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

Bezdek, Máté J.; Chirik, Paul J.

doi:10.1021/jacs.8b08460

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

Journal Article · Thu Sep 27 00:00:00 EDT 2018 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/jacs.8b08460· OSTI ID:1478323

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

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 pK_a and electrochemical data for the complexes [1-C₂H₄]⁺ and [1-NH₃]⁺ 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-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.

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Research Organization:: Princeton Univ., NJ (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES)

Grant/Contract Number:: SC0006498

OSTI ID:: 1478323

Journal Information:: Journal of the American Chemical Society, Vol. 140, Issue 42; ISSN 0002-7863

Publisher:: American Chemical Society (ACS)Copyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 13 works

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A new bis-phenolate mesoionic carbene ligand for early transition metal chemistry Baltrun, Marc; Watt, Fabian A.; Schoch, Roland Dalton Transactions, Vol. 48, Issue 39 https://doi.org/10.1039/c9dt03099a	journal	January 2019

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