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

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