Carbon−Germanium Hyperconjugation: Solid-State and Gas-Phase Investigations of (Trialkylgermyl)methyl-Substituted Pyridinium Ions

A crystallographic and computational study on 2- and 4-((trialkylgermyl)methyl)pyridinium ions 7a−c and 8a−c provides evidence for strong hyperconjugation between the Ge−CH2 bond and the π-deficient aromatic ring. Isodesmic equations show that the trimethylgermyl substituent stabilizes the pyridinium cations by 20−26 kJ mol−1 relative to the germanium-free analogues. Natural bond orbital analysis reveals that the major contributor to this stabilization is hyperconjugation between the Ge−CH2 bond and the aromatic π-system and that the strength of this interaction is greater for the 2-substituted ions 8a compared to the 4-substituted ions 7a. Crystallographic analysis of the 2- and 4-tert-butylgermylmethyl-substituted ions 7c and 8c provides the first structural evidence for carbon−germanium hyperconjugation; thus, the Ge−CH2 bonds are significantly longer (0.03−0.04 Å) than standard values and the CH2−Ar bond distance is shorter. Investigation of the gas-phase unimolecular chemistry of these ions, formed via electrospray ionization (ESI) and subjected to collision-induced dissociation (CID), reveals that the principal fragmentation of these ions involves cleavage of the weak CH2−GeR3 bond, giving an ion−molecule complex between R3Ge+ and a pyridine-enamine, which then undergoes further reactions.