Olivier Roubeau

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 dense metal-organic framework for enhanced magnetic refrigeration

X iv :1 21 2. 28 77 v1 [ co nd -m at .m tr lsc i] 1 2 D ec 2 01 2 Magnetic cryocooling with Gd centers in a light and compact framework G. Lorusso, J. W. Sharples, E. Palacios, O. Roubeau, E. K. Brechin, R. Sessoli, A. Rossin, F. Tuna, E. J. L. McInnes, D. Collison, and M. Evangelisti a) Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC − Universidad de Zaragoza, Departamento de F́ısica de la Materia Condensada, 50009 Zaragoza, Spain School of Chemistry and Photon Science Institute, The University of Manchester, M13-9PL Manchester, United Kingdom School of Chemistry, The University of Edinburgh, EH9-3JJ Edinburgh, United Kingdom Department of Chemistry and INSTM, Università degli Studi di Firenze, 50019 Sesto Fiorentino, Italy Istituto di Chimica dei Composti Organometallici (ICCOM), CNR, 50019 Sesto Fiorentino, ItalyThe three-dimensional metal-organic framework Gd(HCOO)3 is characterized by a relatively compact crystal lattice of weakly interacting Gd(3+) spin centers interconnected via lightweight formate ligands, overall providing a remarkably large magnetic:non-magnetic elemental weight ratio. The resulting magnetocaloric effect per unit volume is decidedly superior in Gd(HCOO)3 than in the best known magnetic refrigerant materials for liquid-helium temperatures and low-moderate applied fields.

Triazole-Based One-Dimensional Spin-Crossover Coordination Polymers

One-dimensional coordination Fe(II) polymers constructed through triple N(1),N(2)-1,2,4-triazole bridges form a unique class of spin-crossover materials, the synthetic versatility of which allows tuning the spin-crossover properties, the design of gels, films, liquid crystals, and nanoparticles and single-particle addressing. This Minireview provides the first complete overview of these very attractive switchable materials and their most recent developments.

Increasing the dimensionality of cryogenic molecular coolers: Gd-based polymers and metal–organic frameworks

The magnetothermal properties of a coordination polymer and a metal-organic framework (MOF) based on Gd(3+) ions are reported. An equally large cryogenic magnetocaloric effect (MCE) is found, irrespective of the dimensionality. This combined with their robustness makes them appealing for widespread magnetic refrigeration applications.

Spin-transition behaviour in chains of FeII bridged by 4-substituted 1,2,4-triazoles carrying alkyl tails

A family of polymeric 1-dimensional chains of iron(II) species showing the spin-crossover phenomenon has been synthesized using 4-n-alkyl-1,2,4-triazoles as bridging ligands. The influence of the length of the alkyl tails on the triazole ligands on characteristic features of the spin transition was studied, showing degrading of steepness with increasing length. A set of four counter ions has been used to access a wider range of transition temperatures. Large hysteresis loops are detected with small tails, mainly for the methyl and ethyl substituted products. In most cases longer tails weaken co-operativity and hysteresis gradually decreases to zero. However it is shown that with certain anions hysteresis remains, even with very long tails on the triazoles. Weakening of the co-operativity mainly arises from a diminution of the length of the polymeric chains with increasing alkyl tails on the triazole. This effect is anion dependent. A strong interaction along the polymeric chains is confirmed.

Molecular prototypes for spin-based CNOT and SWAP quantum gates

We show that a chemically engineered structural asymmetry in [Tb2] molecular clusters renders the two weakly coupled Tb3+ spin qubits magnetically inequivalent. The magnetic energy level spectrum of these molecules meets then all conditions needed to realize a universal CNOT quantum gate. A proposal to realize a SWAP gate within the same molecule is also discussed. Electronic paramagnetic resonance experiments confirm that CNOT and SWAP transitions are not forbidden.

