Lahcène Ouahab

A Molecular-Based Magnet with a Fully Interlocked Three-Dimensional Structure

A compound has been synthesized with the formula (rad)2Mn2[Cu(opba)]3(DMSO)2.2H2O, where rad+ is 2-(4-N-methylpyridinium)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, opba is orthophenylenebis(oxamato), and DMSO is dimethyl sulfoxide. It consists of two nearly perpendicular graphite-like networks with edge-sharing Mn(II)6Cu(II)6 hexagons. The two networks are fully interlocked with the same topological relationship as that between adjacent rings of a necklace. The compound has three kinds of spin carriers: Mn(II) and Cu(II) ions, antiferromagnetically coupled through oxamato bridges, and rad+ radical cations, bridging the Cu(II) ions through the nitronyl nitroxide groups and forming Cu-rad chains. The temperature dependence of the magnetization reveals that below 22.5 K, the compound behaves as a magnet.

Synthesis and Magnetic Behavior of Rare-Earth Complexes with N,O-Chelating Nitronyl Nitroxide Triazole Ligands: Example of a [GdIII{Organic Radical}2] Compound with anS=9/2 Ground State

A ferromagnetically coupled gadolinium–radical compound is described. A series of three lanthanide complexes of general formula [Ln(organic radical)2(NO3)3] (Ln =Y3+, La3+, and Gd3+, shown on the right) have been synthesized. With the paramagnetic GdIII a ferromagnetic interaction with the ligands was found, which gives rise to a S = 9/2 ground-state spin.

Magnetic Memory in an Isotopically Enriched and Magnetically Isolated Mononuclear Dysprosium Complex

The influence of nuclear spin on the magnetic hysteresis of a single-molecule is evidenced. Isotopically enriched Dy(III) complexes are synthesized and an isotopic dependence of their magnetic relaxation is observed. This approach is coupled with tuning of the molecular environment through dilution in an amorphous or an isomorphous diamagnetic matrix. The combination of these approaches leads to a dramatic enhancement of the magnetic memory of the molecule. This general recipe can be efficient for rational optimization of single-molecule magnets (SMMs), and provides an important step for their integration into molecule-based devices.

A Mixed-Valence and Mixed-Spin Molecular Magnetic Material: [MnIIL]6[MoIII(CN)7][MoIV(CN)8]2⋅19.5 H2O

Long-range ferromagnetic ordering at 3 K is observed for the title compound, which may be considered as a fully localized mixed-valence species (Mo(3+) and Mo(4+)) as well as a mixed-spin species (low-spin and high-spin Mn(2+) ions). Its two-dimensional structure consists of heart-shaped 48-membered rings, and each ring contains 16 metal centers (see picture).

Magnetic Poles Determinations and Robustness of Memory Effect upon Solubilization in a DyIII-Based Single Ion Magnet

The [Dy(tta)3(L)] complex behaves as a single ion magnet both in its crystalline phase and in solution. Experimental and theoretical magnetic anisotropy axes perfectly match and lie along the most electro-negative atoms of the coordination sphere. Both VSM and MCD measurements highlight the robustness of the complex, with persistence of the memory effect even in solution up to 4 K.

Tetrathiafulvalene-amido-2-pyridine-N-oxide as Efficient Charge-Transfer Antenna Ligand for the Sensitization of YbIII Luminescence in a Series of Lanthanide Paramagnetic Coordination Complexes

The tetrathiafulvalene-amido-2-pyridine-N-oxide (L) ligand has been employed to coordinate 4f elements. The architecture of the complexes mainly depends on the ionic radii of the lanthanides. Thus, the reaction of L in the same experimental protocol leads to three different molecular structure series. Binuclear [Ln(2)(hfac)(5)(O(2)CPhCl)(L)(3)]·2 H(2)O (hfac(-)=1,1,1,5,5,5-hexafluoroacetylacetonate anion, O(2)CPhCl(-)=3-chlorobenzoate anion) and mononuclear [Ln(hfac)(3)(L)(2)] complexes were obtained by using rare-earth ions with either large (Ln(III)=Pr, Gd) or small (Ln(III)=Y, Yb) ionic radius, respectively, whereas the use of Tb(III) that possesses an intermediate ionic radius led to the formation of a binuclear complex of formula [Tb(2)(hfac)(4)(O(2)CPhCl)(2)(L)(2)]. Antiferromagnetic interactions have been observed in the three dinuclear compounds by using an extended empirical method. Photophysical properties of the coordination complexes have been studied by solid-state absorption spectroscopy, whereas time-dependent density functional theory (TD-DFT) calculations have been carried out on the diamagnetic Y(III) derivative to build a molecular orbital diagram and to reproduce the absorption spectrum. For the [Yb(hfac)(3)(L)(2)] complex, the excitation at 19,600 cm(-1) of the HOMO→LUMO+1/LUMO+2 charge-transfer transition induces both line-shape emissions in the near-IR spectral range assigned to the (2)F(5/2)→(2)F(7/2) (9860 cm(-1)) ytterbium-centered transition and a residual charge-transfer emission around 13,150 cm(-1). An efficient antenna effect that proceeds through energy transfer from the singlet excited state of the tetrathiafulvalene-amido-2-pyridine-N-oxide chromophore is evidence of the Yb(III) sensitization.

