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

Abstract

The pK a 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 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 character 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 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 pK a 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 pK as of the anionic 20-electron complexes 1a - and 1b - are 7 and 12 units, respectively, higher than that of C6Me6. The pK as of 1 2+ are around −10, whereas those of 1a 3+ were estimated to be around −50. In summary, the pK as were determined for the five isostructural oxidation states FeIV to Fe0, those of FeIV being more than 110 pK a units lower than those of Fe0. The benzylic C−H bond dissociation energies (BDEs) of the 18-electron complexes have been determined by means of a thermodynamic cycle using the pK 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.

Organoiron-mediated dendrimer syntheses with 1→3 connectivity and applications

“…5,14 In particular, the activation by the cationic 12-electron CpFe þ group results in the lowering of the pK a in DMSO of the polymethylbenzenes, from 42 to 28 for instance for hexamethylbenzene. 26,27 The interest in such an increase of the acidity of the benzylic protons was first realized thirty years ago when the yellow toluene complex was deprotonated by t-BuOK in THF to a deep-red cyclohexadienylmethylene complex 1 (Scheme 4), especially because deprotonation could be followed by alkylation with MeI and various electrophiles to form C-C and C-element bonds. [28][29][30] Moreover, deprotonation could also be obtained upon reaction of O 2 or air with the corresponding neutral 19-electron Fe I complexes (for instance 2, R¼Me) that resulted from single electron reduction of [Fe II (h 6 -ArMe)(h 5 -C 5 H 5 )] þ [X] (Scheme 5).…”

The Olefin Metathesis Reactions in Dendrimers

“…Two kinds of dendritic effect were found upon analysis of the kinetic data [23,24]: -The dendrimers were more efficient catalysts than the monomeric model complex. This could possibly be due to labilization of metal-phosphine bonds that is facilitated in dendrimers as compared to the monomer for entropic reasons.…”

“…The hexahapto complexation of arenes by the cationic group CpFe + considerably increases the acidity of its benzylic protons (the pKa's of the arenes in DMSO are lowered upon complexation with CpFe + by approximately 15 units, for instance from 43 to 28 for C 6 Me 6 ) [24][25][26]. Therefore, deprotonation of the CpFe(arene) + complexes is feasible under mild conditions with KOH.…”

Metallodendrimers and dendronized gold colloids as nanocatalysts, nanosensors and nanomaterials for molecular electronics

“…The organo-iron activation of simple or functional arenes such as mesitylene and p-ethoxytoluene by the 12-electron fragment CpFe + [21] has provided dendritic cores and building block (dendrons) for the construction of giant dendrimers far beyond the De Gennes dense-packing limit [22] with numbers of branches up to more than 10 5 . The construction consisted in iterating sequences of two reactions: (i) regioselective hydrosilylation of the terminal double bonds with chloromethyl dimethyl silane catalyzed by the Karsted catalyst and (ii) NaI-catalyzed nucleophilic substitution of the chloride by the phenoltriallyl building block [23] (Fig.…”