Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism
Abstract
In this paper, the interconversion of molybdenum ethylene and ethyl complexes by proton-coupled electron transfer (PCET) is described, an unusual transformation in organometallic chemistry. The cationic molybdenum ethylene complex [(PhTpy)(PPh2Me)2Mo(C2H4)][BArF24] ([1-C2H4]+; PhTpy = 4'-Ph-2,2',6',2"-terpyridine, ArF24 = [C6H3-3,5-(CF3)2]4) was synthesized, structurally characterized, and its electronic structure established by a combination of spectroscopic and computational methods. The overall electronic structure is best described as a molybdenum(III) complex with a metallacyclopropane and a redox neutral terpyridine ligand. Addition of the nonclassical ammine complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+) to [1-C2H4]+ resulted in a net C–H bond-forming PCET reaction to yield the molybdenum ethyl [(PhTpy)(PPh2Me)2Mo(CH2CH3)][BArF24] ([1-CH2CH3]+) and amido [(PhTpy)(PPh2Me)2Mo(NH2)][BArF24] ([1-NH2]+) compounds. The reaction was reversed by addition of 2,4,6-tritert-butylphenoxyl radical to [1-CH2CH3]+. The solid-state structure of [1-CH2CH3]+ established a β-agostic ethyl ligand that is maintained in solution as judged by variable temperature 1H and 13C NMR experiments. A combination of variable-temperature NMR experiments and isotopic labeling studies were used to probe the dynamics of [1-CH2CH3]+ and established restricted β-agostic -CH3 rotation at low temperature (ΔG‡ = 9.8 kcal mol–1 at -86 °C) as well as ethyl isomerization by β-hydride elimination-olefin rotation-reinsertion (ΔH‡ = 19.3 ± 0.6 kcal mol–1; ΔS‡ = 3.4 ± 1.7 cal mol–1 K–1). Themore »
- Authors:
-
- Princeton Univ., NJ (United States). Department of Chemistry
- Publication Date:
- Research Org.:
- Princeton Univ., NJ (United States)
- Sponsoring Org.:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- OSTI Identifier:
- 1478323
- Grant/Contract Number:
- SC0006498
- Resource Type:
- Accepted Manuscript
- Journal Name:
- Journal of the American Chemical Society
- Additional Journal Information:
- Journal Volume: 140; Journal Issue: 42; 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
Citation Formats
Bezdek, Máté J., and Chirik, Paul J. Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism. United States: N. p., 2018.
Web. doi:10.1021/jacs.8b08460.
Bezdek, Máté J., & Chirik, Paul J. Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism. United States. doi:10.1021/jacs.8b08460.
Bezdek, Máté J., and Chirik, Paul J. Thu .
"Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism". United States. doi:10.1021/jacs.8b08460. https://www.osti.gov/servlets/purl/1478323.
@article{osti_1478323,
title = {Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism},
author = {Bezdek, Máté J. and Chirik, Paul J.},
abstractNote = {In this paper, the interconversion of molybdenum ethylene and ethyl complexes by proton-coupled electron transfer (PCET) is described, an unusual transformation in organometallic chemistry. The cationic molybdenum ethylene complex [(PhTpy)(PPh2Me)2Mo(C2H4)][BArF24] ([1-C2H4]+; PhTpy = 4'-Ph-2,2',6',2"-terpyridine, ArF24 = [C6H3-3,5-(CF3)2]4) was synthesized, structurally characterized, and its electronic structure established by a combination of spectroscopic and computational methods. The overall electronic structure is best described as a molybdenum(III) complex with a metallacyclopropane and a redox neutral terpyridine ligand. Addition of the nonclassical ammine complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+) to [1-C2H4]+ resulted in a net C–H bond-forming PCET reaction to yield the molybdenum ethyl [(PhTpy)(PPh2Me)2Mo(CH2CH3)][BArF24] ([1-CH2CH3]+) and amido [(PhTpy)(PPh2Me)2Mo(NH2)][BArF24] ([1-NH2]+) compounds. The reaction was reversed by addition of 2,4,6-tritert-butylphenoxyl radical to [1-CH2CH3]+. The solid-state structure of [1-CH2CH3]+ established a β-agostic ethyl ligand that is maintained in solution as judged by variable temperature 1H and 13C NMR experiments. A combination of variable-temperature NMR experiments and isotopic labeling studies were used to probe the dynamics of [1-CH2CH3]+ and established restricted β-agostic -CH3 rotation at low temperature (ΔG‡ = 9.8 kcal mol–1 at -86 °C) as well as ethyl isomerization by β-hydride elimination-olefin rotation-reinsertion (ΔH‡ = 19.3 ± 0.6 kcal mol–1; ΔS‡ = 3.4 ± 1.7 cal mol–1 K–1). The β-(C–H) bond-dissociation free energy (BDFE) in [1-CH2CH3]+ was determined experimentally as 57 kcal mol–1 (THF) supported by a DFT-computed value of 52 kcal/mol–1 (gas phase). Comparison of pKa and electrochemical data for the complexes [1-C2H4]+ and [1-NH3]+ in combination with a deuterium kinetic isotope effect (kH/kD) of 3.5(2) at 23 °C support a PCET process involving initial electron transfer followed by protonation leading to the formation of [1-CH2CH3]+ and [1-NH2]+ or a concerted pathway. Lastly, the data presented herein provides a structural, thermochemical and mechanistic foundation for understanding the PCET reactivity of organometallic complexes with alkene and alkyl ligands.},
doi = {10.1021/jacs.8b08460},
journal = {Journal of the American Chemical Society},
number = 42,
volume = 140,
place = {United States},
year = {2018},
month = {9}
}
Web of Science
Works referencing / citing this record:
CCDC 1875869: Experimental Crystal Structure Determination: MIJDER : chloro-bis[methyl(diphenyl)phosphine]-(4'-phenyl-2,2':6',2''-terpyridine)-molybdenum(ii) tetrakis[3,5-bis(trifluoromethyl)phenyl]borate unknown solvate
dataset, October 2018
- Bezdek, Máté J.; Chirik, Paul J.
- Cambridge Structural Database (CSD)
CCDC 1875870: Experimental Crystal Structure Determination: MIJDIV : (η2-ethene)-bis[methyl(diphenyl)phosphine]-(4'-phenyl-2,2':6',2''-terpyridine)-molybdenum tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
dataset, October 2018
- Bezdek, Máté J.; Chirik, Paul J.
- Cambridge Structural Database (CSD)
CCDC 1875871: Experimental Crystal Structure Determination: MIJDOB : bis[methyl(diphenyl)phosphine]-(4'-phenyl-2,2':6',2''-terpyridine)-ethyl-molybdenum tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
dataset, October 2018
- Bezdek, Máté J.; Chirik, Paul J.
- Cambridge Structural Database (CSD)
CCDC 1875872: Experimental Crystal Structure Determination: MIJDUH : (η2-ethene)-bis[methyl(diphenyl)phosphine]-(4'-phenyl-2,2':6',2''-terpyridine)-molybdenum tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
dataset, October 2018
- Bezdek, Máté J.; Chirik, Paul J.
- Cambridge Structural Database (CSD)
A new bis-phenolate mesoionic carbene ligand for early transition metal chemistry
journal, January 2019
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A new bis-phenolate mesoionic carbene ligand for early transition metal chemistry
journal, January 2019
- Baltrun, Marc; Watt, Fabian A.; Schoch, Roland
- Dalton Transactions, Vol. 48, Issue 39
Evaluation of excited state bond weakening for ammonia synthesis from a manganese nitride: stepwise proton coupled electron transfer is preferred over hydrogen atom transfer
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