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

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• 作者：Asimo Karnezis ; Richard A.J. O’ ; Hair ; Jonathan M. White
• 刊名：Organometallics
• 出版年：2009
• 出版时间：November 23, 2009
• 年：2009
• 卷：28
• 期：22
• 页码：6480-6488
• 全文大小：1046K
• 年卷期：v.28,no.22(November 23, 2009)
• ISSN：1520-6041

文摘

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.