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 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 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.