Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism

doi:10.1021/jacs.8b08460

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)(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 »« less

Authors:

^[1];

^[1]

Princeton Univ., NJ (United States). Department of Chemistry

Publication Date:: 2018-09-27

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.

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                    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.

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                    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.

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@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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DOI: 10.1021/jacs.8b08460

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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)
DOI: 10.5517/ccdc.csd.cc20yzxw

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)
DOI: 10.5517/ccdc.csd.cc20yzyx

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)
DOI: 10.5517/ccdc.csd.cc20yzzy

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)
DOI: 10.5517/ccdc.csd.cc20z001

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
DOI: 10.1039/c9dt03099a

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
DOI: 10.1039/c9dt03099a

Evaluation of excited state bond weakening for ammonia synthesis from a manganese nitride: stepwise proton coupled electron transfer is preferred over hydrogen atom transfer
journal, January 2019

Loose, Florian; Wang, Dian; Tian, Lei
Chemical Communications, Vol. 55, Issue 39
DOI: 10.1039/c9cc01046g

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Title: Proton-Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism

Abstract

Citation Formats

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

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

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

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

A new bis-phenolate mesoionic carbene ligand for early transition metal chemistry journal, January 2019

A new bis-phenolate mesoionic carbene ligand for early transition metal chemistry journal, January 2019

Evaluation of excited state bond weakening for ammonia synthesis from a manganese nitride: stepwise proton coupled electron transfer is preferred over hydrogen atom transfer journal, January 2019

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dataset, October 2018

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dataset, October 2018

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

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journal, January 2019

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