Marco Evangelisti

Co–Ln Mixed-Metal Phosphonate Grids and Cages as Molecular Magnetic Refrigerants

The synthesis, structures, and magnetic properties of six families of cobalt-lanthanide mixed-metal phosphonate complexes are reported in this Article. These six families can be divided into two structural types: grids, where the metal centers lie in a single plane, and cages. The grids include [4 × 3] {Co(8)Ln(4)}, [3 × 3] {Co(4)Ln(6)}, and [2 × 2] {Co(4)Ln(2)} families and a [4 × 4] {Co(8)Ln(8)} family where the central 2 × 2 square is rotated with respect to the external square. The cages include {Co(6)Ln(8)} and {Co(8)Ln(2)} families. Magnetic studies have been performed for these compounds, and for each family, the maximum magnetocaloric effect (MCE) has been observed for the Ln = Gd derivative, with a smaller MCE for the compounds containing magnetically anisotropic 4f-ions. The resulting entropy changes of the gadolinium derivatives are (for 3 K and 7 T) 11.8 J kg(-1) K(-1) for {Co(8)Gd(2)}; 20.0 J kg(-1) K(-1) for {Co(4)Gd(2)}; 21.1 J kg(-1) K(-1) for {Co(8)Gd(4)}; 21.4 J kg(-1) K(-1) for {Co(8)Gd(8)}; 23.6 J kg(-1) K(-1) for {Co(4)Gd(6)}; and 28.6 J kg(-1) K(-1) for {Co(6)Gd(8)}, from which we can see these values are proportional to the percentage of the gadolinium in the core.

Recipes for enhanced molecular cooling

Molecular nanomagnets are considered valid candidates for magnetic refrigeration at low temperatures. Designing these materials for enhanced cooling requires the control and optimization of the quantum properties at the molecular level, in particular: spin ground state, magnetic anisotropy, and presence of low-lying excited spin states. Herein, we present the theoretical framework together with a critical review of recent results, and perspectives for future developments.

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 A 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).

Cryogenic Magnetocaloric Effect in a Ferromagnetic Molecular Dimer

Over the last few years, great interest has emerged in the synthesis and magnetothermal studies of molecular clusters based on paramagnetic ions, often referred to as molecular nanomagnets, in view of their potential application as lowtemperature magnetic refrigerants. What makes them promising is that their cryogenic magnetocaloric effect (MCE) can be considerably larger than that of any other magnetic refrigerant, for example, lanthanide alloys and magnetic nanoparticles. The MCE is the change of magnetic entropy (DSm) and related adiabatic temperature (DTad) in response to 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) a large spin ground state S, since the magnetic entropy amounts to R ln(2S+1); 2) a negligible magnetic anisotropy, which permits easy polarization of the net molecular spins in magnetic fields of weak or moderate strength; 3) the presence of low-lying excited spin states, which enhances the field dependence of the MCE owing to the increased number of populated spin states; 4) dominant ferromagnetic exchange, favoring a large S and hence a large field dependence of the MCE; 5) a relatively low molecular mass (or a large metal/ligand mass ratio), since the nonmagnetic ligands contribute passively to the MCE. Although this last point is crucial for obtaining an enhanced effect, it has beenmostly 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 multinuclear magnetic core. Herein we propose a drastically different approach by focusing on the simple and well-known ferromagnetic molecular dimer gadolinium acetate tetrahydrate, [{Gd(OAc)3(H2O)2}2]·4H2O (1). [4a,b] The structure of 1 (Figure 1) com-

A calix[4]arene 3d/4f magnetic cooler.

