Generating Potent C–H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII–H Complex Demonstrates a Promising Strategy

Schild, Dirk J.; Drover, Marcus W.; Oyala, Paul H.; Peters, Jonas C.

doi:10.1021/jacs.0c09363

Title: Generating Potent C–H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*Fe^III–H Complex Demonstrates a Promising Strategy

Journal Article · Mon Oct 26 00:00:00 EDT 2020 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/jacs.0c09363· OSTI ID:1710227

Schild, Dirk J. ^[1];

^[1];

^[1]

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*Fe^III–H species, [Fe^III(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, [Fe^III(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, [Fe^I(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 (BDFE_C–H ≈ 29 kcal mol^–1 and 25 kcal mol^–1, respectively) cf. BDFE_Fe–H of 56 kcal mol^–1 for [Fe^III(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 [Fe^II(η⁵-Cp*)(dppe)CO]⁺ byproduct.

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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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Title: Generating Potent C–H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII–H Complex Demonstrates a Promising Strategy

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Title: Generating Potent C–H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*Fe^III–H Complex Demonstrates a Promising Strategy