Ferrocene Acidity and C-H Bond Dissociation Energy via Experiment and Theory

摘要

The gas-phase acidity of ferrocene (Delta H degrees(acid)(1) = 391.5 +/- 1.3 kcal mol(-1)) and electron affinity of the ferrocenyl radical (EA(1r) = 1.74 +/- 0.08 eV) were measured in a Fourier transform mass spectrometer and combined in a thermodynamic cycle with the known ionization energy of the hydrogen atom to afford the C-H bond dissociation energy of 1 (BDE(1) = 118.0 +/- 2.3 kcal mol(-1)). Companion M06-2X but not B3LYP computations reproduce each of these thermodynamic quantities and are in accord with an unusually strong aromatic C-H BDE. Natural population analysis and atomic polar tensor charges indicate that a covalent description of ferrocene with a neutral iron atom is a better representation of this compound than an ionic one with a doubly charged Fe2+ center. Predicted structural differences upon deprotonation of MCp2, M = Fe and Mg, are also in accord with this view.