Large magnetocaloric effect in a Wells-Dawson type {Ni6Gd6P6} cage.

Magnetic refrigeration is based on the magnetocaloric effect (MCE) which relies on the entropy change of a material when placed in a magnetic field.1 Molecular magnets have recently been examined in this context, especially high spin isotropic magnetic molecules.2-8 The very large MCE observed for some of these cages suggests they could be used as a replacement for helium-3 in some applications; the expense and rarity of helium-3 makes this worth further investigation. Recently, paramagnetic metal ions have been used as vertices in high-symmetry cages such as the Keplerate,9 and this has led to the observation of exotic magnetic phenomena associated with the perfect spin-frustrated topology.10 Moreover, such a geometrically frustrated pattern enhances the field dependence of the MCE due to the increased number of populated spin states.2,11 Phosphonate as a tridentate ligand has its potential to form spherical cages due to the appropriate O-P-O angles (usually between 100o and 120o when coordinated).12 We have been able to use it to construct cobaltgadolinium grid-like complexes,13 in which the presence of fourcoordinate cobalt(II) ions are vital in the formation of the planartype structures rather than cages. We thought it worth investigating similar chemistry with nickel(II). Reacting benzylphosphonic acid (H2O3PCH2Ph) with two precursors, [Ni2(μ-OH2)(O2CBu)4]·(HO2CBu)4 and [Ln2(O2CBu)6(HO2CBu)6] (Ln = Gd, Dy and Y),15 we are able to obtain a family of molecular cages [Ni6Ln6(OH)2(O3PCH2Ph)6(O2CBu)16(MeCO2H)2](MeCN)4, where Ln = Gd 1, Dy 2 and Y 3. While the structures are heterometallic, in many ways the closest structural analogues in the literature are the very beautiful homometallic diamagnetic WellsDawson polyoxometallates.16 Compounds 1-3 crystallise in the same space group P21/n. Since they are isomorphous we will describe the structure of 1 only. The crystal structure of 1 features a centrosymmetric rugby-ball shaped core (Figure 1).17 The two ends of the rugby ball are capped by a {Ni3(μ3-OH)} triangle, in which the μ3-OH group is displaced by ca. 0.48 Å out of the Ni3 plane. There are two 2.1118 pivalates and one 2.20 acetate ligands that bridge the edges of these {Ni3(μ3-OH)} triangles (see also Figure S1 in supporting information).