Self-Assembled Ln(III)₄ (Ln = Eu, Gd, Dy, Ho, Yb) [2 × 2] Square Grids: a New Class of Lanthanide Cluster

Self-assembly of the Ln­(III) ions (Ln = Eu, Gd, Dy, Ho, Yb) into square [2 × 2] grid-like arrays has been readily effected using simple, symmetric ditopic ligands based on a carbohydrazone core. The metal ions are connected via single atom bridges (e.g., μ₂-O_hydrazone, μ₂-OH, μ₂-OMe, μ₂-1,1-N₃ ^–, μ₄-O), depending on reaction conditions. The Gd­(III)₄ examples exhibit intramolecular antiferromagnetic exchange (−J < 0.11 cm^–1), and in one Dy­(III)₄ example, with a combination of μ₂-1,1-N₃ ^–, and μ₄-O bridges linking adjacent metal ions, SMM behavior is observed. One thermally driven relaxation process is observed in the temperature range 10–25 K (τ₀ = 6.5(1) × 10^–7 s, U _eff = 110(1) K) in the presence of an 1800 Oe external field, employed to suppress a second quantum based relaxation process. The extended group of Ln­(III) ions which submit to this controlled self-assembly, typical of the transition metal ions, indicates the general applicability of this approach to the lanthanides. This occurs despite the anticipated limitations based on larger ionic radii and coordination numbers, and is an encouraging sign for extension to larger grids with appropriately chosen polytopic ligands.