Chemists from the University of Bonn (Germany) have synthesized extremely unusual compounds. Their central building block is a silicon atom. However, the arrangement of the four bound partners of the atom is different from the usual, which are not in the shape of a tetrahedron around it, but are flat like a trapezoid. This arrangement is usually extremely energy unfavorable, but the molecules are very stable. Their properties are so far completely unknown; researchers now want to explore them. The results will be published in Journal of the American Chemical Society, but are already available online.
Like its relative carbon, silicon generally forms four bonds with other atoms. When this happens, the result is usually a tetrahedron. The silicon atom is in the center, its bonding partners (so-called ligands) at tetrahedral angles. This arrangement is the most energy efficient. Therefore, it is formed quasi automatically, just as a soap bubble is usually spherical.
Researchers led by prof. Dr. Alexander C. Filippou of the Institute of Inorganic Chemistry at the University of Bonn has now constructed molecules that contain silicon and are as unusual as cube-shaped soap bubbles. In them, the four ligands do not form a tetrahedron, but a distorted square, a trapezoid. They lie in one plane together with silicon. “Despite this, the joints are so stable that they can be easily bottled and stored for weeks,” explains Dr. Priyabrata Ghana, a former doctoral student who has since moved to RWTH Aachen University.
Molecular exotics are unusually stable
The researchers themselves were surprised by this unusual stability. They discovered the reason by modeling molecules on a computer. Ligands also form bonds with each other. In the process, they form a solid framework. This seems to be so strong that it completely prevents the trapezoidal arrangement from “jumping” into the tetrahedron. “Our computer calculations show that there is no structure for molecules that would be more energy-efficient than a flat trapezoidal shape,” emphasizes Jens Rump, a doctoral student at the Institute of Inorganic Chemistry.
The researchers grew crystals of the substances and then mined them with X-rays. X-ray light is scattered from atoms and changes its direction. Therefore, these deviations can be used to calculate the spatial structure of molecules in a crystal. Together with spectroscopic measurements, this method confirmed that ligands and silicon are indeed in the same plane of new molecules.
Although the synthesis of exotic compounds must be carried out under an inert gas, it is otherwise relatively simple. On the other hand, the production of starting materials is complex; one of them was first synthesized just over ten years ago and was already a source for the synthesis of several new classes of silicon compounds.
The influence of the unusual structure on the properties of silicon, an important element for the electronics industry, is currently completely unclear. In any case, it has long been considered completely impossible to produce such compounds.
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