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

Chalkley, Matthew J. and Oyala, Paul H. and Peters, Jonas C. (2019) Cp* Noninnocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation. Journal of the American Chemical Society, 141 (11). pp. 4721-4729. ISSN 0002-7863. http://resolver.caltech.edu/CaltechAUTHORS:20190221-125557747

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Abstract

Metallocenes, including their permethylated variants, are extremely important in organometallic chemistry. In particular, many are synthetically useful either as oxidants (e.g., Cp_2Fe^+) or as reductants (e.g., Cp_2Co, Cp*_2Co, and Cp*_2Cr). 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*_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-η^4-C_5Me_5H)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-η^4-C_5Me_5H)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-η^4-C_5Me_5H)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-η^4-C_5Me_5H)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.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1021/jacs.9b00193DOIArticle
https://pubs.acs.org/doi/suppl/10.1021/jacs.9b00193PublisherSupporting Information
ORCID:
AuthorORCID
Chalkley, Matthew J.0000-0002-0484-7335
Oyala, Paul H.0000-0002-8761-4667
Peters, Jonas C.0000-0002-6610-4414
Alternate Title:Cp* Non-innocence Leads to a Remarkably Weak C−H Bond via Metallocene Protonation
Additional Information:© 2019 American Chemical Society. Received: January 7, 2019; Published: February 21, 2019. This work was supported by Department of Energy (DOE-0235032). MJC acknowledges support from the Center for Environmental Microbial Interactions (CEMI) and the Resnick Sustainability Institute at Caltech. The Caltech EPR facility was supported by the National Science Foundation (NSF MRI-153194) and the Dow Next Generation Educator Fund. The authors declare no competing financial interest.
Group:Caltech Center for Environmental Microbial Interactions (CEMI), Resnick Sustainability Institute
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DOE-0235032
Caltech Center for Environmental Microbial Interactions (CEMI)UNSPECIFIED
Resnick Sustainability InstituteUNSPECIFIED
NSFMRI-153194
Dow Next Generation Educator FundUNSPECIFIED
Record Number:CaltechAUTHORS:20190221-125557747
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20190221-125557747
Official Citation:Cp* Noninnocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation. Matthew J. Chalkley, Paul H. Oyala, and Jonas C. Peters. Journal of the American Chemical Society 2019 141(11), 4721-4729. DOI: 10.1021/jacs.9b00193
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:93165
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:22 Feb 2019 16:53
Last Modified:20 Mar 2019 15:41

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