Tricoordination NMR Evidence for Free Stannylium Ions

Lambert and Zhao reported preparation of a free, tricoordinate silylium ion in 1997, based on NMR chemical shift data. They used their technique of nucleophilic isolation provided by aromatic solvents and by the TPFPB anion. In addition, the substituents on silicon were bulked up by the use of 2,4,6-trimethylphenyl (mesityl) to prevent access of even the weak solvent and anion nucleophiles to the silicon atom. Hydride abstraction had proved impossible with trimesitylsilane, because of steric hindrance to access of electrophiles. Consequently, they developed a novel method to generate the cation by using allyl as the leaving group instead of hydride. This procedure is illustrated in Equation (2.7.1) with tin instead of silicon. 

The allyl group extends outside the steric sphere imposed by the mesityl groups, so that an electrophile such as trityl has easy access to the double bond. The tetracoordinate tin intermediate shown in the equation is sterically relieved as it expels the neutral leaving group to produce the stannylium (or silylium 

To test whether the chemical shift was being shielded by interaction with solvent, the authors altered the nucleophilicity of the solvent. They found the value of 5 806 to be invariant to changes to better donors such as toluene and p-xylene or to worse donors such as 1,2-dichlorobenzene. These results seemed to indicate little solvent interaction. They also succeeded in synthesizing allylphenyldi(2,4,6-triisopropylphenyl)stannane, which on treatment with electrophiles produced a cation with a chemical shift of S 697, more shielded than the trimesityl case. The presence of the single phenyl group, however, may permit better access of solvent, even with the two bulkier groups. 

The NMR results alone were inconclusive. Clearly, highly deshielded chemical shifts were being observed, indicative of high stannylium ion character. In the absence of a good point of reference from reliable calculations, however, it was impossible to conclude whether these were entirely free (1) or partially solvated (8) stannylium ions. 

5 Tricoordination Crystallographic Evidence and Computational Confirmation of Free Stannylium Ions 

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