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<i>^x</i>(η⁵-C₅R₅)(η⁶-arene)]<i>ⁿ</i>,<i>x</i>= 0−IV, R = H or Me,<i>n</i>= −1 to +3

Abstract

The p K a of the 18-electron complexes [Fe ( η 5 -C 5 R 5 )(η 6 -C 6 Me 6 )][PF 6 ] { 1a [PF 6 ] (R = H) and 1b [PF 6 ] (R = Me)} and [Fe(η 5 -C 5 R 5 )(η 6 -C 6 H 5 CHPh 2 )][PF 6 ] { 1c [PF 6 ]} have been determined by the direct method in DMSO using bases with known p K a values and found to be 12−14 p K a units lower than the p K a of the free arenes, illustrating the electron-withdrawing character of the CpFe + and Cp*Fe + groups. Access to the p K a 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 p K a of the 19-electron iron complex 1a (43.5) is about the same as that of free C 6 Me 6 (43−44) whereas that of 1b is even slightly higher (46.4). The p K a s of the anionic 20-electron complexes 1a - and 1b - are 7 and 12 units, respectively, higher than that of C 6 Me 6 . The p K a s of 1 2+ are around −10, whereas those of 1a 3+ were estimated to be around −50. In summary, the p K a s were determined for the five isostructural oxidation states Fe IV to Fe 0, those of Fe IV being more than 110 p K a units lower than those of Fe 0 . The benzylic C−H bond dissociation energies (BDEs) of the 18-electron complexes have been determined by means of a thermodynamic cycle using the p K a values and standard oxidation potentials of the deprotonated forms measured in DMSO. These BDE values are between 81 and 86 kcal/mol, i.e. approximately 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 thermodynamic 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 well 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 than in the starting paramagnetic complexes. For the 17-electron complexes, the BDEs were estimated to be only between 47 and 51 kcal/mol. We propose this stabilization of the corresponding dicationic C−H activated species to be due to coordination of the exocyclic double bond reminiscent of the structure of ferrocenyl carbonium cations.