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

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]^{+ Ph}Tpy = 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 variableerature 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 â86 °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. The data presented herein provides a structural, thermochemical and mechanistic foundation for understanding the PCET reactivity of organometallic complexes with alkene and alkyl ligands.