Synthesis, structure, photoluminescence and magnetic properties of new 3-D lanthanide-pyridine-2,4,6-tricarboxylate frameworks

Abstract

Seven 3-D (3D) metal–organic framework solids [Nd₂(pyta)₂(H₂O)₆]_n1 and {[Ln₂(pyta)₂(H₂O)_x]·yH₂O}_n (Ln = Sm 2, Eu 3, Gd 4 with x = 3, y = 2.5; Ln = Tb 5 with x = 2, y = 2.5, Ln = Dy 6, Ho 7 with x = 2, y = 2), H₃pyta = 2,4,6-pyridinecarboxylic acid) have been prepared under hydrothermal conditions. X-Ray structural analysis shows that compound 1 crystallises in monoclinic space groupPc and all Ln(III) ions are nine-coordinated. The 3D framework of 1 is constructed from [Nd₄(µ₄-pyta)₂]_n chains with 1D small channels along the a axis. Isomorphous compounds 2–7 crystallise in C2/c space group and are composed of alternative [Ln₄(µ₅-pyta)₂]_n chains and [Ln₆(µ₄-pyta)₂]_n helical chains. The completely deprotonated pyta³⁻ ligands adopt µ₄-bridging coordination modes in 1, and µ₄-, µ₅-bridging modes in 2–7. The Sm(III) 2, Eu(III) 3, Tb(III) 5 and Dy(III) 6 compounds exhibit characteristic photoluminescence (PL) in the visible region and the Nd(III) 1 and Ho(III) 7 compounds show photoluminescence in the near-infrared (NIR) region. The magnetic properties of three heavy lanthanide ions Gd(III) 4, Dy(III) 6 and Ho(III) 7 compounds were investigated in the temperature range of 2–300 K.