Peroxide Ligands Support Tetranuclear Lanthanide Ensembles: Synthesis, Structure, Magnetism, and Theoretical Studies

We report the synthesis, structure, magnetism, and theoretical studies of the peroxide-assisted formation of tetranuclear assemblies, [Ln₄(LH₂)₂(η¹-NO₃)₂(μ₃-O₂)₂(DMF)₄]·2NO₃·2DMF; 1, Ln = Dy(III); 2, Ln = Er(III); 3, Ln = Gd(III); DMF = dimethylformamide, LH₄ = (E)-6-((bis(2-hydroxyethyl)amino)methyl)-N'-(2-hydroxy-3-methoxybenzylidene)picolinohydrazide. In these complexes, the peroxide ions coordinate in a side-on manner to metal centers in a μ₃-η²:η²:η² fashion. DC magnetic behavior revealed the presence of weak intramolecular antiferromagnetic interactions taking place between the Gd(III) ions in 3, while additional ferromagnetic interactions are highlighted in 1. The ac susceptibility measurements demonstrated a field-induced single-molecule magnet (SMM) behavior of both Dy(III) and Er(III) analogues. 1 showed a multirelaxation mechanism involving mainly Raman and direct processes. Finally, the Gd(III) analogue, 3, displayed a magnetocaloric effect with the value of maximum entropy change of -ΔS = 27.93 J K^-1 kg^-1 at 4 K and ΔH = 0-13 T. Detailed ab initio CASSCF and DFT calculations were performed on these complexes to elucidate their observed magnetic behavior. Calculations suggest that the peroxide bridges support the anisotropic nature of oblate Dy(III) ions and lead to the SMM behavior for 1; however, they do not support the anisotropic nature of prolate Er(III) ions in 2.