Single-Molecule Magnet Behaviour in a Tetrathiafulvalene-Based Electroactive Antiferromagnetically Coupled Dinuclear Dysprosium(III) Complex

The reactions between the [Ln(tta)(3)]·2H(2)O precursors (tta(-)=2-thenoyltrifluoroacetonate anion) and the tetrathiafulvalene-3-pyridine-N-oxide ligands (L(1)) lead to dinuclear complexes of formula [{Ln(tta)(3)(L(1))}(2)]·xCH(2)Cl(2) (x=0.5 for Ln=Dy(III) (1) and x=0 for Ln=Gd(III) (2)). The crystal structure reveals that two {Ln(tta)(3)} moieties are bridged by two donors through the nitroxide groups. The Dy(III) centre adopts a distorted square antiprismatic oxygenated polyhedron structure. The antiferromagnetic nature of the exchange interaction between the two Dy(III) ions has been determined by two methods: 1) an empirical method using the [Dy(hfac)(3)(L(2))(2)] mononuclear complex as a model (3) (hfac(-)=1,1,1,5,5,5-hexafluoroacetylacetonate anion, L(2)=tetrathiafulvaleneamido-2-pyridine-N-oxide ligand), and 2) assuming an Ising model for the Dy(III) ion giving an exchange energy of -2.30 cm(-1), g=19.2 in the temperature range of 2-10 K. The antiferromagnetic interactions have been confirmed by a quantitative determination of J for the isotropic Gd(III) derivative (J=-0.031 cm(-1), g=2.003). Compound 1 displays a slow magnetisation relaxation without applied external magnetic fields. Alternating current susceptibility shows a thermally activated behaviour with pre-exponential factors of 5.48(4)×10(-7) s and an energy barrier of 87(1) K. The application of an external field of 1.6 kOe compensates the antiferromagnetic interactions and opens a new quantum tunnelling path.

Delicate Crystal Structure Changes Govern the Magnetic Properties of 1D Coordination Polymers Based on 3d Metal Carboxylates

Homo- and heterometallic 1D coordination polymers of transition metals (Co II, Mn II, Zn II) have been synthesized by an in-situ ligand generation route. Carboxylato-based complexes [Co(PhCOO)2]n (1 a, 1 b), [Co(p-MePhCOO)2]n (2), [ZnMn(PhCOO)4]n (3), and [CoZn(PhCOO)4]n (4) (PhCOOH=benzoic acid, p-MePhCOOH=p-methylbenzoic acid) have been characterized by chemical analysis, single-crystal X-ray diffraction, and magnetization measurements. The new complexes 2 and 3 crystallize in orthorhombic space groups Pnab and Pcab respectively. Their crystal structures consist of zigzag chains, with alternating M(II) centers in octahedral and tetrahedral positions, which are similar to those of 1 a and 1 b. Compound 4 crystallizes in monoclinic space group P2 1/c and comprises zigzag chains of M II ions in a tetrahedral coordination environment. Magnetic investigations reveal the existence of antiferromagnetic interactions between magnetic centers in the heterometallic complexes 3 and 4, while ferromagnetic interactions operate in homometallic compounds (1 a, 1 b, and 2). Compound 1 b orders ferromagnetically at TC=3.7 K whereas 1 a does not show any magnetic ordering down to 330 mK and displays typical single-chain magnet (SCM) behavior with slowing down of magnetization relaxation below 0.6 K. Single-crystal measurements reveal that the system is easily magnetized in the chain direction for 1 a whereas the chain direction coincides with the hard magnetic axis in 1 b. Despite important similarities, small differences in the molecular and crystal structures of these two compounds lead to this dramatic change in properties.

Coordination complexes in conducting and magnetic molecular materials

Abstract In this paper we present organic–inorganic molecular materials investigated in our group during the last decade. These materials result from the chemical and electrochemical molecular assemblies of coordination complexes such as tetracyanometallates, hexacyanometallates, metal bis-(dithiolates), metal bis-(dithiolenes), metallocenium complexes and polyoxometalates and organic precursors such as TTF and BEDT-TTF derivatives and paramagnetic nitronyl nitroxide radicals. Taking advantage of the fascinating flexibility of coordination chemistry, the goal of our work is to prepare compounds with particular physical properties such as electrical conductivity, magnetism, spin crossover, and also materials combining two or more of these properties. In the light of the observed results, we present different approaches which we are currently investigating with the aim of increasing interactions between the organic and inorganic components.

Synthesis, X-ray crystal structures and magnetic properties of Cu(II)(NITpPy)2[N(CN)2]2·solv (NITpPy=nitronyl nitroxide radical, solv=H2O or CH3CN). From discrete molecules to 2-D polymeric coordination compounds

Abstract The synthesis, X-ray crystal structures and magnetic properties of five new copper(II) complexes containing nitroxide radicals Cu(NITpPy)2(NO3)2 (1), Cu(NITpPy)2(CH3COO)2 (2), Cu(NITpPy)2[N(CN)2]2·(H2O)2 (3), Cu(NITpPy)2[N(CN)2]2·3CH3CN (4) and Cu(NITpPy)2[N(CN)2]2·2CH3CN (5) are reported. Compounds 1–3 consists of discrete molecules in the solid state, but in compounds 4 and 5 the Cu(NITpPy)22+ units are connected through N(CN)2− bridging ligands in μ2 coordination developing, respectively, into one-dimensional and two-dimensional polymeric networks. For all compounds, the magnetic properties have been investigated and antiferromagnetic interactions dominate at low temperature.