Cyrille Train

Molecules to build solids: high TC molecule-based magnets by design and recent revival of cyano complexes chemistry

Abstract This paper was presented as a session lecture at the XXXIII ICCC in Florence (29 August–4 September 1998). It intends to point out some recent achievements in the chemistry and physics of transition metal polycyanides in the field of molecular magnetism. Prussian blue is sometimes considered as the first coordination compound and the paper shows how it is possible to obtain brand new results with Prussian blue analogues when looking at these antique systems with fresh eyes. Hexacyanometalates revealed in the last few years as very flexible molecular precursors to build three-dimensional molecule-based magnets with tunable and high Curie temperatures or to grow high nuclearity clusters with tunable high spins and anisotropy. The use of a localized electron orbital model allowed the authors’ team to push the Curie temperatures from 5.6 K in the Prussian blue itself to above room temperature in a vanadium–chromium Prussian blue analogue. Several groups confirmed the result and are improving it. In the same way, high spin molecules with ground spin states ranging from S=3/2 to 27/2 were obtained. The paper reviews some of the steps which lead to these spectacular findings and some of the prospects opened in molecular materials by this revival of polycyanide chemistry.

Strong magneto-chiral dichroism in enantiopure chiral ferromagnets

As materials science is moving towards the synthesis, the study and the processing of new materials exhibiting well-defined and complex functions, the synthesis of new multifunctional materials is one of the important challenges. One of these complex physical properties is magneto-chiral dichroism which arises, at second order, from the coexistence of spatial asymmetry and magnetization in a material. Herein we report the first measurement of strong magneto-chiral dichroism in an enantiopure chiral ferromagnet. The ab initio synthesis of the enantiopure chiral ferromagnet is based on an enantioselective self-assembly, where a resolved chiral quaternary ammonium cation imposes the absolute configurations of the metal centres within chromium-manganese two-dimensional oxalate layers. The ferromagnetic interaction between Cr(III) and Mn(II) ions leads to a Curie temperature of 7 K. The magneto-chiral dichroic effect is enhanced by a factor of 17 when entering into the ferromagnetic phase.

High proton conduction in a chiral ferromagnetic metal-organic quartz-like framework.

A complex-as-ligand strategy to get a multifunctional molecular material led to a metal-organic framework with the formula (NH(4))(4)[MnCr(2)(ox)(6)]·4H(2)O. Single-crystal X-ray diffraction revealed that the anionic bimetallic coordination network adopts a chiral three-dimensional quartz-like architecture. It hosts ammonium cations and water molecules in functionalized channels. In addition to ferromagnetic ordering below T(C) = 3.0 K related to the host network, the material exhibits a very high proton conductivity of 1.1 × 10(-3) S cm(-1) at room temperature due to the guest molecules.

Large Magnetization-Induced Second Harmonic Generation in an Enantiopure Chiral Magnet

The absence of centrosymmetry in the enantiopure chiral magnet [N(CH(3))(n-C(3)H(7))(2)(C*H(CH(3))C(2)H(5))][Mn(II)Cr(III)(ox)(3)] allows the observation of bulk second harmonic generation (SHG) in this material. At low temperature, the onset of magnetization gives birth to a magnetization-induced SHG (MSHG) contribution. With an angular shift of 13.1 degrees upon magnetization reversal, the MSHG effects appear to be much larger than the corresponding linear magneto-optical effects. Thanks to the single-crystalline state of the sample, the variation of the signal with the orientation of the magnetic field and/or the angle between the polarization of the incident radiation and the outgoing SHG signal in the paramagnetic and ferromagnetic phases is reproduced and well-understood through the use of a symmetry-based analysis of the nonlinear susceptibility tensor.

Multiferroics by Rational Design: Implementing Ferroelectricity in Molecule‐Based Magnets

Multiferroics (MF) are materials that exhibit simultaneouslyseveral ferroic order parameters. Among the multiferroicmaterials, those combining antiferro- or ferroelectricity (FE)and antiferro-, ferri-, or ferromagnetism (FM) within thesame material are highly desirable: the coexistence of thepolar and magnetic orders paves the way towards four-levelmemories while their interactions through the magnetoelec-tric effect makes it possible to control the magnetization byelectric fields and hence to develop electronically tuneablemagnetic devices, which are an essential feature for spin-tronics.

Polyfunctional two- (2D) and three- (3D) dimensional oxalate bridged bimetallic magnets

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.

Magnetocaloric effect in hexacyanochromate Prussian blue analogs

We report on the magnetocaloric properties of two molecule-based hexacyanochromate Prussian blue analogs, nominally

Spin transition and exchange interaction: Janus visions of supramolecular spin coupling between face-to-face verdazyl radicals.

{\mathrm{CsNi}}^{II}[{\mathrm{Cr}}^{III}(\mathrm{CN}{)}_{6}]∙({\mathrm{H}}_{2}\mathrm{O})

Crystal‐Packing‐Induced Antiferromagnetic Interactions of Metallocenes: Cyanonickelocenes, ‐cobaltocenes, and ‐ferrocenes

and

Verdazyl-based extended structures: synthesis, structures and magnetic properties of silver(I) one dimensional compounds

{\mathrm{Cr}}_{3}^{II}[{\mathrm{Cr}}^{III}(\mathrm{CN}{)}_{6}{]}_{2}∙12({\mathrm{H}}_{2}\mathrm{O})