Magnetic and magnetocaloric properties of an unusual family of carbonate-panelled [Ln III6Zn II2] cages

Abstract

The reaction of the pro-ligand H₄L, which combines the complementary phenolic oxime and diethanolamine moieties within the same organic framework, with Zn(NO₃)₂·6H₂O and Ln(NO₃)₃·6H₂O in a basic methanolic solution generates a family of isostructural heterometallic coordination compounds of general formula [Ln₆Zn₂(CO₃)₅(OH)(H₂L)₄(H₃L)₂(H₄L)]NO₃·xMeOH [Ln = Gd, x = 30 (1), Ln = Dy, x = 32 (2), Ln = Sm, x = 31 (3), Ln = Eu, x = 29 (4), Ln = Tb, x = 30 (5)]. The octametallic skeleton of the cage describes a heavily distorted [Gd^III₆] octahedron capped on two faces by Zn^II ions. The metal core is stabilised by a series of μ₃- and μ₄-CO₃²⁻ ions, originating from the serendipitous fixation of atmospheric CO₂. The magnetic properties of all family members were examined via SQUID magnetometry, with the χ_MT product and VTVB data of the Gd analogue (1) being independently fitted by numerical diagonalisation to afford the same best-fit parameter J_Gd–Gd = −0.004 cm⁻¹. The MCE of complex 1 was elucidated from specific heat data, with the magnetic entropy change reaching a value of 22.6 J kg⁻¹ K⁻¹ at T = 1.7 K, close to the maximum entropy value per mole expected from six Gd^III spins (S_Gd = 7/2), 23.7 J kg⁻¹ K⁻¹.