ISTOCK
A team of Russian researchers has successfully synthesised a new class of complex compounds of rare earth elements. Unlike other lanthanide compounds, the resulting substances are highly soluble in most organic solvents. These novel compounds can find application in organic and organometallic synthesis, as well as in the production of new luminescent materials. The study has been published in Inorganic Chemistry.
The research was carried out with support from the Russian Science Foundation (RSF). Project 22-13-00312.
Luminescent materials are substances capable of absorbing energy from an external source and emitting it in the form of visible or invisible infrared light. These materials are used in fluorescent thermometers, where the intensity or colour of the emitted glow varies according to the temperature. They are employed in LEDs, which emit light when an electric current passes through them, and in labels that can only be seen with special devices, helping to identify authentic products. For instance, fluorescent labels on Russian banknotes remain invisible under natural light but fluoresce under ultraviolet light.
Scientists continue to search for novel methods to produce luminescent materials which are both cost-effective and more efficient than current ones. A team of researchers including students of the HSE Faculty of Chemistry Svetlana Degtyareva, Daniil Bardonov and Anna Afanaseva, led by Dmitrii Roitershtein, have discovered a new class of stable lanthanide compounds exhibiting luminescent properties. The researchers coined the term 'pseudosandwiches' to describe complex compounds of rare earth elements with the formula [LnCl3L2].
For 70 years, the term 'sandwich' has been employed to describe metal compounds featuring two planar organic ligands. The metal acts as a 'burger' positioned between two 'slices of bread'—the two ligands—above and below, forming a complex with the central atom. In our substance, positioned between the two 'slices of bread' is a self-contained chemical compound—lanthanide chloride—thus, we call this configuration a pseudosandwich.
To synthesise pseudosandwiches, the scientists used dehydrated lanthanide chlorides, to which tetrahydrofuran was added. At the next stage, an accessible organic compound was introduced—a six-membered ring consisting of three nitrogen atoms and three carbon atoms. The resultant substance was dissolved in organic solvents, and crystals were subsequently grown from this solution. The reactions were conducted within a controlled environment, inside a sealed chamber free of moisture and oxygen and filled with argon to maintain an inert atmosphere.
The scientists have shown the method to be effective across the entire row of rare earth elements, obtained 30 new compounds featuring different rare earth metals, and established the molecular structure for 20 of them.
The obtained crystals were analysed using four different methods. One of these methods is X-ray diffraction analysis, which examines how X-rays are diffracted in the crystal lattice of a substance. Using this data, scientists can determine how atoms and molecules are arranged in a crystal. According to the authors of the paper, the obtained compounds exhibit an atypical planar trigonal structure of the LnCl3 fragment, wherein the atoms reside in the same plane, forming a triangle with the metal at the centre.
'Atoms within molecules obey specific rules of connectivity, and there are numerous examples illustrating these principles, amounting to tens of thousands of different structures. However, we have never observed such an arrangement of atoms in a lanthanide compound previously,' according to Svetlana Degtyareva.
After determining the structure and arrangement of the compounds in the solid phase, the scientists investigated their behaviour in solution. Solubility is a critical property for industrial material synthesis as it facilitates the blending of multiple substances, with liquids being particularly convenient for mixing purposes. A solution is also inherently easier to apply to a surface.
It turns out that the obtained substances are highly soluble in organic solvents and exhibit luminescence in both states, far more effectively than the scientists had anticipated. Furthermore, the substances crystallise well and maintain a consistent composition.
'The compounds we have synthesised are the least sensitive to air and the most stable among all we have worked with. The obtained compounds hold promise for use in material manufacturing due to their excellent solubility in hydrocarbons and luminescent properties,' notes Daniil Bardonov.
The scientists are currently preparing a new study in collaboration with colleagues from the Faculties of Chemistry and Physics at Lomonosov Moscow State University. 'Together with colleagues from MSU, we are investigating whether the complex compounds of rare earth elements we have obtained could potentially serve as promising molecular magnets. The study is still at an early stage, but we have already been able to observe some positive results,' according to Anna Afanaseva.
IQ
Dmitrii Roitershtein
Co-author of the paper, Academic Supervisor of the programme 'Chemistry of Molecular Systems and Materials'