Cédric Desplanches

A symmetry-breaking spin-state transition in iron(III)

Stepping up: A two-step magnetic spin transition with accompanying structural phase transitions is reported for the first time for Fe III. The transitions are observed at 187 K and 90 K on cooling with a hysteretic transition recorded upon heating during the first crossover at 106 K. The intermediate phase persists over 97 K and contains an unprecedented [HS-HS-LS] motif with tripling of the unit cell. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A hybrid magnet with coexistence of ferromagnetism and photoinduced Fe(III) spin-crossover

The insertion of a [Fe(sal2-trien)]+ complex cation into a 2D oxalate network results in a hybrid magnet with coexistence of magnetic ordering and photoinduced spin-crossover (LIESST effect) in compound [FeIII(sal2-trien)][MnIICrIII(ox)3]·(CH2Cl2) (1). A complete photomagnetic characterization together with a detailed structural analysis of the low-spin (LS) and high-spin (HS) structures of 1 is presented in order to understand such unusual behavior. This very rare and unexpected property in a FeIII spin-crossover complex, has been attributed to the strong distortion exhibited by the metastable HS state. Furthermore, 1 has shown that, in contrast to what has been previously proposed, a cooperative spin-crossover is not a necessary condition to observe LIESST effect in FeIII compounds. The photo-induced spin conversion of the inserted FeIII complex has shown to have a negligible influence on the cooperative magnetic behaviour of the 2D oxalate network.

Spin-state ordering on one sub-lattice of a mononuclear iron(III) spin crossover complex exhibiting LIESST and TIESST

The two-step spin crossover in mononuclear iron(III) complex [Fe(salpm)2 ]ClO4 ⋅0.5 EtOH (1) is shown to be accompanied by a structural phase transition as concluded from (57) Fe Mössbauer spectroscopy and single crystal X-ray diffraction, with spin-state ordering on just one of two sub-lattices in the intermediate magnetic and structural phase. The complex also exhibits thermal- and light-induced spin-state trapping (TIESST and LIESST), and relaxation from the LIESST and TIESST excited states occurs via the broken symmetry intermediate phase. Two relaxation events are evident in both experiments, that is, two T(LIESST) and two T(TIESST) values are recorded. The change in symmetry which accompanies the TIESST effect was followed in real time using single crystal diffraction. After flash freezing at 15 K the crystal was warmed to 40 K at which temperature superstructure reflections were observed to appear and disappear within a 10 000 s time range. In the frame of the international year of crystallography, these results illustrate how X-ray diffraction makes it possible to understand complex ordering phenomena.

New Molecule-Based Magnets: From Hexacyano to Octacyanometalates

Abstract Prussian blue molecule-based magnets can be designed to exhibit various magnetic properties. One of them based on chromicyanide and vanadium is a magnet at room temperature. Its properties are very sensitive to the synthetic reaction conditions, solvent effects and counterions which can be used to tune the Tc. To study the interaction between nearest metallic neighbours we explore also the influence of diffuse and high energy magnetic orbitals, i.e. 4d or 5d metallic orbitals. Two derivatives of [Wv(CN)8]3- are presented.

Does cooperativity influence the lifetime of the photo-induced HS state ?

We have first recalled the T(LIESST) procedure which consists to determine the temperature above which the photo-magnetic effect is erased. In addition we have selected to series of iron(II) spin crossover complexes, the [Fe(PM-L)2(NCS)2] and [Fe(bpp)2]X2·nH2O families, to analyse the influence of the cooperativity on the stability of the photo-induced HS state. Some of these complexes exhibit gradual thermal spin crossover behaviours while some others undergo an abrupt thermal transition, with and without hysteresis. Interestingly, whatever the cooperativity effect on the thermal spin crossover transition, the lifetime of the metastable state of all these derivates remains governed by the T(LIESST) = T0 - 0.31 T1/2 relation. Finally, we have investigated the magnetic and the photomagnetic properties of a [Fe(bpp)2]-Nafion film. Once more the role of the cooperativity on the stability of the photoinduced HS state appears minor. Conversely, the influence of the nature and the geometry of the inner coordination sphere appears preponderant.

