Cryogenic magnetocaloric effect in a ferromagnetic molecular dimer.

Over the last few years, great interest has emerged in the synthesis and magnetothermal studies of polymetallic molecular clusters based on paramagnetic ions, often referred to as molecular nanomagnets, in view of their potential application as lowtemperature magnetic refrigerants.[1,2] What makes them promising is that their cryogenic magnetocaloric effect (MCE) can be considerably larger than that of any other magnetic refrigerant, e.g. lanthanide alloys and magnetic nanoparticles.[3] The MCE is the change of magnetic entropy (∆Sm) and related adiabatic temperature (∆Tad) following the change of applied magnetic field and it can be exploited for cooling applications via a field removal process called adiabatic demagnetization. Although the MCE is intrinsic to any magnetic material, in only a few cases are the changes sufficiently large to make them suitable for applications. The ideal molecular refrigerant comprises the following key characteristics:[1] (i) a large spin ground state S, since the magnetic entropy amounts to Rln(2S+1); (ii) a negligible magnetic anisotropy, which permits easy polarization of the net molecular spins in magnetic fields of weak or moderate strength; (iii) the presence of low-lying excited spin states, which enhances the field dependence of the MCE due to the increased number of populated spin states; (iv) dominant ferromagnetic exchange,[3(c)] favouring a large S and hence a large field dependence of the MCE; (v) a relatively low molecular mass (or a large metal:ligand mass ratio) since the non-magnetic ligands contribute passively to the MCE. Although this last point is crucial for obtaining an enhanced effect, it has been mostly ignored to date. Molecular cluster compounds tend to have a very low magnetic density because of the large complex structural frameworks required to encase the multi-metallic core. In this communication we propose a drastically different approach by focusing on the simple and well-known ferromagnetic molecular dimer gadolinium acetate tetrahydrate,[4] [{Gd(OAc)3(H2O)2}2]•4H2O (1). The structure of 1 is depicted in Figure 1 and comprises a dimer of Gd3+ ions bridged through two of the six carboxylate groups which bond in a η:η:μ2-fashion. The remaining acetates are chelating with the nine-coordinate [capped square anti-prismatic] geometry of the metal centres being completed by the presence of two terminally bound H2O molecules. These partake in intra-molecular H-bonding to the neighbouring chelating acetate ligands, and are responsible for both the direct inter-molecular H-bonds in the a-b plane and the inter-plane Hbonds mediated by the lattice H2O molecules (Fig. S1 and Table S1).