Four orotate-based lanthanide coordination polymers with rhomboid [Ln4(μ3-OH)2]-based 1D chainlike structure exhibiting luminescence and large magnetocaloric effect

Abstract

Four isostructural lanthanide coordination polymers with orotic acid (H3OA) as building ligand, {[Ln(HOA)(μ3-OH)(H2O)2]·H2O}n (Ln = Eu (1); Gd (2); Tb (3); Dy (4)), were synthesized under hydrothermal conditions. Single-crystal X-ray diffraction analyses demonstrate that complexes 1–4 have an one-dimensional (1D) chainlike structure consisting of rhomboid [Ln4(μ3-OH)2] unit and HOA2- linker. Magnetic studies indicate that 2 displays a large magnetocaloric effect with magnetic entropy value of 37.11 J kg−1 K−1 at 2.0 K for ΔH = 7.0 T. This is a considerable value among the reported molecular magnetic coolants, and has a great application potential in the field of ultra-low temperature refrigeration. In addition, the solid-state photophysical properties of 1 and 3 exhibit obvious characteristic Eu(III) and Tb(III) photoluminescence emission in the visible region, suggesting that Eu- and Tb-based luminescence were sensitized effectively by the energy transfer from the ligand to the metal centers.