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

Abstract

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 (BDFEC-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.