Motivation behind the research
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. To better understand how this happens, consider table salt: NaCl is solid at room temperature, but it melts at 801ºC. Potassium chloride, KCl, is a very similar substance, but due to the bulkier potassium cation, it forms weaker interionic bonds and melts only at 770ºC. Following this trend, scientists have long discovered that we can prepare salts with bulkier and less symmetric ions that melt at even lower temperatures. The chloride salt of a peculiar organic cation, with the long name of 1-butyl-3-methylimidazolium (bmim+) melts only at 70ºC, i.e. below the boiling point of water… it is an Ionic Liquid (IL).
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. But where does magnetism come from?
Electrons can be viewed as the smallest magnets, due to a fundamental property they possess, called “spin”. This is, for instance, the source of magnetism of a permanent magnet.
The spin of a single electron acts like a tiny magnet, but when it “pairs” to the spin of a neighbouring electron within the same atom, this magnetism is quenched as the two spins are netted out. If the electron remains “unpaired”, the atom itself becomes a tiny magnet, very much like the electron. 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.
From previous research, at the Florida International University, we knew that reaction of copper salts with certain organic molecules, called pyrazoles, can lead to larger molecules, called “complexes”, which are negatively charged and, most importantly, magnetic. These larger molecules contain three magnetic copper ions and these ions interact between them like small bar magnets. We also knew how to orient them, so that their interactions are ferromagnetic or antiferromagnetic. Finally, we 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.
Study Limitations and future studies
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).