The success with which coordination chemists have produced (often aesthetically pleasing) molecules with fascinating physical properties is derived from the systematic exploration of the effects of ligand design, metal identity, and heating regime upon cluster symmetry, topology, and nuclearity. The design of molecular nanomagnets—model systems with which to investigate the possible implementation of spinbased solid-state qubits and molecular spintronics—has been the subject of much interest in recent years because their molecular nature and inherent physical properties allow the crossover between classical and quantum physics to be observed. The synthesis of new types of molecular nanomagnets therefore remains an exciting challenge, but the range of organic ligands employed thus far is surprisingly restricted. Undoubtedly the most successful route has been to employ small, flexible polydentate ligands in self-assembly. An alternative approach would be to entirely encapsulate the magnetic skeleton within a large rigid organic or inorganic sheath whose dual role could also include the introduction of redox activity, surface compatibility, or simply the removal/control of dipolar interactions. Calix[4]arenes (C4s) are typically bowl-shaped molecules which have been exploited in the formation of various nanometer-scale supramolecular architectures. Their rigid conformations can be utilized in self-assembly, or combined with functionalization at the upper rim to present binding sites for assembly-directing metal centers. 10] The polyphenolic nature of these molecules therefore renders them good ligand candidates for the isolation of paramagnetic cluster compounds. In this regard only one cluster compound, having greater than four transition metals, has been shown to form with methylene-bridged para-tert-butylcalix[4]arene 1 (Figure 1a). Thiacalix[4]arenes and their oxidized derivatives 2–4 (Figure 1b) possess additional donor atoms, and these have been used in the formation of a number of polynuclear transition-metal or Ln clusters. The additional donor atoms (relative to 1) around the molecular skeleton play a key role in supporting complex formation by taking part in the bonding within the metal-cluster framework. For our purposes, readily accessible methylene-bridged C4s present the potential to 1) form novel cluster compounds at the lower rim of the bowl-shaped macrocycles, and 2) easily alter the upper-rim properties to access a vast library of new metal clusters containing supramolecular building blocks. These features may therefore allow control of the interactions between clusters (thereby modulating their orientation in the solid state), or variation of the degree of cluster isolation (or encapsulation) through alteration of the upper rim of the calix[4]arene. We have recently reported the formation of the first Mn cluster and the first single-molecule magnet (SMM) to be isolated using any methylene-bridged C4 (Figure 1c). The mixed-valent Mn2Mn II 2 complex is housed between two Figure 1. a) para-tert-Butylcalix[4]arene sused in transition-metal cluster formation. b) Thiacalix[4]arenes used in transitionand lanthanidemetal cluster formation. c) Mn2Mn II 2 SMM formed with 1. [14] Hydrogen atoms omitted for clarity.

Magnetothermal properties of molecule-based materials

We critically review recent results obtained by studying the low-temperature specific heat of some of the most popular molecule-based materials. After introducing the experimental techniques and basic theoretical framework needed for heat capacity determination and understanding, we report on the magnetothermal properties of molecular antiferromagnetic wheels. For selected molecular high-spin clusters, particular emphasis is devoted to magnetic quantum tunnelling and coherence as well as collective phenomena as probed by heat capacity experiments. We discuss also the possibilities of application of molecule-based materials for magneto-cooling at low temperatures and the limitations in other temperature ranges. Perspectives for future developments are mentioned as well.

Co-Gd Phosphonate Complexes as Magnetic Refrigerants

Three 3d–4f phosphonate complexes, [CoII8GdIII8(μ3-OH)4(NO3)4(O3PtBu)8(O2CtBu)16], [CoII8GdIII4(O3PtBu)6(O2CtBu)16] and [CoII4GdIII6(O3PCH2Ph)6(O2CtBu)14(MeCN)2], have been synthesized and have structures that can be related to molecular grids. Magnetic studies show they have promise as low temperature magnetic refrigerants.

Mixed-Valent Mn Supertetrahedra and Planar Discs as Enhanced Magnetic Coolers

The syntheses and structures of two decametallic mixed-valent Mn supertetrahedra using 2-amino-2-methyl-1,3-propanediol (ampH2), two decametallic mixed-valent Mn planar discs using 2-amino-2-methyl-1,3-propanediol (ampH2) and 2-amino-2-ethyl-1,3-propanediol (aepH2), and a tetradecametallic mixed-valent Mn planar disc using pentaerythritol (H4peol) are reported. The decametallic complexes display dominant ferromagnetic exchange and spin ground states of S = 22, and the tetradecametallic complex displays dominant antiferromagnetic exchange and a spin ground state of S = 7 +/- 1. All display large (the former) and enormous (the latter) magnetocaloric effect--the former as a result of negligible zero-field splitting of the ground state, and the latter as a result of possessing a high spin-degeneracy at finite low temperatures--making them the very best cooling refrigerants for low-temperature applications.

Lanthanoid single-ion magnets based on polyoxometalates with a 5-fold symmetry: the series [LnP5W30O110]12- (Ln3+ = Tb, Dy, Ho, Er, Tm, and Yb).

A robust, stable and processable family of mononuclear lanthanoid complexes based on polyoxometalates (POMs) that exhibit single-molecule magnetic behavior is described here. Preyssler polyanions of general formula [LnP(5)W(30)O(110)](12-) (Ln(3+) = Tb, Dy, Ho, Er, Tm, and Yb) have been characterized with static and dynamic magnetic measurements and heat capacity experiments. For the Dy and Ho derivatives, slow relaxation of the magnetization has been found. A simple interpretation of these properties is achieved by using crystal field theory.

Molecular coolers: The case for [CuII5GdIII4]

The use of triethanolamine (teaH3) in 3d/4f chemistry produces the enneanuclear cluster compound [CuII5GdIII4O2(OMe)4(teaH)4(O2CC(CH3)3)2(NO3)4]·2MeOH·2Et2O (1·2MeOH·2Et2O) whose molecular structure comprises a series of vertex- and face-sharing {GdIIICuII3} tetrahedra. Magnetic studies reveal a large number of spin states populated even at the lowest temperatures investigated. Combined with the high magnetic isotropy, this enables 1 to be an excellent magnetic refrigerant for low temperature applications.