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 organo-metallic chemistry. The cationic molybdenum ethylene complex [((Ph)Tpy)(PPh2Me)(2)Mo(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 [((Ph)Tpy)(PPh2Me)(2)Mo(NH3)][BArF24] ([1-NH3](+)) to [1-C2H4](+) resulted in a net C-H bond-forming PCET reaction to yield the molybdenum ethyl [((Ph)Tpy)(PPh2Me)(2)Mo(CH2CH3)][BArF24] ([1-CH2CH3](+)) and amido [((Ph)Tpy)(PPh2Me)(2)Mo(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 beta-agostic ethyl ligand that is maintained in solution as judged by variable temperature H-1 and C-13 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 beta-agostic -CH3 rotation at low temperature (Delta G(double dagger) = 9.8 kcal mol(-1) at -86 degrees C) as well as ethyl isomerization by beta-hydride elimination-olefin rotation-reinsertion (Delta H-double dagger = 19.3 +/- 0.6 kcal mol(-1); Delta S-double dagger = 3.4 +/- 1.7 cal mol(-1) K-1). The beta-(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 degrees 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.