Magnetic and magnetocaloric properties of an unusual family of carbonate-panelled [Ln6^IIIZn2^II] cages

The reaction of the pro-ligand HL, which combines the complementary phenolic oxime and diethanolamine moieties within the same organic framework, with Zn(NO)·6HO and Ln(NO)·6HO in a basic methanolic solution generates a family of isostructural heterometallic coordination compounds of general formula [LnZn(CO)(OH)(HL)(HL)(HL)]NO·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] octahedron capped on two faces by Zn ions. The metal core is stabilised by a series of ?- and ?-CO ions, originating from the serendipitous fixation of atmospheric CO. The magnetic properties of all family members were examined via SQUID magnetometry, with the T product and VTVB data of the Gd analogue (1) being independently fitted by numerical diagonalisation to afford the same best-fit parameter J = -0.004 cm. The MCE of complex 1 was elucidated from specific heat data, with the magnetic entropy change reaching a value of 22.6 J kg K at T = 1.7 K, close to the maximum entropy value per mole expected from six Gd spins (S = 7/2), 23.7 J kg K.