Generating Potent C–H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII–H Complex Demonstrates a Promising Strategy
- California Institute of Technology (CalTech), Pasadena, CA (United States)
Highly reactive organometallic species that mediate reductive proton-coupled electron transfer (PCET) reactions are an exciting area for development in catalysis, where a key objective focuses on tuning the reactivity of such species. Here, this work pursues ligand-induced activation of a stable organometallic complex toward PCET reactivity. This is studied via the conversion of a prototypical Cp*FeIII–H species, [FeIII(η5-Cp*)(dppe)H]+ (Cp* = C5Me5–, dppe = 1,2-bis(diphenylphosphino)ethane), to a highly reactive, S = 1/2 ring-protonated endo-Cp*H–Fe relative, triggered by the addition of CO. Our assignment of the latter ring-protonated species contrasts with its previous reported formulation, which instead assigned it as a hypervalent 19-electron hydride, [FeIII(η5-Cp*)(dppe)(CO)H]+. Herein, pulse EPR spectroscopy (1,2H HYSCORE, ENDOR) and X-ray crystallography, with corresponding DFT studies, cement its assignment as the ring-protonated isomer, [FeI(endo-η4-Cp*H)(dppe)(CO)]+. A less sterically shielded and hence more reactive exo-isomer can be generated through oxidation of a stable Fe0(exo-η4-Cp*H)(dppe)(CO) precursor. Both endo- and exo-ring-protonated isomers are calculated to have an exceptionally low bond dissociation free energy (BDFEC–H ≈ 29 kcal mol–1 and 25 kcal mol–1, respectively) cf. BDFEFe–H of 56 kcal mol–1 for [FeIII(η5-Cp*)(dppe)H]+. These weak C–H bonds are shown to undergo proton-coupled electron transfer (PCET) to azobenzene to generate diphenylhydrazine and the corresponding closed-shell [FeII(η5-Cp*)(dppe)CO]+ byproduct.
- Research Organization:
- California Institute of Technology (CalTech), Pasadena, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); National Science Foundation (NSF); Dow Next Generation Educator Fund
- Grant/Contract Number:
- SC0019136
- OSTI ID:
- 1710227
- Journal Information:
- Journal of the American Chemical Society, Vol. 142, Issue 44; ISSN 0002-7863
- Publisher:
- American Chemical Society (ACS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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