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Invited Reviews

Polyfunctional Two- (2D) and Three- (3D) Dimensional Oxalate Bridged Bimetallic Magnets

Summary
We report major results concerning polyfunctional two- (2D) and three- (3D) dimensional oxalate bridged bimetallic magnets. As a consequence of their specific organisation they are composed of an anionic sub-lattice and a cationic counter-part. These bimetallic polymers can accommodate various counter-cations possessing specific physical properties in addition to the magnetic ones resulting from the interactions between the metallic ions in the anionic sub-lattice. Thus, molecular magnets possessing paramagnetic, conductive and optical properties are presented in this review.
René Clément, Silvio Decurtins, Michel Gruselle, Cyrille Train

Ferromagnetism in Metallocene-Doped Fullerenes

Summary
Ferromagnetism in fullerene-based systems doped with metallocenes is reviewed. These compounds form a ferromagnetic state by spin-coupling between π electrons on fullerene units, while the metallocene molecules do not contribute to the spin ordering. One of these compounds has the highest critical temperature (19 K) for this class of compound. The magnetic properties of these materials are very strongly dependent on the crystallization conditions.
Dragan Mihailovic

High Spin and Anisotropic Molecules Based on Polycyanometalate Chemistry

Summary
This paper points out some recent achievements in the chemistry and physics of high spin and anisotropic molecules based on polycyanometalate complexes. Following a step by step synthetic strategy and using a localized electron orbital model, isotropic high spin molecules were obtained with ground spin states ranging from S = 9/2 to 27/2. In the same way, anisotropie molecules with various nuclearities (bi, tri, tetra, hexa, and hepta-nuclear complexes) have been synthesized. Mixing these two approaches, it has been possible to obtained anisotropic high spin molecules that behave as single molecule magnets. The paper reviews some of the steps that lead to these findings and some of the prospects opened in the field of single molecule magnets.
Valérie Marvaud, Juan M. Herrera, Thomas Barilero, Fabien Tuyeras, Raquel Garde, Ariane Scuiller, Caroline Decroix, Martine Cantuel, Cédric Desplanches

Spin Crossover Properties of the [Fe(PM-BiA)2(NCS)2] Complex — Phases I and II

Summary
In the present review, we reexamine the photomagnetic properties of the [Fe(PMBiA)2(NCS)2], cis-bis(thiocyanato)-bis[(N-2′-pyridylmethylene)-4-(aminobiphenyl)]iron(II),compound which exhibits, depending on the synthetic method, an exceptionally abrupt spin transition (phase I) with a very narrow hysteresis (T ½ ↓ = 168K and T ½ ↑ = 173 K) or a gradual spin conversion (phase II) occurring at 190 K. In both cases, light irradiation in the tail of the 1MLCT-LS absorption band, at 830 nm, results in the population of the high-spin state according to the light-induced excited spin-state trapping (LIESST) effect. The capacity of a compound to retain the light-induced HS information, estimated through the T(LIESST) experiment, is determined for both phases. Interestingly, the shape of the T(LIESST) curve is more gradual for the phase II than for the phase I and the T(LIESST) value is found considerably lower in the case of the phase II. The kinetics parameters involved in the photo-induced high-spin → low-spin relaxation process are estimated for both phases. From these data, the experimental T(LIESST) curves are simulated and the particular influence of the cooperativity as well as of the parameters involved in the thermally activated and tunneling regions are discussed. The Light-Induced Thermal Hysteresis (LITH), originally described for the strongly cooperative phase I, is also reinvestigated. The quasi-static LITH loop is determined by recording the photostationary points in the warming and cooling branches.
Jean-François Létard, Guillaume Chastanet, Olivier Nguyen, Silvia Marcén, Mathieu Marchivie, Philippe Guionneau, Daniel Chasseau, Philipp Gütlich

Spin Transition of 1D, 2D and 3D Iron(II) Complex Polymers The Tug-of-War between Elastic Interaction and a Shock-Absorber Effect

Summary
The structures of linear chain Fe(II) spin-crossover compounds of α,β-and α,ω-bis (tetrazol-l-yl)alkane type ligands are described in relation to their magnetic properties. The first threefold interlocked 3-D catenane Fe(II) spin-transition system, [µ-tris(1,4-bis(tetrazol-l-yl)butane-N1,N1′) iron(II)] bis(perchlorate), will be discussed. An analysis is made among the structures and the cooperativity of the spin-crossover behaviour of polynuclear Fe(II) spin-transition materials.
Petra J. van Koningsbruggen, Matthias Grunert, Peter Weinberger

Is There a Need for New Models of the Spin Crossover?

