Cp~* Noninnocence Leads to a Remarkably Weak C-H Bond via Metallocene Protonation

摘要

Metallocenes, including their permethylated variants, are extremely important in organometallic chemistry. In particular, many are synthetically useful either as oxidants (e.g., Cp2Fe+) or as reductants (e.g., Cp2Co, Cp-2*Co, and Cp-2*Cr). The latter have proven to be useful reagents in the reductive protonation of small-molecule substrates, including N-2. As such, understanding the behavior of these metallocenes in the presence of acids is paramount. In the present study, we undertake the rigorous characterization of the protonation products of Cp-2*Co using pulse electron paramagnetic resonance (EPR) techniques at low temperature. We provide unequivocal evidence for the formation of the ring-protonated isomers Cp*(exo/endo-eta(4)-C5Me5H)Co+. Variable temperature Q-band (34 GHz) pulse EPR spectroscopy, in conjunction with density functional theory (DFT) predictions, are key to reliably assigning the Cp*(exo/endo-eta(4)-C5Me5H)Co+ species. We also demonstrate that exo-protonation selectivity can be favored by using a bulkier acid and suggest this species is thus likely a relevant intermediate during catalytic nitrogen fixation given the bulky anilinium acids employed. Of further interest, we provide physical data to experimentally assess the C-H bond dissociation free energy (BDFEC-H) for Cp*(exo-eta(4)-C5Me5H)Co+. These experimental data support our prior DFT predictions of an exceptionally weak C-H bond (29 kcal mol(-1)), making this system among the most reactive (with respect to C-H bond strength) to be thoroughly characterized. These data also point to the propensity of Cp*(exo-eta(4)-C5Me3H)Co to mediate hydride (H-) transfer. Our findings are not limited to the present protonated metallocene system. Accordingly, we outline an approach to rationalizing the reactivity of arene-protonated metal species, using decamethylnickelocene as an additional example.