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Title: 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 reactivemore » (with respect to C–H bond strength) to be thoroughly characterized. Furthermore, 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.« less

Authors:
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]
  1. California Institute of Technology (CalTech), Pasadena, CA (United States)
Publication Date:
Research Org.:
California Institute of Technology (CalTech), Pasadena, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1698028
Grant/Contract Number:  
SC0019136
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of the American Chemical Society
Additional Journal Information:
Journal Volume: 141; Journal Issue: 11; Journal ID: ISSN 0002-7863
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Bond dissociation free energy; Reaction mechanisms; Proton coupled electron transfer; Sandwich compounds; Electron paramagnetic resonance spectroscopy

Citation Formats

Chalkley, Matthew J., Oyala, Paul H., and Peters, Jonas C. Cp* Noninnocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation. United States: N. p., 2019. Web. doi:10.1021/jacs.9b00193.
Chalkley, Matthew J., Oyala, Paul H., & Peters, Jonas C. Cp* Noninnocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation. United States. https://doi.org/10.1021/jacs.9b00193
Chalkley, Matthew J., Oyala, Paul H., and Peters, Jonas C. 2019. "Cp* Noninnocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation". United States. https://doi.org/10.1021/jacs.9b00193. https://www.osti.gov/servlets/purl/1698028.
@article{osti_1698028,
title = {Cp* Noninnocence Leads to a Remarkably Weak C–H Bond via Metallocene Protonation},
author = {Chalkley, Matthew J. and Oyala, Paul H. and Peters, Jonas C.},
abstractNote = {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. Furthermore, 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.},
doi = {10.1021/jacs.9b00193},
url = {https://www.osti.gov/biblio/1698028}, journal = {Journal of the American Chemical Society},
issn = {0002-7863},
number = 11,
volume = 141,
place = {United States},
year = {Thu Feb 21 00:00:00 EST 2019},
month = {Thu Feb 21 00:00:00 EST 2019}
}

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