>4</b>), Ho (<b>5</b>)) and [Ln­(CPOA)­(C₂O₄)_0.5(H₂O)]<i>_n</i> (Ln = Eu (<b>1a</b>), Gd (<b>2a</b>), Tb (<b>3a</b>), Dy (<b>4a</b>), Ho (<b>5a</b>)) (CPOA^2– = 4-carbomethoxybenzoate), have been synthesized by a hydrothermal method under different temperatures. The compounds <b>1</b>–<b>5</b> were obtained at 140 °C. When the temperature went up to 180 °C, the ligands partly decomposed to C₂O₄^2–, then came into being compounds <b>1a</b>–<b>5a</b>. The two series of compounds displayed the differences not only in structure but also in thermal stability and luminescent and magnetic properties. Compounds <b>1</b>–<b>5</b> crystallize in the triclinic system, space group <i>P</i>1̅, whereas compounds <b>1a</b>–<b>5a</b> belong to the monoclinic system with space group <i>P</i>2₁/<i>c</i>. Thermal analyses were carried out on <b>1</b>–<b>5</b> and <b>1a</b>–<b>3a</b>. Solid luminescence studies of compounds <b>1</b> and <b>1a</b>, <b>3</b> and <b>3a</b>, and <b>4</b> and <b>4a</b> show the characteristic bands of Eu³⁺, Tb³⁺, and Dy³⁺, respectively. Magnetic properties were investigated on <b>2</b>–<b>4</b> and <b>2a</b>–<b>4a</b>. The out-of-phase signals (χ_M″) of <b>4</b> exhibit a strong frequency dependence, suggesting the existence of slow magnetic relaxation behavior in <b>4