Self-assembly of rare octanuclear quad(double-stranded) cluster helicates showing slow magnetic relaxation and the magnetocaloric effect

Abstract

Four rare quad(double-strand) cluster helicates containing Ln8 polyhedron cores, namely [Ln₈(HL)₄L₄(OH)₆(H₂O)₂(acac)₂](ClO₄)₄·2CH₃OH·12H₂O (Ln = Gd for 1, Tb for 2, Dy for 3, and Ho for 4, respectively), have been assembled by using lanthanide salts and flexible ligands of H₂L obtained from a 1 : 2 condensation reaction between 1,1′-(2,6-pyridyl)bis-1,3-butanedione and 2-aminophenol. In each complex, four pairs of organic ligands (HL⁻ and L²⁻) wrap the Ln8 polyhedron presenting a bicapped face-sharing double cubane configuration in a propeller-like form, resulting in the formation of octanuclear quad(double-stranded) helicates by coordination interactions of Ln–O/N bonds. The dc magnetic susceptibility measurements of complex 1 suggested the existence of weak antiferromagnetic couplings between neighboring Gd^III ions and the magnetocaloric effect with −ΔS^max_m = 22.9 J kg⁻¹ K⁻¹ at 3.5 K for ΔH = 7 T. Ac magnetic susceptibility measurements of complex 3 at zero dc field demonstrated a slow magnetic relaxation behavior below 8.0 K with an effective energy barrier of 2.14 K.