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]+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 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 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. The data presented herein provides a structural, thermochemical and mechanistic foundation for understanding the PCET reactivity of organometallic complexes with alkene and alkyl ligands. © 2018 American Chemical Society.