Lanthanide-Base Helical Chain Constructed by In Situ Schiff Base Reaction: Structures and Magnetic Properties

Schiff base ligands with multiple chelating coordination sites are usually able to quickly capture lanthanide metal ions to form complexes, and it is difficult to twist and construct helical chains. In this work, we used 2-hydroxy-1-naphthaldehyde, propylenediamine, and Ln(NO3)3·6H2O to react at 80 °C in solvothermal to obtain chains [Ln = Dy (1); Ln = Gd (2)] with a helical structure. It is worth noting that 2-hydroxy-1-naphthaldehyde and propylenediamine undergo an in situ reaction under one-pot conditions to form ligand 1,1′-((1E,1′E)-(propane-1,3-diylbis(azaneylylidene))bis(methaneylylidene))bis(naphthalen-2-ol) (H2L). The ligand (L)2− bridges the two metal center Ln(III) ions with a monodentate, causing it to twist into an “S” shape and further form a helical chain. In addition, the independent unit of the helical chain is a single core, and its metal center is in the O9 coordination environment provided by the ligand and the end group coordination NO3− ions. What’s more noteworthy is that chain 1 exhibits single-molecule magnet (SMM) behavior, and its SMM performance is significantly improved under the DC field induction condition of 600 Oe. By Debye model fitting, the effective energy barrier of 1 is 17.8 K, and the relaxation time is 2.4 × 10–6 s. The magnetocaloric effect test of chain 2 is further carried out. When ΔH = 7 T and T = 2 K, its − ΔSm reaches the maximum value of 16.19 J K−1 kg−1.