Summary
The existing models of the low-spin to high-spin transition (spin crossover) are briefly reviewed. Experimental data pointing to a need of new models are displayed. A statistical model with the distribution of the solid-state cooperativeness is outlined. A modeling is shown as well as its application to a spin crossover system [Fe(bzimpy)2](C1O4)2] · 0.25H2O. This shows an abrupt spin crossover at temperature as high as 403 K with a hysteresis width of 12 K. The angled walls of the hysteresis loop can be followed by the outlined statistical model.
Roman Boča, Wolfgang Linert

Quantum Spin Dynamics in Molecular Magnets

Summary
The detailed theoretical understanding of quantum spin dynamics in various molecular magnets is an important step on the roadway to technological applications of these systems. Quantum effects in both ferromagnetic and antiferromagnetic molecular clusters are, by now, theoretically well understood. Ferromagnetic molecular clusters allow one to study the interplay of incoherent quantum tunneling and thermally activated transitions between states with different spin orientation. The Berry phase oscillations found in Fe8 are signatures of the quantum mechanical interference of different tunneling paths. Antiferromagnetic molecular clusters are promising candidates for the observation of coherent quantum tunneling on the mesoscopic scale. Although challenging, application of molecular magnetic clusters for data storage and quantum data processing are within experimental reach already with present day technology.
Michael N. Leuenberger, Florian Meier, Daniel Loss

8. Ab Initio Calculations Versus Polarized Neutron Diffraction for the Spin Density of Free Radicals

Summary
The determination of the magnetization distribution using polarized neutron diffraction has played a key role during the last twenty years in the field of molecular magnetism. This distribution can also be obtained by first principle ab initio calculations. Such calculations always rely on approximations and the question that arises is to know whether the obtained results are reliable enough to represent accurately the properties of these molecules. The comparison between polarized neutron experimental results and ab initio calculations has turned to provide stringent tests for these methods. In the present article a comparison between experimental and theoretical results is made and is illustrated by examples based on magnetic free radicals.
E. Ressouche, J. Schweizer

Contributions

9. A New Layered Compound Containing [PMo12O40]3- and Both 5- and 6-Coordinated Homoleptic (1-(2-Chloroethyl)tetrazole)Copper(II) Cations

Summary
The synthesis, crystal structure and physical properties of the complex obtained from the reaction between the polyoxometalate anion [PMo12O40]3- copper(II) and the ligand 1-(2-chloroethyl)tetrazole (teec) are described. This compound has been synthesized as a model for designing materials containing both magnetic polyoxometalate anions and iron(II) spin-crossover cations. Keywords. Coordination chemistry; Heterocycles; Polyoxometalates; Structure elucidation; Tetrazole.
The compound, with formula [Cu(teec 5]2[Cu(teec 6][PMo12O40]2 · 2H2O, consists of alternating layers of polyoxometalates and cationic complexes. Both, five and six coordinated Cu(II) ions are present,each positioned in different layers. Despite these layers having a similar width, the layer of pentacoordinated Cu(II) ions contains twice as many cationic complexes as the layer of hexacoordinated Cu(II) ions. This unusual coexistence of complexes with different coordination number is most likely caused by the steric hindrance induced by the bulky polyoxometalates in the layer of pentacoordinated Cu(II) which prevents the presence of a sixth teec ligand.
Arno F. Stassen, Eugenia Martínez Ferrero, Carlos Giménez-Saiz, Eugenio Coronado, Jaap G. Haasnoot, Jan Reedijk

Synthesis and Characterization of a [Mn12 O12(O2CR)16(H2O)4] Complex Bearing Paramagnetic Carboxylate Ligands. Use of a Modified Acid Replacement Synthetic Approach

Summary
new modified approach for the synthesis of Mn12 clusters, based on the use of complex [Mn12O12(O2C’Bu)16(H2O)4] (2) as starting material to promote the acidic ligand replacement, is presented here. This new synthetic approach allowed us to obtain complex [Mn12O12(O2CC6H4N(O)tBU)16(H2O)2] (3),, whose preparation remained elusive by direct replacement of the acetate groups of Mn12Ac (1). Complex 3 bearing open-shell radical units, was prepared to increase the total spin number of its ground state, and consequently, to increase T B , with the expectation that the radical ligands may couple ferromagnetically with the Mn12 core. Unfortunately, magnetic measurements of complex 3 revealed that the sixteen radical carboxylate ligands interact antiferromagnetically with the Mn12 core to yield a S = 2 magnetic ground state.
Philippe Gerbier, Daniel Ruiz-Molina, Neus Domingo, David B. Amabilino, José Vidal-Gancedo, Javier Tejada, David N. Hendrickson, Jaume Veciana

Size Effect on Local Magnetic Moments in Ferrimagnetic Molecular Complexes: An XMCD Investigation

Summary
Molecular chemistry allows to synthesize new magnetic systems with controlled properties such as size, magnetization or anisotropy. The theoretical study of the magnetic properties of small molecules (from 2 to 10 metallic cations per molecule) predicts that the magnetization at saturation of each ion does not reach the expected value for uncoupled ions when the magnetic interaction is antiferromagnetic. The quantum origin of this effect is due to the linear combination of several spin states building the wave function of the ground state and clusters of finite size and of finite spin value exhibit this property. When single crystals are available, spin densities on each atom can be experimentally given by Polarized Neutron Diffraction (PND) experiments. In the case of bimetallic MnCu powdered samples, we will show that X-ray Magnetic Circular Dichroism (XMCD) spectroscopy can be used to follow the evolution of the spin distribution on the MnII and CuII sites when passing from a dinuclear MnCu unit to a one dimensional (MnCu) n compound.
Guillaume Champion, Marie-Anne Arrio, Philippe Sainctavit, Michele Zacchigna, Marco Zangrando, Marco Finazzi, Fulvio Parmigiani, Françoise Villain, Corine Mathonière, Christophe Cartier dit Moulinl

