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

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*₂Co, and Cp*₂Cr). The latter have proven to be useful reagents in the reductive protonation of small-molecule substrates, including N₂. 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*₂Co 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-η⁴-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-η⁴-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 (BDFE_C-H) for Cp*( exo-η⁴-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-η⁴-C₅Me₅H)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.