Solution and Solid-State Structure of 2- and 4-Bis(trimethylsilyl)methylpyridinium Cations

N-Methylation of the 2- and 4-bis(trimethylsilyl)methyl pyridines 7 and 9 gives the ions 5 and 6 and
results in a significant decrease in the 13CH−29SiMe3 coupling constant, consistent with increased C−Si−π
hyperconjugation between the trimethylsilyl substituents and the electron-deficient aromatic system
of the pyridinium cations. The rotation barrier about the Cipso−CH bond, which was measured for
both 5 and 7 using variable-temperature 13C NMR and calculated at the B3LYP/6-311G** level,
reveals that steric effects make a larger contribution than hyperconjugative effects to this barrier. The
crystal structure of the protonated salt 11 shows that the conformation of the CH(SiMe3)2 substituent is
similar to that predicted by calculations and allows for both C−SiMe3 bonds to interact with the aromatic
π-system.