Chemists from Germany have synthesized and characterized several halogenated silylium salts with a carborane moiety as the anion. They are stronger Lewis acids than alkylated silylium salts, but they gradually decompose in solution. The study was published in the journal Nature Chemistry.
In many reactions involving halogenosilanes, silylium salts are formed as intermediates — ionic particles with a cation containing trivalent silicon. For a long time, chemists were unable to obtain such a salt in pure form, but in 2002, scientists finally succeeded and reliably characterized the silylium salt for the first time. It consisted of a trimethylsilyl cation and a non-coordinating carborane anion. Since then, chemists have been trying to obtain silylium salts in which not only carbon atoms but also atoms of other elements will be bound to silicon.
Martin Oestreich and his colleagues from the Technical University of Berlin decided to take on the production of halogen-substituted silylium salts. To do this, they took diisopropylchlorosilane, in which two isopropyl groups, a hydrogen atom, and a chlorine atom are attached to the silicon atom, and mixed it with a carborane salt of a triphenylmethyl cation in deuterobenzene. The scientists' idea was that the cation would tear off a hydrogen atom from the silane, forming the desired chlorine-substituted salt. Instead, the researchers observed the formation of a salt with another cation, in which two isopropyl groups and a hydrogen atom were attached to the silicon.
Then they decided to use a different method of synthesis: the chemists mixed the original silane with a salt in which the cation was protonated benzene. In this case, the reaction went as expected - hydrogen was released, and a salt with a cation containing a silicon-chlorine bond and two isopropyl groups was formed. The chemists characterized it using NMR spectroscopy and X-ray structural analysis.
Using the same method, scientists obtained fluorine-, bromine-, and iodine-substituted silylium salts. They were stable in solid form in an inert atmosphere, but gradually decomposed in solution, and decomposed especially quickly in the light.
Next, the chemists decided to check how strong the obtained silylium salts are Lewis acids, that is, how high their affinity is for unshared electron pairs. To do this, the scientists used NMR spectroscopy of adducts of their salts with fluorinated benzonitrile, as well as quantum-chemical calculations. It turned out that among all previously obtained silylium salts, halogenated salts are the strongest Lewis acids.
Thus, scientists synthesized halogen-substituted silylium salts for the first time and studied their acidic properties. As the authors of the article write, the obtained compounds are among the strongest Lewis acids ever obtained in pure form.
Most often, organosilicon compounds, including silanes, are used to obtain polymers. Recently, chemists from France have learned to convert such polymers back into monomeric chlorosilanes, in particular, into dimethylchlorosilane.
