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 (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.