Lanthanoid Coordination Polymers Based on Homoditopic Picolinate Ligands: Synthesis, Structure and Magnetic Properties

A ditopic ligand (H2L1), containing picolinate subunits segmented by ethynylene bridges, has been used in the synthesis of a series of isostructural coordination polymers, formulated as [(CH3)2NH2][Ln(L1)2]·H2O·CH3COOH, where Ln = Eu (1), Gd (2), Tb (3), Dy (4) and Ho (5). The single-crystal structures show that these compounds crystallise in the orthorhombic Pna21 space group and form 3D anionic lattices with triangular cavities. AC magnetic susceptibility measurements show that the Gd, Tb and Dy derivatives (2–4) present a slow relaxation in their magnetisation under an applied DC magnetic field. The detailed study of the AC susceptibility in compounds 2 and 4 shows that they relax following direct and Orbach mechanisms under these conditions. The Dy derivative (4) retains this behaviour in the absence of an external field, relaxing via quantum tunnelling and Orbach mechanisms. Compound 2 is one of the very few reported Gd(III) compounds showing slow relaxation in its magnetisation.