Magnetocaloric effect and slow magnetic relaxation in peroxide-assisted tetranuclear lanthanide assemblies

Abstract

The synthesis, structure and magnetism of tetranuclear complexes, [Ln₄(LH₂)₂(η¹-Piv)₂(η²-Piv)₂(μ₃-O₂)₂(H₂O)₂]·2MeOH (1, Ln = Gd^III; 2,Ln = Tb^III; 3, Ln = Dy^III; and 4, Ln = Er^III) (LH₄ = 6-((bis(2-hydroxyethyl)amino)-N′-(2-hydroxybenzylidene)picolinohydrazide) (piv = pivalate ion) are reported. Interestingly, the formation of these tetranuclear assemblies is assisted by two side-on coordinating peroxide ligands which bind in a μ₃-η²:η²:η² fashion. DC magnetic behaviour revealed the existence of a dominating but weak intramolecular antiferromagnetic interaction in the cases of 1, 2, and 4, while a small structural change in complex 3 turns this into a ferromagnetic interaction. We have investigated the magnetothermal behaviour of 1 which shows a magnetocaloric effect with the value of maximum entropy change of −ΔS_m = 33.60 J K⁻¹ kg⁻¹ at 4 K (ΔH = 13–0 T). This is close to the calculated value of 36.45 J K⁻¹ kg⁻¹. The ac magnetic susceptibility study confirms zero-field out-of-phase magnetic susceptibility signals only for 3 which get well resolved on the application of a 1 kOe dc magnetic field. The energy barrier for the Orbach process of spin reversal was found to be 23 K (τ₀ = 9 × 10⁻⁷ s). We also report detailed theoretical studies to rationalize and understand the observed magnetic behaviour.