Dalton Transactions

Can you melt a magnet?

Researchers from the Institut de Chimie, have reported magnetic liquids that are neither solutions nor suspensions, but low-melting magnetic salts. This work was featured by the science blog "The Surg" with the title "Can you melt a magnet?" (https://thesurg.com/can-you-melt-magnet/) and hopefully will provide ideas for the deployment of magnetic materials in the liquid state.

Background

Ionic liquids are an intriguing class of substances: they are salts, but they have very low melting points, and some are liquid at room temperature or even below. E.g., the chloride salt of 1-butyl-3-methylimidazolium (bmim+) melts only at 70ºC.

Ionic Liquids have many fascinating properties: e.g., although they are salts, they are also liquid, which makes them excellent solvents at room temperatures.

Metal-containing ILs have long been known, but a key property of metal ions has gained attention only recently: magnetism. In other words, the ability of a material to interact with magnetic fields.

Various metallic ions contain one more more unpaired electrons in their outer shells which makes them also act as tiny magnets: these ions, and their compounds, are called paramagnetic. When those ions approach each other, entirely new magnetic behaviours are observed. E.g. antiferromagnetism, when the spins of the ions align in an antiparallel fashion or ferromagnetism, when the spins align in a parallel fashion.

Such behaviours are very useful for practical applications: e.g. ferromagnetism is what allows magnetic hard disks to store information. But such properties are observed in solids. What if we need to work with liquids? We could prepare solutions of certain magnetic materials, but then they would be diluted and not in their pure form. To understand the difference, compare a bar of chocolate to a hot chocolate beverage: the former is pure, the latter is diluted and tastes differently. If we could melt this bar of chocolate, we could work with it in entirely new ways, without diluting it or changing its taste. Ionic Liquids give us a solution to this problem.

The research results

From previous research, at the Florida International University, we knew that eaction of copper halides with pyrazolate ligands can lead to trinuclear anionic complexes which can exhibit intramolecular ferro- or antiferromagnetic interactions. We also knew that we can use those negatively charged complexes to form salts with cations of our choice. This seemed like the ideal system to prepare magnetic Ionic Liquids.

We used the previously mentioned bmim+ cation to prepare salts of those complexes and we studied their magnetic and thermal properties at the University of Strasbourg. We synthesized a ferromagnetic salt that melts at 140ºC and which remains molten upon cooling down to 70ºC. We also synthesized an antiferromagnetic salt, which was obtained as a viscous paste at room temperature and which does not to crystallise upon cooling, but simply become more and more viscous, just like glasses do.

 

This study was focused on a specific cation, bmim+, and complexes of unsubstituted pyrazole. Moreover, it was mostly focused on pure samples of these salts and did not examine in depth the behaviour of their mixtures, which could melt at even lower temperatures. We will be expanding our research to include additional cations, substituted pyrazoles and mixtures of different such salts.

Research Article:

Towards ionic liquids with tailored magnetic properties: bmim+ salts of ferro- and antiferromagnetic CuII3 triangles. Dalton Transactions, Volume 46, 12263-12273 (doi: 10.1039/C7DT02472J).

Authors:

Athanassios K. Boudalis,*^ab Guillaume Rogez,^c Benoît Heinrich,^c  Raphael G. Raptis,^a and Philippe Turek^b

* Corresponding authors

^a Department of Chemistry and Biochemistry and the Biomolecular Sciences Institute, Florida International University, Miami, USA

^b Institut de Chimie de Strasbourg (UMR 7177, CNRS-Unistra), Université de Strasbourg, 4 rue Blaise Pascal, CS 90032, F-67081 Strasbourg, France
E-mail:   bountalis@unistra.fr. 

^c Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504, F-67000 Strasbourg, France