Novel Three-Dimensional Pillared-Layer Ln(III)−Cu(I) Coordination Polymers Featuring Spindle-Shaped Heterometallic Building Units

The hydrothermal reaction of rare earth oxide, CuO, 2,6-pyridinedicarboxylic acid, and 4,4′-bipyridine in the presence of H3PO3 resulted in the formation of a new series of 3d-4f heterometallic coordination polymers [Ln(pydc)3Cu3(bipy)3·m(H2O)]n (Ln = Pr (1), Nd (2), m = 5; Ln = Sm (3), Eu (4), Gd (5), Tb (6), Er (7), Yb (8), m = 4; pydc = 2,6-pyridinedicarboxylate anion; bipy = 4,4′-bipyridine). Complexes 1−8 are isostructural and structurally characterized by elemental analysis, FT-IR spectroscopy, thermogravimetry-differential thermal analysis (TG-DTA), single-crystal X-ray diffraction, X-ray powder diffraction (XRPD), and nitrogen adsorption/desorption techniques. The synthesis results show that the addition of H3PO3 in the reaction plays an important role in the formation of the compounds. Single-crystal X-ray diffraction analysis reveals that the heterometallic ions are first interconnected by mixing bridging ligands to produce a spindle-shaped heterometallic ring [Ln6(pydc)6Cu12(bipy)6], which is used as the second building unit (SBUs) and finally pillared by bridging bipy molecules to form the rare 3D pillared-layer porous Ln(III)−Cu(I) coordination polymers. Luminescence measurements made under excitation by UV rays reveal that Sm−Cu, Eu−Cu, and Tb−Cu compounds exhibit the characteristic emission bands of Sm3+, Eu3+, and Tb3+ ions in the visible regions, respectively; near-infrared (NIR) emission bands from Nd(III) and Yb(III) ions can also be obtained in Nd−Cu and Yb−Cu compounds, respectively; while Pr−Cu, Gd−Cu, and Er−Cu compounds all display similar emission spectra of Cu(I) coordination compounds in the visible regions.