Polymorphism and Pressure Driven Thermal Spin Crossover Phenomenon in [Fe(abpt)2(NCX)2] (X=S, and Se): Synthesis,Structure and Magnetic Properties

Summary
The monomeric compounds [Fe(abpt 2(NCX)2(X = S (1), Se (2) and abpt = 4-amino- 3,5-bis(pyridin-2-yl)-1,2,4-triazole) have been synthesized and characterized. They crystallize in the monoclinic P21/n space group with a = 11.637(2) Å, b = 9.8021(14) Å, c = 12.9838(12) Å, β = 101.126(14)°, and Z=2 for 1, and a= 11.601(2) Å, b = 9.6666(14) Å, c = 12.883(2) Å, β = 101.449(10)°, and Z = 2 for 2. The unit cell contains a pair mononuclear [Fe(abpt)2(NCX)21 units related by a center of symmetry. Each iron atom, located at a molecular inversion center, is in a distorted octahedral environment. Four of the six nitrogen atoms coordinated to the Fe(II) ion belong to the pyridine-N(1) and triazole-N(2) rings of two abpt ligands. The remaining trans positions are occupied by two nitrogen atoms, N(3), belonging to the two pseudo-halide ligands. The magnetic susceptibility measurements at ambient pressure have revealed that they are in the high-spin range in the 2 K300 K temperature range. The pressure study has revealed that compound 1 remains in high-spin as pressure is increased up to 4.4kbar, where an incomplete thermal spin crossover appears at around T 1/2 = 65 K.. Quenching experiments at 4.4 kbar have shown that the incomplete character of the conversion is a consequence of slow kinetics. Relatively sharp spin transition takes place at T 1/2 = 106, 152 and 179 K, as pressure attains 5.6, 8.6 and 10.5 kbar, respectively.
Ana B. Gaspar, M. Carmen Muñoz, Nicolás Moliner, Vadim Ksenofontov, Georgii Levchenko, Philipp Gütlich, José Antonio Reall

Substituent Effects on the Spin-Transition Temperature in Complexes with Tris(pyrazolyl) Ligands

Summary
Iron (II) complexes with substituted tris(pyrazolyl) ligands, which exhibit a thermally driven transition from a low-spin state at low temperatures to a high-spin state at elevated temperatures, have been studied by Mössbauer spectroscopy and magnetic susceptibility measurements. From the observed spectra the molar high-spin fraction and the transition temperature have been extracted. All substituents, except for bromine, lead to a decrease of the transition temperature. Density functional calculations have been carried out to compare the experimentally observed shifts of the transition temperature with those derived from theory.
Hauke Paulsen, Lars Duelund, Axel Zimmermann, Frédéric Averseng, Michael Gerdan, Heiner Winkler, Hans Toftlund, Alfred X. Trautwein

Magneto-Structural Correlations in Trinuclear Cu(II) Complexes: A Density Functional Study

Summary
Density functional theoretical methods have been used to study magneto-structural correlations for linear trinuclear hydroxo-bridged copper(II) complexes. The nearest-neighbor exchange coupling constant shows very similar trends to those found earlier for dinuclear compounds for which the Cu-O-Cu angle and the out of plane displacement of the hydrogen atoms at the bridge are the two key structural factors that determine the nature of their magnetic behavior. Changes in these two parameters can induce variations of over 1000 cm-1 in the value of the nearest-neighbor coupling constant. On the contrary, coupling between next-nearest neighbors is found to be practically independent of structural changes with a value for the coupling constant of about - 60 cm-1. The magnitude calculated for this coupling constant indicates that considering its value to be negligible, as usually done in experimental studies, can lead to considerable errors, especially for compounds in which the nearest-neighbor coupling constant is of the same order of magnitude.
Antonio Rodríguez-Fortea, Eliseo Ruiz, Pere Alemany, Santiago Alvarez

Localized Orbitals vs. Pseudopotential-Plane Waves Basis Sets: Performances and Accuracy for Molecular Magnetic Systems

Summary
Density functional theory, in combination with a) a careful choice of the exchange-correlation part of the total energy and b) localized basis sets for the electronic orbitals, has become the method of choice for calculating the exchange-couplings in magnetic molecular complexes. Orbital expansion on plane waves can be seen as an alternative basis set especially suited to allow optimization of newly synthesized materials of unknown geometries. However, little is known on the predictive power of this scheme to yield quantitative values for exchange coupling constants J as small as a few hundredths of eV (50–300 cm-1). We have used density functional theory and a plane waves basis set to calculate the exchange couplings J of three homodinuclear Cu-based molecular complexes with experimental values ranging from + 40 cm -1 to + 300 cm -1. The plane waves basis set proves as accurate as the localized basis set, thereby suggesting that this approach can be reliably employed to predict and rationalize the magnetic properties of molecular-based materials.
Carlo Massobrio, Eliseo Ruiz

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