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Title: 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)(PPh 2Me) 2Mo(C 2H 4)][BArF 24] ([1-C 2H 4] +; PhTpy = 4'-Ph-2,2',6',2"-terpyridine, ArF 24 = [C 6H 3-3,5-(CF 3) 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)(PPh 2Me) 2Mo(NH 3)][BArF 24] ([1-NH 3] +) to [1-C 2H 4] + resulted in a net C–H bond-forming PCET reaction to yield the molybdenum ethyl [( PhTpy)(PPh 2Me) 2Mo(CH 2CH 3)][BArF 24] ([1-CH 2CH 3] +) and amido [( PhTpy)(PPh 2Me) 2Mo(NH 2)][BArF 24] ([1-NH 2] +) compounds. The reaction was reversed by addition of 2,4,6-tritert-butylphenoxyl radical to [1-CH 2CH 3] +. The solid-state structure of [1-CH 2CH 3] + 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 tomore » probe the dynamics of [1-CH 2CH 3] + and established restricted β-agostic -CH 3 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-CH 2CH 3] + was determined experimentally as 57 kcal mol –1 (THF) supported by a DFT-computed value of 52 kcal/mol –1 (gas phase). Comparison of pK a and electrochemical data for the complexes [1-C 2H 4] + and [1-NH 3] + in combination with a deuterium kinetic isotope effect (k H/k D) of 3.5(2) at 23 °C support a PCET process involving initial electron transfer followed by protonation leading to the formation of [1-CH 2CH 3] + and [1-NH 2] + 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.« less

Authors:
ORCiD logo [1]; ORCiD logo [1]
  1. 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) (SC-22)
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.
@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}
}

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