Generating Potent C-H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII-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*Few -H species, [Fe-III(eta(5)-Cp*)(dppe)H](+) (Cp* = CsMes-, 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 (eta(5)-Cp*)(dppe)(CO)H+. Herein, pulse EPR spectroscopy (H-1,H-2 HYSCORE, ENDOR) and X-ray crystallography, with corresponding DFT studies, cement its assignment as the ring-protonated isomer, [Fe-I(endo-eta(4)-Cp*H)(dppe)(CO)](+). A less sterically shielded and hence more reactive exo-isomer can be generated through oxidation of a stable Fe-0(exo-eta(4)-Cp*H)(dppe)(CO) precursor. Both endo- and exo-ring-protonated isomers are calculated to have an exceptionally low bond dissociation free energy (BDFECH approximate to 29 kcal mol(-1) and 25 kcal mol(-1), respectively) cf. BDFEFe-H of 56 kcal mol(-1) for [Fe-III(eta Cp5-*)(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(eta 5-Cp*)(dppe)CO](+) byproduct.