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 [Fe-x(eta(5)-C5R5)(eta(6)-arene)](n), x=0-IV, R = H or Me, n = -1 to +3

The pK(a) of the 18-electron complexes [Fe(eta(5)-C5R5)(eta(6)-C6M6)][PF6] {1a[PF6] (R = H) and 1b[PF6] (R = Me)} and [Fe(eta(5)-C5R5)(eta(6)-C6H5CHPh 2)][PF6] {1c[PF6]} have been determined by the direct method in DMSO using bases with known pK(a) values and found to be 12-14 pK(a) units lower than the pK(a) of the free arenes, illustrating the electron-withdrawing charact er of the CpFe+ and Cp*Fe+ groups. Access to the pK(a) values for 16-, 17-, 19-, and 20-electron iron-sandwich complexes of this type with various are ne structures was available by means of thermodynamic diagrams using the st andard redox potentials of the oxidation and reduction of the Is-electron c ations 1(+) and the deprotonated complexes 2. For instance, the pK(a) of th e 19-electron iron complex 1a (43.5) is about the same as that of free C6Me 6 (43-44) whereas that of 1b is even slightly higher 46.4). The pK(a)s of t he anionic 20-electron complexes 1a(-) and 1b(-) are 7 and 12 units, respec tively, higher than that of C6Me6. The pK(a)s of 1(2+) are around -10, wher eas those of 1a(3+) were estimated to be around -50. In summary, the pK(a)s were determined for the five isostructural oxidation states Fe-IV to Fe-0, those of Fe-IV being more than 110 pK(a) units lower than those of FeO. Th e benzylic C-H bond dissociation energies (BDEs) of the 18-electron complex es have been determined by means of a thermodynamic cycle using the pK(a) v alues and standard oxidation potentials of the deprotonated forms measured in DMSO. These BDE values are between 81 and 86 kcal/mol, i.e. approximatel y the same as that of the free arene. The benzylic C-H BDEs in the 19- and 20-electron complexes 1 and 1(-) have been determined using other thermodyn amic diagrams and are 20 kcal/mol lower than in the 18-electron complexes, indicating the ease of these H-atom abstraction reactions. This trend is we ll taken into account by the fact that the valence shells of the metals are one unit closer to 18 in the products resulting from H-atom abstraction th an in the starting paramagnetic complexes. For the 17-electron complexes, t he BDEs were estimated to be only between 47 and 51 kcal/mol. We propose th is stabilization of the corresponding dicationic C-H activated species to b e due to coordination of the exocyclic double bond reminiscent of the struc ture of ferrocenyl carbonium cations.