A New Three-dimensional Lanthanide Framework Constructed by Oxalate and 3,5-pyridinedicarboxylate

Abstract

ABSTRACTHydrothermal reactions of rare-earth metal acetates and nitrates with oxalic acid (H2C2O4) and 3,5-pyridinedicarboxylic acid (H2pddc) in triethylamine/water solutions have yielded five isostructural polymers with the general formula: [Ln(pddc)(C2O4)1/2(H2O)2]·H2O. [Ln = La(1), Pr(2), Nd(3), Eu(4) and Er(5)]. These compounds crystallize in monoclinic crystal system, space group P21/n, Z = 4, with only slight variations in their unit cell parameters: 1 a = 7.747(2), b = 9.954(2), c = 15.134(3) Å, = 98.64(3)°, V = 1153.8(4) Å3; 2 a = 7.707(2), b = 9.895(2), c = 15.006(3) Å, β = 98.54(3)°, V = 1131.7(4) Å3; 3 a = 7.688(2), b = 9.897(2), c = 14.955(3) Å, β = 98.43(3)°, V = 112 5.6(4) Å3; 4 a = 7.638(2), b = 9.842(2), c = 14.809(3) Å, β = 98.42(3)°, V = 1101.2(4) Å3; 5 a = 7.573(2), b = 9.761(2) Å, c = 14.630(3) Å, β = 98.10(3)°, V = 1070.7(4) Å3. The structure of these compounds is composed of 2D Ln(pddc) layers that are interconnected by chelating oxalate. Within the layer, each rare-earth metal forms a monodentate bond with each of the four pddc groups. The metal centers in the neighboring layers are bridged through µ4-oxalate, resulting in a three-dimensional framework. The remaining two sites around the eight-coordinate Ln are occupied by water molecules. Compounds 2, 3, and 5 exhibit paramagnetic behavior and 1 is diamagnetic. They are thermally stable up to 250°C.