Magnetic refrigeration and slow magnetic relaxation in tetranuclear lanthanide cages (Ln = Gd, Dy) with in situ ligand transformation

Abstract

Three new Ln³⁺ coordination compounds having formulae, [Gd₄(μ₃-OH)₂(L)₂L₁L₂(HOCH₃)₂]·11H₂O (1), [Dy₄(μ₃-OH)₂(L)₂L₁L₂(H₂O)₂]·11H₂O (2) and [Dy₄(μ₄-O)(OMe)(HOMe)₂(CH₃COO)₃(L₃)₂]·2H₂O (3), have been synthesized in a one pot synthesis from O-vanillin, diaminomaleonitrile (DAMN), LnCl₃·6H₂O (Ln = Gd³⁺, Dy³⁺) and sodium acetate for 3, {H₂L = 2,3-bis((E)-(2-hydroxy-3-methoxy benzylidene) amino)maleonitrile, HL₁ = (2-amino-3-((E)-(2-hydroxy-3-methoxy benzylidene)amino)maleonitrile), H₃L₂ = ((1E,3Z,8Z,10E)-1,6,11-tris(2-hydroxy-3-methoxyphenyl)-2,5,7,10-tetraazaundeca-1,3,8,10-tetraene-3,4,8,9-tetracarbonitrile) and H₂L₃ = 2-((cyano(2-hydroxy-3-methoxyphenyl)methyl)amino)-3-((E)-(2-hydroxy-3-methoxybenzylidene)amino)maleonitrile. Single-crystal X-ray diffraction studies reveal that compounds 1 and 2 are quasi-isostructural, exhibiting tetranuclear hemicubane-like cores. For 3 the metal centers are arranged in a tetrahedral arrangement. Complexes 1–3 were formed with the ligands (L₁–L₃), which resulted in situ during synthesis. Magnetic studies reveal that compound 1 shows significant magnetocaloric effect (ΔS_m = −27.2 J kg⁻¹ K⁻¹) at 3 K and 7 T. The magnetic properties of 2 and 3 are considerably different. Indeed, no out-of-phase alternating current (ac) signal is noticed for 2, whereas 3 shows a slow relaxation of magnetization. These differences are most likely due to the different Dy–O–Dy angles observed for the respective cores.