Magnetic and magnetocaloric properties of an unusual family of carbonate-panelled [Ln(6)(III)Zn(2)(II)] cages

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)center dot 6H(2)O and Ln(NO3)(3)center dot 6H(2)O in a basic methanolic solution generates a family of isostructural heterometallic coordination compounds of general formula [Ln(6)Zn(2)(CO3)(5)(OH)(H2L)(4)(H3L)(2)(H4L)]NO3 center dot 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-6(III)] octahedron capped on two faces by Zn-II ions. The metal core is stabilised by a series of mu(3)- and mu 4-CO32- ions, originating from the serendipitous fixation of atmospheric CO2. The magnetic properties of all family members were examined via SQUID magnetometry, with the chi T-M 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.