Snapshots of a Migrating H-Atom: Characterization of a Reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Iron(I) Isomer

Abstract

We report the characterization of an S= (Formula presented.) iron π-complex, [Fe(η6-IndH)(depe)]+ (Ind=Indenide (C9H7−), depe=1,2-bis(diethylphosphino)ethane), which results via C−H elimination from a transient FeIII hydride, [Fe(η3:η2-Ind)(depe)H]+. Owing to weak M−H/C−H bonds, these species appear to undergo proton-coupled electron transfer (PCET) to release H2 through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open-shell π-arene complex to have a BDFEC-H value of ≈50 kcal mol−1, roughly equal to the BDFEFe-H of its FeIII−H precursor (ΔG°≈0 between them). Markedly, this reactivity differs from related Fe(η5-Cp/Cp*) compounds, for which terminal FeIII−H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated ring (indene). Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines a valuable approach for the differentiation of a ring- versus a metal-bound H-atom by way of continuous-wave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.