In its natural form, the crystalline solid silicon bonds with four other elements, creating a 3D structure called a tetrahedron. Tetrahedral silicon compounds are considered the building blocks of Earth’s crust. The crust is formed from different kinds of rocks such as granite, basalt, schist and sandstone, most of which consist mainly of minerals that are compounds of silicon – the second most abundant element in the crust after oxygen. In the past, synthesising and characterising a 2D equivalent of a silicon tetrahedron seemed unattainable. Now, scientists supported by the EU-funded pCx4All project have succeeded in creating such a crystalline complex. This achievement will pave the way for new uses of silicon in catalysis and materials research in the field of molecular chemistry.
Overcoming tetrahedral stability
Silicon is a metalloid and one of the six chemical elements that make up Group 14, the carbon group of the periodic table. Like carbon, silicon bonds with four elements to form a tetrahedron, a solid with four triangular sides. In geometric terms, the 2D equivalent of a tetrahedron would be a square. However, the tetrahedron is so stable that there are no other known structures in natural silicon with four bonds, or silicon(IV). According to a news release posted on ‘ScienceDaily’, Dr Lutz Greb of pCx4All project host Heidelberg University, Germany, noted that not even 40 years of intensive research were enough for scientists to produce a square-planar structure in the field of silicon(IV) chemistry. For the first time, Dr Greb’s team has succeeded in synthesising and completely characterising a square-planar silicon(IV) species. Their findings have been published in the journal ‘Chem’. The team grew a silicon monocrystal – a single-crystal solid whose crystal lattice is continuous and unbroken throughout – and then irradiated it using a focused beam of X-rays. When they encountered the monocrystal’s atoms, the X-rays were diffracted, resulting in an unmistakeable pattern from which the researchers could determine the position of the atomic nuclei. This made it possible to establish the existence of molecules with square-planar silicon(IV). Further analyses revealed unexpected physical and chemical properties in the structure, such as colour in a naturally colourless substance. “Synthesising this configuration from the components we chose is comparatively simple once you have understood the key conditions,” observed the study’s first author Dr Fabian Ebner, also from Heidelberg University, in the news release. However, the fact that the square-planar silicon(IV) molecule is a stable, isolable compound was reportedly a surprising discovery. “Due to the high reactivity, there are many conceivable ways of decomposition. Still, we have always believed that it is possible to isolate this compound,” Dr Greb concluded. The pCx4All (Calixpyrrole for p-block elements: anti-van’t Hoff-Le Bel configuration and ligand-element cooperativity revive the standard oxidation states.) project ends in October 2025. For more information, please see: pCx4All project
pCx4All, silicon, tetrahedron, square-planar silicon(IV), 2D, molecular chemistry