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 [( Ph Tpy)(PPh 2 Me) 2 Mo(C 2 H 4 )][BArF 24 ] ([1-C 2 H 4 ] + ; Ph Tpy = 4'-Ph-2,2',6',2″-terpyridine, ArF 24 = [C 6 H 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 [( Ph Tpy)(PPh 2 Me) 2 Mo(NH 3 )][BArF 24 ] ([1-NH 3 ] + ) to [1-C 2 H 4 ] + resulted in a net C-H bond-forming PCET reaction to yield the molybdenum ethyl [( Ph Tpy)(PPh 2 Me) 2 Mo(CH 2 CH 3 )][BArF 24 ] ([1-CH 2 CH 3 ] + ) and amido [( Ph Tpy)(PPh 2 Me) 2 Mo(NH 2 )][BArF 24 ] ([1-NH 2 ] + ) compounds. The reaction was reversed by addition of 2,4,6-tri tert-butylphenoxyl radical to [1-CH 2 CH 3 ] + . The solid-state structure of [1-CH 2 CH 3 ] + established a β-agostic ethyl ligand that is maintained in solution as judged by variable temperature 1 H and 13 C NMR experiments. A combination of variable-temperature NMR experiments and isotopic labeling studies were used to probe the dynamics of [1-CH 2 CH 3 ] + and established restricted β-agostic -CH 3 rotation at low temperature (Δ G ‡ = 9.8 kcal mol -1 at -86 °C) as well as ethyl isomerization by β-hydride elimination-olefin rotation-reinsertion (Δ H ‡ = 19.3 ± 0.6 kcal mol -1 ; Δ S ‡ = 3.4 ± 1.7 cal mol -1 K -1 ). The β-(C-H) bond-dissociation free energy (BDFE) in [1-CH 2 CH 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 p K a and electrochemical data for the complexes [1-C 2 H 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 2 CH 3 ] + and [1-NH 2 ] + 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.