79973-42-5Relevant articles and documents
Cp? Noninnocence Leads to a Remarkably Weak C-H Bond via Metallocene Protonation
Chalkley, Matthew J.,Oyala, Paul H.,Peters, Jonas C.
, (2019)
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 (-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.
Permethylmetallocene, II. Decamethylcobaltocen: Synthese und Umwandlung in methylierte (Aren)(cyclopentadienyl)cobalt-Kationen
Koelle, Ulrich,Khouzami, Fouad
, p. 2929 - 2937 (2007/10/02)
Decamethylcobaltocene (1b) is obtained from dibromo(1,2-dimethoxyethane)cobalt and pentamethylcyclopentadienyllithium. 1b shows enhanced reactivity in oxidative addition reactions with electrophiles RX.This reaction, when performed with chloroiodoalkanes, leads to exo-chloroalkyl complexes, which undergo a solvolytic ring expansion reaction, again more smoothly than the analogous derivatives of cobaltocene (1a), to form permethylated (cyclohexadienyl)(cyclopentadienyl)cobalt cations +.The latter are oxidized with nitric acid to yield for the first time (arene)(cyclopentadienyl)cobalt dications, 2+, methylated in both ligand rings.
