Fragmenting gadolinium: mononuclear polyoxometalate-based magnetic coolers for ultra-low temperatures

The polyoxometalate clusters with formula [Gd(W(5) O(18) )(2) ](9-) and [Gd(P(5) W(30) O(110) )](12-) each carry a single magnetic ion of gadolinium, which is the most widespread element among magnetic refrigerant materials. In an adiabatic demagnetization, the lowest attainable temperature is limited by the presence of magnetic interactions that bring about magnetic order below a critical temperature. We demonstrate that this limitation can be overcome by chemically engineering the molecules in such a way to effectively screen all magnetic interactions, suggesting their use as ultra-low-temperature coolers.

Observation of divergent isotope effects as well as metal ion-modulated T(C) and spin-canting nature in isostructural supramolecular magnets

The ion-pair complexes of [4-NH(2)-PyH][M(mnt)(2)] (M = Pt for 1 and Ni for 3) and their deuterated analogues [4-NH(2)-PyD][M(mnt)(2)] (M = Pt for 2 and Ni for 4) are isostructural with each other. Four complexes crystalline in monoclinic space group C2/c, whose asymmetric unit consists of two halves of [M(mnt)(2)](-) anions and one cation, show quite similar cell parameters and almost identical packing structures as well. In the crystals of 1-4, two types of crystallographically inequivalent [M(mnt)(2)](-) anions construct individual layers, which are separated by the cation layer; the supramolecular networks are formed via the H-bonding interactions between the [M(mnt)(2)](-) and 4-NH(2)-PyH(+) (or 4-NH(2)-PyD(+)) ions as well as the weakly ππ stacking interactions between the [M(mnt)(2)](-) anions. The four isostructural complexes exhibit canted antiferromagnetism, arising from the non-collinearity of the magnetic moments between the crystallographically inequivalent anion layers, with T(C) ≈ 14.8 K for 1, 13.6 K for 2, 7.7 K for 3 and 8.8 K for 4, respectively. Ac magnetic susceptibility measurements revealed that 1 and 2 show spin canting, while 3 and 4 show hidden-spin canting characteristics. The isostructural 1 and 3 were deuterated to give the divergent isotope effects on the cell volume and T(C).

High-nuclearity 3d-4f clusters as enhanced magnetic coolers and molecular magnets

Four 52-metal-ion 3d-4f cluster complexes featuring a common core of Ln(42)M(10) (Ln = Gd(3+), Dy(3+); M = Co(2+/3+), Ni(2+)) were obtained through self-assembly of the metal ions templated by mixed anions (ClO(4)(-) and CO(3)(2-)). Magnetic studies revealed that the Gd(42)Co(10) and Gd(42)Ni(10) clusters exhibit the largest magnetocaloric effect (MCE) among any known 3d-4f complexes. Replacement of Gd(3+) ions with anisotropic Dy(3+) ions caused significant changes in the magnetic behavior of the clusters; both Dy(42)Co(10) and Dy(42)Ni(10) displayed slow relaxation of the magnetization.

Carboxylate-free Mn(III)2Ln(III)2 (Ln = lanthanide) and Mn(III)2Y(III)2 complexes from the use of (2-hydroxymethyl)pyridine: analysis of spin frustration effects

The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [Mn(III)(2)M(III)(2)(OH)(2)(NO(3))(4)(hmp)(4)(H(2)O)(4)](NO(3))(2) (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1-4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic Ln(III) atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5-300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J(1) = -0.16(2) cm(-1) and J(2) = -0.12(1) cm(-1) for the Mn(III)···Mn(III) and Mn(III)···Gd(III) interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = -0.75(3) cm(-1) for the Mn(III)···Mn(III) interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J(1) and J(2).

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.

A family of double-bowl pseudo metallocalix[6]arene discs

We report the synthesis and magnetic characterisation of a series of planar [M₇] (M= Ni(II), Zn(II)) disc complexes [Ni₇(OH)₆(L₁)₆](NO₃)₂ (1), [Ni₇(OH)₆(L₁)₆](NO₃)₂·2MeOH (2), [Ni₇(OH)₆(L₁)₆](NO₃)₂·3MeNO₂ (3), [Ni₇(OH)₆(L₂)₆](NO₃)₂·2MeCN (4), [Zn₇(OH)₆(L₁)₆](NO₃)₂·2MeOH·H₂O (5) and [Zn₇(OH)₆(L₁)₆](NO₃)₂·3MeNO₂ (6) (where HL₁ = 2-iminomethyl-6-methoxy-phenol, HL₂ = 2-iminomethyl-4-bromo-6-methoxy-phenol). Each member exhibits a double-bowl pseudo metallocalix[6]arene topology whereby the individual [M₇] units form molecular host cavities which are able to accommodate various guest molecules (MeCN, MeNO₂ and MeOH). Magnetic susceptibility measurements carried out on complexes 1 and 4 indicate weak exchange between the Ni(II) centres.