A Novel μ1,1-Azido-, μ2-Alkoxo-, and μ2-Phenoxo-Bridged Tetranuclear Copper(ii) Complex with a Quinquedentate Schiff-Base Ligand: Magneto-Structural and DFT Studies

A novel tetranuclear copper(ii) complex [Cu4L2(μ1,1- N3)2]·CH3CN (1) has been synthesized using a symmetrical quinquedentate N2O3-donor Schiff-base ligand H3L (N,N′-(2-hydroxypropane-1,3-diyl)bis(2- hydroxyacetophenimine)) and characterized by elemental analysis, Fourier-transform (FT)-IR, UV-visible, electron paramagnetic resonance spectroscopy, and cyclic voltammetry. The X-ray single-crystal structure of 1 reveals three kinds of bridges among the four metal centres: one from the exogenous azido ligand and two from the phenoxo and alkoxo moieties of the Schiff base. The same structure highlights the remarkable versatility of copper(ii) to adopt an array of stereochemistries. Four unusual eight-membered metallacycles exist in 1. The μ1,1-azido, μ2-alkoxo, and μ2-phenoxo bridges among the four copper centres have facilitated interesting magnetic interactions that have been elucidated by variable-temperature magnetic susceptibility measurements and backed up by density functional theory calculations. Detailed analyses have shown that the antiferromagnetic effect through the alkoxo-bridged CuII centres combined with the ferromagnetic interaction through the azido-bridged metal centres results in an S = 0 ground state.

Kinetics of photo-induced phase transition and relaxation in the spin-crossover complexes [FexZn1-x(phen)2(NCS)2], influence of metal dilution

The kinetics of light-induced spin transition and relaxation in [FexZn1-x(phen)2(NCS)2] (phen=1,10-phenanthroline) has been investigated from time/temperature dependent x-ray powder diffraction with in situ optical excitation. We show that the phase transformation is driven by a heterogeneous nucleation and growth mechanism with phase separation, in both the thermally induced and light-induced regimes. The high spin to low spin isothermal relaxation curves strongly differ from first-order kinetics, and are interpreted using the Kolmogorov-Johnson-Mehl-Avrami model of phase transformation, from which the activation energy to domain growth is derived. Dilution tends to increase the activation energy, and slows down the whole relaxation process. Non-linear photo-induced kinetics as a function of laser power is evidenced, resulting from a light-induced phase separation process; a laser power threshold effect is pointed out. It is found that dilution speeds up the photo-conversion kinetics and reduces the laser power threshold.

Reversible switching of the electronic ground state in a pentacoordinated Cu(II) complex.

An easy reversible switching of the electronic ground state in a pentacoordinated copper(II) complex is reported for the first time. The simple protonation of a carboxylic group in a Cu(II) complex with a {dx(2)-y(2)}(1) electronic configuration leads to a flip of the ground electronic configuration from {dx(2)-y(2)}(1) to {dz(2)}(1) in the metal ion.

Towards synergy between spin-crossover and metal–ligand bond break in molecular crystals: structural investigations of eight seven-coordinated Fe(II) macrocyclic complexes

Sixteen structures of eight analogical Fe(II) macrocyclic complexes have been investigated exhibiting distinguishable structural properties even though no spin-crossover is observed. Notwithstanding, due to a synergistic effect between ligand field strength and intermolecular interaction, two of these complexes present very rare evidence for metal–ligand bond break at low temperature.

Complete temperature study of the relaxation from HS to LS state in the mixed [FexZn1-x(Phen)2(NCS)2] systems (with x = 1, 0.73, 0.5, 0.32, 0.19 and 0.04)

The thermal and light-induced spin transitions in [FexZn1-x(Phen)2(NCS)2] (Phen = 1,10-phenantholine) have been investigated by magnetic susceptibility, photomagnetism and diffuse reflectivity measurements. These complexes display a thermal spin transition and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. For each compound, the thermal spin transition temperature, T1/2, and the relaxation temperature of the photo-induced high-spin state, T(LIESST), have been systematically determined. It appears that T1/2 decreases with the metal dilution while T(LIESST) remains unchanged. This behaviour is discussed on the basis of the kinetic study performed by combining the use of SQUID magnetometer, up to the T(LIESST) value, and the dynamic diffuse reflectivity setup, from the T(LIESST) temperature up to the thermal spin transition.