The importance of being exchanged: [Gd(III)4M(II)8(OH)8(L)8(O2CR)8]4+ clusters for magnetic refrigeration.

One of the most promising applications for molecules built from paramagnetic metal ions is low-temperature magnetic refrigeration. Indeed, recent studies have suggested that molecular coolers can outperform any conventionally employed solid-state refrigerant material by orders of magnitude. In order to do so, molecules must possess a combination of a large spin ground state (S), with negligible anisotropy (Dcluster= 0), weak magnetic exchange between the constituent metal ions, and a relatively large metal:non-metal mass ratio (that is, a large magnetic density). These molecular prerequisites suggest the use of lanthanide ions and, in particular, the f ion Gd in the construction of homoand heterometallic (Gd-3d) clusters, and a sensible starting point is the synthesis of Gd–Cu clusters, as previous studies have shown this combination favors ferromagnetic exchange. Herein we introduce a rather remarkable new family of compounds of general formula [Ln4M II 8(OH)8(L)8(O2CR)8](X)4 in which almost all the constituent parts, namely the lanthanide ions Ln, the transition-metal ions M, the bridging ligand L, the carboxylates, and the counterions X can be exchanged. In each case, the structure remains essentially the same and this allows for a thorough understanding of the individual contributions to the magnetocaloric effect (MCE). Herein we describe the three family members [Gd4M II 8(OH)8(L)8(O2CR)8](ClO4)4 (M=Zn, R=CHMe2, 1; M=Cu, R=CHMe2, 2 ; M=Ni, R=CH2Me, 3 ; LH= 2(hydroxymethyl)pyridine) and show how the identity of the transition metal and the sign of the magnetic exchange are vital components to consider when designing molecular coolers.