Chemists Synthesize Extremely Unusual ‘Flat’ Silicon Compounds

Every field of study gives us many new things to ponder on every day. Chemistry is one such field. Chemists at the University of Bonn (Germany) have synthesized extremely unusual compounds this time.

Silicon is the central building block in this case. Usually, we have the arrangement of atoms around silicon in a tetrahedral arrangement, whereas this time the arrangement is flat like a trapezoid.

If you have an idea about such an arrangement, you would argue that the molecule is energetically extremely unfavourable. But these molecules seem to be very stable. Such properties are not yet known, thus grabbing the interest of researchers to explore them.

We all know about how Carbon has its four bonds with other atoms. It is in a tetrahedron form. Silicon, also being a relative of Carbon (same group) shows the same property. The main silicon atom is at the centre with the bonding atoms (ligands) at the tetrahedral corners. This arrangement is most favoured energy-wise, thus being more stable just like a soap bubble is usually spherical.

Led by Prof. Dr. Alexander C. Filippou of the Institute for Inorganic Chemistry at the University of Bonn, researchers have created silicon-containing molecules that are as unusual as a cube-shaped soap bubble. In these molecules, the four ligands form a distorted square, trapezoid rather than forming a tetrahedron. All the atoms including silicon lie in one plane.

Dr. Priyabrata Ghana, a former doctoral student who has since moved to RWTH Aachen University, explains that despite the fact of the shape of the molecule, this current compound is so stable that they can be filled into bottles and stored for weeks with no problems.

Researchers were surprised by such unusual stability. The reason was found by modelling the molecules on the computer. The ligands also form bonds with each other, thus forming a solid framework. The bond appears so strong that it completely prevents the trapezoidal arrangement from “snapping” into a tetrahedron.

Jens Rump, a doctoral student at the Institute for Inorganic Chemistry, said that the computer calculations showed there was no structure for other molecules that are more energetically favourable than the planar trapezoidal shape.

The researchers grew the crystals of substances and then blasted them with X-rays. The atoms scatter the X-ray light and change its direction. These deviations are used to calculate the spatial structure of molecules in the crystal. This method confirmed that ligands and silicon are in the same plane in the new molecules. The spectroscopic measurements confirmed this. The synthesis of the compounds requires to be done under inert gas. The process is comparatively simple.

But producing the starting material used for production is a bit complex. One of them was first synthesized only just over ten years ago and has already been the source for the synthesis of several novel classes of silicon compounds. For a long time, it was considered completely impossible to produce such compounds.

Journal Reference:
Priyabrata Ghana, Jens Rump, Gregor Schnakenburg, Marius I. Arz, Alexander C. Filippou. Planar Tetracoordinated Silicon (ptSi): Room-Temperature Stable Compounds Containing Anti-van’t Hoff/Le Bel Silicon. Journal of the American Chemical Society, 2020; DOI: 10.1021/jacs.0c11628

Press Release: University of Bonn