Aggressive helper: Chemists have created a new group of so-called Lewis superacids – highly reactive acids that can snatch electrons from other molecules and thus break strong bonds. The new superacids in the form of boronic acid esters and ferrocenylboranes contain boron, iron and benzene rings and can be produced electrochemically. In the future, they could help to break down persistent organic pollutants and convert them into usable chemical raw materials.
Chemical bonds are formed when atoms share some of their outer electrons or give them all to the bonding partner. The strength of these bonds determines, among other things, how stable a molecule is and how easily it can be broken down again in the environment, for example. The synthesis of new substances from molecular starting materials is also made more difficult by too tight bonds.
Bond rupture through electron stealing
In such cases, chemists often use so-called Lewis acids as catalysts. These highly reactive acids can snatch electron pairs from other molecules and thus also break stable bonds. Antimony pentafluoride (SbF 5 ) was long thought to be the strongest Lewis acid , but chemists have now discovered some phosphorus- and boron-containing organic compounds that are even more aggressive. They are therefore referred to as Lewis superacids.
“For strong bonds you need very reactive reagents, i.e. substances that react extremely readily,” explains senior author Jan Paradies from the University of Paderborn. However, such Lewis superacids are very difficult to synthesize and isolate because of their high reactivity. However, with the help of electrochemical methods, the chemists have now succeeded in producing a new group of these superacids. “Using a trick, we managed to produce such molecules and use them in catalytic reactions,” says Paradies.
New boron-based Lewis superacids
The new superacids are a complex molecule made of boron, a halogen-containing radical and ferrocenyl – two benzene rings linked by an iron atom. In the presence of weak silver salts, these complex molecules form positively charged cations that are found to be extremely reactive: “The estimated fluoride ion affinities (a measure of Lewis acid activity) are 240 and 209 kilojoules per mole greater than that of antimony pentafluoride,” write the chemists. “Thus, these boranes can be classified as Lewis superacids.”
The reactivity of the new superacids is therefore sufficient to cleave particularly stable carbon-fluorine or sulfur-fluorine bonds. As a result, they could be used, for example, to “crack” fluorine-containing hydrocarbons – and thus render these pollutants, which are difficult to degrade in nature, harmless. With the help of such Lewis superacids, climate-damaging greenhouse gases such as sulfur hexafluoride could also be converted back into chemicals that can be used in a sustainable manner. (Angewandte Chemie, 2023; doi:10.1002/anie.202216959 )