Laboratoire de Chimie de Coordination UPR 8241


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Thématiques

  • Matériaux magnétiques


A l’interface de la chimie et de la physique, nous exploitons des propriétés électroniques - interaction d’échange, anisotropie magnétique - de composés de coordination des éléments de transition et des ions de terres-rares pour réaliser des aimants et des nano-aimants de type SMM ou SCM (Single-Molecule ou Single- Chain Magnets).


Quelques exemples récents :


○ Cyano-Bridged Fe(II)−Cr(III) Single-Chain Magnet Based on Pentagonal Bipyramid Units : On the Added Value of Aligned Axial Anisotropy


The relative orientation of the local uniaxial anisotropies can have a significant contribution to the overall energy barrier for magnetization reversal, Δeff/kB, and should be a parameter seriously taken into account in the search of SCM with augmented performances. This statement is substantiated by the energy gap of 113 K and magnetic hysteresis with a coercive field of 1400 Oe at 2 K achieved for a cyano-bridged Fe(II)−Cr(III) single-chain magnet designed to ensure a parallel orientation of the axial anisotropy of the building units. This ferromagnetic chain compound consists of a pentagonal bipyramid Fe(II) complex with Ising-type anisotropy and a dicyanide Cr(III) complex interlinked through their apical positions. The quite remarkable performances of this single-strand SCM positions this compound among the very few examples of SCMs with spin reversal barriers above 100 K.
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○ Pentagonal Bipyramid Fe(II) Complexes : Robust Ising-spin Units Towards Heteropolynuclear Nano-Magnets


Pentagonal bipyramid Fe(II) complexes have been investigated in order to evaluate their potential as Ising-spin building units for the preparation of hetero-polynuclear complexes that are likely to behave as SMMs. The considered monometallic complexes result from association of a divalent metal ion with pentadentate ligands having 2,6-diacetylpyridine bis(hydrazone) core (H2LN3O2R). Their magnetic anisotropy established by magnetometry revealed zero-field splitting (ZFS) parameter D ranging between -4 and -13 cm-1 which was found to be modulated by the apical ligands (ROH versus Cl).
The alteration of D by N-bound CN ligands in axial positions upon association with cyanometallates was also assessed for heptacoordinated Fe(II) as well as for related Ni(II) and Co(II) derivatives. In all cases N-coordinated cyanides lead to large magnetic anisotropy (i.e. -8 to -18 cm-1 for Fe and Ni, +33 cm-1 for Co). Ab initio calculations performed on three Fe(II) complexes enable one to rationalize the role of the ligand on the nature and magnitude of the magnetic anisotropy. Starting from the preexisting heptacoordinated complexes, a series of pentanuclear complexes have been obtained by reactions with paramagnetic [W(CN)8]3-. Magnetic studies revealed the occurrence of ferromagnetic interactions between the spin carriers in all the heterometallic systems. Field-induced slow magnetic relaxation was observed for mononuclear Fe(II) complexes and SMM behavior was evidenced for a heteronuclear [Fe3W2] derivative confirming the parent complexes as robust Ising-type building units. High-field EPR (HF-EPR) spectroscopic investigation confirmed the ZFS parameters for a Ni derivative.
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○ On the importance of ferromagnetic exchange between transition metals in field-free SMMs : example of ring-shaped hetero-trimetallic [(LnNi2)W(CN)8]2 compounds.


Rare case of genuine (i.e. field-free) SMM has been found for mixed 3d-4f-5d ring-shaped compounds with Tb(III) (Ueff/kB = 23.0 K) or Dy(III) (Ueff/kB = 26.4 K). The ferromagnetic interactions between the transition metals are shown to play an essential role in this feature. More importantly, the present results suggest that 3d-4f SMMs with enlarged energy barriers for magnetization reversal can be expected when polynuclear units of transition metal ions, with substantial ferromagnetic interactions between them, are interacting with the anisotropic 4f centres.
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○ Novel bi-compartmental Schiff-base with peripheral ester functionalization


A novel ester-functionalized bi-compartmental Schiff-base ligand and its dinuclear Zn-Ln complexes [Ln = Dy, Tb, Gd] have been investigated. Although electron-withdrawing substituents at the para position of the phenol do not change the coordination environment of the Ln centers in comparison with the non-functionalized ligand, they have a clear steric effect on the crystal packing and alter the coordination strength of the central ligand core. This is illustrated by the atypical reactivity of the bi-compartmental Schiff-base ligand towards Ln coordination and corroborated by its longer bond lengths to the rare-earth ions in comparison with the non-functionalized ligand. A slow relaxation of the magnetization with an energy barrier of 96 K was found for Dy derivative in zero field.
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  • Edifices à structures ouvertes


Nous explorons une approche originale vers des architectures microporeuses aux propriétés zéolitiques spécifiques utilisant la liaison hydrogène ionique comme ciment de structures 3-D ouvertes.


Quelques exemples récents :


○ Hydrogen-Bonded Open-Framework with Pyridyl-Decorated Channels : Straightforward Preparation and Insight into its Affinity for Acidic Molecules in Solution


An hydrogen-bonded open framework with pores decorated by pyridyl groups has been constructed following an off-charge-stoichiometry assemblage of protonated tetrakis(4-pyridyl-oxymethyl)methane and [Al(oxalate)3]3-, respectively the H-bond donor and acceptor of the ionic H-bond interactions. This supramolecular porous architecture (SPA-2) possesses 1 nm-large pores interconnected in 3D with high solvent accessible void (53%). It demonstrated remarkable affinity for acidic organic molecules in solution, which was investigated by the means of various carboxylic acids including larger drug molecules. Noteworthy, competing sorption between acetic acid and its halogenated homologues evidenced good selectivity of the porous material for the halogenated acids. In most of the examples considered here the acid-to-pyridine ratio exceeded one, revealing that, though the acid-base association is decisive in the efficiency of the sorption process, several other supramolecular interactions contribute to the accommodation of the guests. These include H-bonding involving the carboxylic groups but also weaker interactions such as halogen bonding with the oxalate units as revealed by crystal structure and HRMAS-NMR investigations. This variety of host-guests interactions points to the importance of the overall composition of the channels’ walls in the uptake capacity of the porous material. The facile release of these guests’ molecules in neutral conditions makes this SPA a carrier of acidic molecules.
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○ Nano-porous supramolecular framework based on ionic H-bonds


SPA-1 is a rare example of robust H-bonded supramolecular metal-organic framework with permanent porosity. Its large (1 nm) and polar 1-D channels have been found to adsorb a wide variety of guest molecules (I2 in figure) with no alteration of the framework. As demonstrated for I2, the guests can be released in smooth conditions by a polar solvent such as EtOH. Remarkably, the porous architecture is sufficiently robust to allow reactions within the channels as shown for aniline polymerization. The chemical approach followed for its preparation involves rather simple building units, i.e. an anionic metal-oxalate complex and a cationic pyridine derivative, that efficiently assemble to a crystalline material in ambient conditions and water.
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○ Supramolecular open-framework architectures based on dicarboxylate H-bond acceptors and polytopic cations with three/four N-H+ donor units


Supramolecular assemblages based on anionic H-acceptors and cationic H-donors have been envisioned to elaborate open frameworks maintained by ionic H-bonds. Combinations of di-anionic chloranilate (CA2-), oxalate (Ox2-), or terephtalate (BDC2-) and trisimidazolium or tetrapyridinium derivatives (respectively three and four N-H+ donors), yielded open framework architectures. Their assembling patterns and framework dimensionalities are mainly governed by the chemical features of the cation.
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