A team of researchers led by Oldenburg chemist Thomas Müller has found important clues as to how novel, non-toxic catalysts - so-called "frustrated Lewis pairs" - cause hydrogen to react.
Hydrogen is an important raw material for the chemical industry - and an environmentally friendly energy source, for example for vehicles with fuel cells. However, reaction accelerators are needed to make the inert molecule usable for chemical reactions. These so-called catalysts usually contain expensive precious or toxic heavy metals. Together with colleagues from TU Berlin, the Oldenburg chemists led by Prof Dr Thomas Müller have now used targeted experiments to find the first important clues as to how a certain group of catalysts, "frustrated Lewis pairs", react with hydrogen, for example. The researchers have published their findings in the international edition of the renowned journal Angewandte Chemie.
Lewis pairs consist of an acid and a base and thus of two molecules that attract each other very strongly and neutralise each other in their effect. However, if the Lewis pairs consist of particularly large molecules with a special structure, they cannot come together - they are "frustrated". A good ten years ago, scientists discovered that these frustrated pairs are able to form a complex with hydrogen molecules, for example - which naturally consist of two atoms - and ultimately split the molecule. The Lewis acid consists of a compound containing the element boron; the Lewis base contains the element phosphorus.
A few years ago, the Oldenburg chemists also discovered that instead of boron, compounds with silicon are also suitable as Lewis acids. "Using these molecules to get hydrogen to react would be wonderful," says Müller. The advantage is that, unlike precious and heavy metals, phosphorus and silicon are very common in nature. They are also cheap and non-toxic. However, scientists have not yet understood how frustrated Lewis pairs help reactions with hydrogen to take off.
This is where the current work of the Oldenburg chemists comes in: In experiments, they were able to identify the first intermediate steps of the unusual reaction. "In our investigations, we were able to clearly show that the frustrated Lewis pairs form very reactive compounds, so-called radicals," says Anastasia Merk, a doctoral student at the university's Institute of Chemistry and first author of the study. According to Müller, the findings are an important step towards better understanding the reactivity of Lewis pairs. However, the researchers still need to carry out further investigations in order to be able to generalise their conclusions.
It is not yet clear when frustrated Lewis pairs could also play a decisive role in industrial processes, says Müller. This depends above all on economic factors. However, he sees other applications for the reactive molecules - in the pharmaceutical industry: in future, drugs that require hydrogen to react during production could be manufactured without toxic metal residues.