Hydrothermal Syntheses, Crystal Structures, and Magnetic Properties of a Series of Unique Three-Dimensional Lanthanide-Organic Coordination Frameworks with a N-Protonated 2,6-Dihydroxypyridine-4-Carboxylate Tecton

A series of novel three-dimensional (3-D) lanthanide-organic frameworks (LnOFs), with the general formula [Ln(2)(H-L)(3)(H2O)(4)](infinity) (Ln = Dy-III for 1, Gd-III for 2, Tb-III for 3, Eu-III for 4, and Ho-III for 5; H-L = N-protonated 2,6-dihydroxypyridine-4-carboxylate), have been synthesized under hydrothermal conditions. X-ray structural analysis reveals that complexes 1-5 are isostructural and show the unique 3-D coordination framework with a trinodal (3,4,5)-connected (4.6(2))(4(2).6)(4(2).8(4))(4(3).6.8(6))(4(2).6(5).8(3)) net topology, in which the H-L ligands adopt different mu(4)- and mu(3)-bridging forms. Variable-temperature magnetic susceptibility studies reveal that complexes 1-3 and 5 show ferromagnetic behaviors. Notably, complex 1 (Dy-III) is the first instance leading to ferromagnetic coupling for a lanthanide ion through a spin-polarization mechanism. The alternating current (ac) signal observed for 1 and 5 should originate from the long-range ferromagnetic ordering, or it could reflect the splitting of the J multiplet for Dy-III or Ho-III under a low symmetry crystal field, which would give rise to slow relaxation of the magnetization. Additionally, thermal stability of these crystalline materials has also been investigated by thermogravimetric analysis of mass loss.