A carborane anion stabilizes a highly Lewis-acidic silylium cation that catalyses the hydrodefluorination of polyfluoroalkanes
Perfluorinated compounds have some very special properties, most notably that they are often immiscible with both oil and water. These properties have led to their widespread use in household items such as stain repellents and non-stick cookware, and hydrofluorocarbons have also been used as refrigerator coolants in place of chlorofluorocarbons. However, they have recently been shown to be toxic, and the high strength of the carbon–fluorine bond means that such compounds are persistent in the environment. They are also thought to be super-greenhouse gases. Methods that can convert them into less harmful compounds are therefore of great interest.
Now, Christos Douvris and Oleg Ozerov from Brandeis University in the USA have developed a catalyst that efficiently substitutes fluorine for hydrogen in hydrofluorocarbons1. Previous methods have relied on reductive processes using electron-rich metal complexes, but Ozerov and Douvris were attracted to a conceptually different approach, which relies on the abstraction of fluoride by a silylium cation — a very strong Lewis acid — followed by reaction of the resultant carbocation with a silane to form a new silylium ion. The silylium species therefore acts as a catalyst in the process, which is extremely thermodynamically favourable because the C–H and Si–F bonds formed are significantly stronger than the C–F and Si–H bonds that are broken. The key to developing the process was to identify a counterion that could stabilize the silylium cation intermediate and the highly stable carborane anion [HCB11H5Cl6]− proved very successful.
The process was shown to have a much higher turnover — the number of C–F bonds broken per molecule of catalyst — than previous systems. The high efficiency may be beneficial in large-scale remediative applications.
Douvris, C. & Ozerov, T. Hydrodefluorination of perfluoroalkyl groups using silylium carborane catalysts. Science 321, 1188 (2008).
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Davey, S. Polyfluoroalkanes come unstuck. Nature Chem (2008). https://doi.org/10.1038/nchem.60