The role of hyperconjugative π-aromaticity in the enhanced acidity of methyl-, silyl and germylcyclopentadienes

Abstract

The relative stability of the different isomers of cyclopentadienyl derivatives CpXH₃ (X = C, Si, Ge) and their intrinsic acidities have been investigated by means of B3LYP/6-311+G(3df,2p)//CCSD/6-311+G(d,p) density functional theory calculations. Whereas for the methylcyclopentadiene the 1- and 2- substituted isomers are almost equally stable and much more stable than the 5-substituted isomer, for the germyl derivatives the 5-substituted compound is the global minimum, due to the stabilization of the system through a hyperconjugative π-aromaticity effect, which is the larger the more electropositive the XH₃ substituent is. As a consequence CpXH₃ (X = Si, Ge) are more aromatic than cyclopentadiene. The silyl and germyl derivatives are more fluxional than the methyl derivative, the 1,2-XH₃ shift activation barriers being around 60 kJ mol⁻¹. For all the isomers, the most favourable deprotonation process corresponds to the loss of the proton attached to the sp³ carbon atom of the five membered ring. For Si and Ge containing compounds this behaviour differs from that observed for saturated and α,β-unsaturated compounds, which behave as Si or Ge acids in the gas phase. CpXH₃ (X = C, Si, Ge) compounds are predicted to be stronger acids in the gas phase than the unsubstituted parent compound, due to a significant anionic hyperconjugation effect which reinforces the C–X bond upon deprotonation and favours the conjugation of the C–X π-bond with the π-system associated to the five membered ring.

Keywords:

• cyclopentadienyl derivatives
• intrinsic acidity
• density functional theory calculations
• hyperconjugative π-aromaticity

Acknowledgements

This work has been partially supported by the DGI Project No. CTQ2009-13129-C01, by the Project MADRISOLAR2, Ref.: S2009PPQ/1533 of the Comunidad Autónoma de Madrid and by Consolider on Molecular Nanoscience CSD2007-00010. A generous allocation of computing time at the CCC of the UAM is also acknowledged, J.-C. G. thanks the CNES for financial support. MH acknowledges The Ministry of Science and Innovation of Spain for a postdoctoral contract (Project No. CTQ2009-07197-E).

Notes

Note

Supplementary Material can be viewed online.