The Importance of Being Exchanged: [Gd4M8(OH)8(L)8(O2CR)8] Clusters for Magnetic Refrigeration

One of the most promising applications for molecules built from paramagnetic metal ions is low temperature magnetic refrigeration.[1] Indeed recent studies have suggested that molecular coolers can outperform any conventionally-employed solid-state refrigerant material by orders of magnitude.[2] 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 (i.e. a large magnetic density).[1b] These molecular pre-requisites suggest the use of lanthanide ions and, in particular, the f7 ion Gd3+ in the construction of homoand heterometallic (Gd-3d) clusters, and a sensible starting point is the synthesis of GdIII-CuII clusters since previous studies have shown this combination favours ferromagnetic exchange.[3] Here we introduce a rather remarkable new family of compounds of general formula [Ln4M8(OH)8(L)8(O2CR)8](X)4 in which almost all the constituent parts – the lanthanide ions (Ln3+), the transition metal ions (M2+), the bridging ligand L, the carboxylates and the counter anions (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 magneto-caloric effect (MCE). In this communication we describe the three family members [Gd4M8(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. For the sake of brevity we provide a generic structure description, highlighting any differences. The core (Figure 1 shows complex 2) of the molecule consists of a square (or wheel) of four cornersharing {Gd2M2O4} cubanes. The shared corners are the Gd ions which thus themselves form an inner {Gd4} square, each edge of which is occupied by two μ3-OH ions which further bridge to a MII ion. The μ3-L ions chelate the M2+ ions and use their O-arm to further bridge to the second M2+ ion in the same cubane and to one Gd ion. There are two carboxylates per cubane, each μ-bridging across a M2+...Gd square face, alternately above and below the plane of the {Gd4} square.