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Synthesis

Silylated Ammonium ‘Trophy Molecule’

Chemists finally succeed in preparing the elusive silicon analog of a quaternary alkyl ammonium compound

by Stephen K. Ritter
June 16, 2014 | A version of this story appeared in Volume 92, Issue 24

A research team in Germany has prepared a long-awaited silicon analog of a tetraalkyl ammonium cation. The persilylated ammonium salt (shown), heralded as a “trophy molecule” by one journal editor, was reported by Axel Schulz and coworkers of the University of Rostock (Organometallics 2014, DOI: 10.1021/om500519j). The achievement comes after more than 20 years of research by multiple groups on silylium cations, SiR3+ (where R is an alkyl or aryl group), which have been pursued by chemists interested in comparing the properties of silicon and carbon compounds. Along the way an array of silylium-containing species has been reported, including S[Si(CH3)3]3+ and P[Si(CH3)3]4+, but never a persilylated ammonium cation such as N[Si(CH3)3]4+. One approach adopted by Schulz and his team, which had been unsuccessful by others in the past, was to attempt direct silylation of N[Si(CH3)3]3 with a silylium cation. The Rostock researchers pulled it off, but not without a hitch: Using the bridging hydride compound [(CH3)3SiHSi(CH3)3][B(C6F5)4] as a silylating reagent resulted in the loss of a methyl group, leaving the molecule one methyl shy of a full house. The researchers determined that the unexpected loss of the methyl group results from a methyl-hydrogen exchange reaction catalyzed by the silylating agent.

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