Thermodynamics of C−H Activation in Multiple Oxidation States: Comparison of Benzylic C−H Acidities and C−H Bond Dissociation Energies in the Isostructural 16−20-Electron Complexes [Fex(η5-C5R5)(η6-arene)]n,x= 0−IV, R = H or Me,n= −1 to +3

Abstract

The pKa of the 18-electron complexes [Fe(η5-C5R5)(η6-C6Me6)][PF6] {1a[PF6] (R = H) and 1b[PF6] (R = Me)} and [Fe(η5-C5R5)(η6-C6H5CHPh2)][PF6] {1c[PF6]} have been determined by the direct method in DMSO using bases with known pKa values and found to be 12−14 pKa units lower than the pKa of the free arenes, illustrating the electron-withdrawing character of the CpFe+ and Cp*Fe+ groups. Access to the pKa values for 16-, 17-, 19-, and 20-electron iron-sandwich complexes of this type with various arene structures was available by means of thermodynamic diagrams using the standard redox potentials of the oxidation and reduction of the 18-electron cations 1+ and the deprotonated complexes 2. For instance, the pKa of the 19-electron iron complex 1a (43.5) is about the same as that of free C6Me6 (43−44) whereas that of 1b is even slightly higher (46.4). The pKas of the anionic 20-electron complexes 1a- and 1b- are 7 and 12 units, respectively, higher than that of C6Me6. The pKas of 12+ are around −10, whereas thos...