Heterodimetallic [LnLn′] Lanthanide Complexes: Toward a Chemical Design of Two-Qubit Molecular Spin Quantum Gates

A major challenge for realizing quantum computation is finding suitable systems to embody quantum bits (qubits) and quantum gates (qugates) in a robust and scalable architecture. An emerging bottom-up approach uses the electronic spins of lanthanides. Universal qugates may then be engineered by arranging in a molecule two interacting and different lanthanide ions. Preparing heterometallic lanthanide species is, however, extremely challenging. We have discovered a method to obtain [LnLn′] complexes with the appropriate requirements. Compound [CeEr] is deemed to represent an ideal situation. Both ions have a doubly degenerate magnetic ground state and can be addressed individually. Their isotopes have mainly zero nuclear spin, which enhances the electronic spin coherence. The analogues [Ce2], [Er2], [CeY], and [LaEr] have also been prepared to assist in showing that [CeEr] meets the qugate requirements, as revealed through magnetic susceptibility, specific heat, and EPR. Molecules could now be used for quantum information processing.

An unprecedented 1D ladder coordination polymer based on a pentanuclear copper(ii) 2,4,6-tris(dipyridin-2-ylamino)-1,3,5-triazine building blockElectronic supplementary information (ESI) available: additional characterization data for 1 (µeff vs. T and B curves), TGA, XRD. See http://www.rsc.org/suppdata/cc/b2/b203394a/

Copper(II) nitrate reacts with the rigid polydentate triple-connecting dpyatriz ligand in acetonitrile to an unprecedented infinite molecular ladder in which five-coordinated copper pseudo-dimer are bridged by nitrate anions and the coordination polymer chains are linked by hexacoordinated copper ions leading to the formation of large guest cavities.

A Molecular Pair of [GdNi3] Tetrahedra Bridged by Water Molecules

The authors thank the Generalitat de Catalunya for the prize ICREA Academia 2008 (G.A.) and Grant 2009SGR-1459 (E.C. and E.R.) and Spanish MCI through CTQ2009–06959 (G.A., L.B.), MAT2009-13977- C03 (M.E.), and CTQ2008–06670-C02–01 (E.C., ER). The advanced light source (S.J.T.) is supported by the U.S. Department of Energy (DE-AC02–05CH11231).

An S-Shaped [Fe4Dy2] Complex Exhibiting Slow Relaxation of Magnetization: Synthesis, Magnetism, and Crystal Structures of a Family of [Fe4Ln2][Ln] Coordination Compounds (Ln = Nd, Gd, Tb, Dy, and Ho)

A series of heterobimetallic 3d-4f cluster coordination compounds has been synthesized using a Schiff-base ligand (H(3)L), a trinuclear iron(III) precursor complex and lanthanide nitrates as reactants. Five new isostructural complexes were prepared with the ligand in 4 different forms, i.e., neutral and 3 different levels of deprotonation, and with the general formula [Fe(III)(4)Ln(III)(2)(H(2)L)(2)(HL)(2)L(2)(CH(3)OH)(2)((CH(3))(3)CCOO)(2)(NO(3))(2)][Ln(III)(NO(3))(4)(H(3)L)(CH(3)OH)].NO(3).H(2)O, where Ln is Nd, Gd, Tb, Dy, and Ho for compounds 1, 2, 3, 4, and 5, respectively. The single-crystal structures of two complexes were determined by X-ray diffraction, consisting of an original [Fe(III)(4)Ln(III)(2)](2+) moiety with a linear S-shaped [Ln-Fe(4)-Ln] core and an isolated Ln(III) ion coordinated by nitrate anions and the neutral Schiff-base ligand. The isostructural nature of all five coordination compounds is further illustrated both by XRPD and IR analysis. Magnetic properties of all five compounds have been studied and are discussed in light of magnetostructural considerations. Among these five compounds, the Dy (4) cluster shows frequency-dependent ac-susceptibility indicative of probable single-molecule magnet behavior.