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

Schild, Dirk J. and Drover, Marcus W. and Oyala, Paul H. and Peters, Jonas C. (2020) 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 of the American Chemical Society, 142 (44). pp. 18963-18970. ISSN 0002-7863. doi:10.1021/jacs.0c09363.

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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. 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 (^(1,2)H 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.

Item Type:Article
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URLURL TypeDescription
Drover, Marcus W.0000-0002-2186-1040
Oyala, Paul H.0000-0002-8761-4667
Peters, Jonas C.0000-0002-6610-4414
Additional Information:© 2020 American Chemical Society. Received: August 31, 2020; Published: October 26, 2020. The authors are grateful to the Department of Energy Basic Energy Sciences for support via Grant No. DOE-0235032. The Caltech EPR facility was previously supported by the National Science Foundation via Grant No. NSF MRI-153194 and is currently supported by the Dow Next Generation Educator Fund. The Beckman Institute is thanked for X-ray support. M.W.D. acknowledges NSERC (Banting PDF award), and M.W.D. and D.J.S. thank the Resnick Sustainability Institute at Caltech for fellowships. Author Contributions: D.J.S. and M.W.D. contributed equally. The authors declare no competing financial interest.
Group:Resnick Sustainability Institute
Funding AgencyGrant Number
Department of Energy (DOE)DOE-0235032
Dow Next Generation Educator FundUNSPECIFIED
Caltech Beckman InstituteUNSPECIFIED
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Resnick Sustainability InstituteUNSPECIFIED
Issue or Number:44
Record Number:CaltechAUTHORS:20201027-151729690
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Official Citation:Generating Potent C–H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII–H Complex Demonstrates a Promising Strategy. Dirk J. Schild, Marcus W. Drover, Paul H. Oyala, and Jonas C. Peters. Journal of the American Chemical Society 2020 142 (44), 18963-18970; DOI: 10.1021/jacs.0c09363
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106306
Deposited By: Tony Diaz
Deposited On:27 Oct 2020 22:32
Last Modified:16 Nov 2021 18:52

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