Essential Roles of Cp Ring Activation and Coordinated Solvent During Electrocatalytic H₂ Production with Fe(Cp^N3) Complexes

Cyclopentadienyl (Cp), a classic ancillary ligand platform, can be chemically noninnocent in electrocatalytic H-H bond formation reactions via protonation of coordinated η⁵-Cp ligands to form η⁴-CpH moieties. However, the kinetics of η⁵-Cp ring protonation, ligand-to-metal (or metal-to-ligand) proton transfer, and the influence of solvent during H₂ production electrocatalysis remain poorly understood. We report in-depth kinetic details for electrocatalytic H₂ production with Fe complexes containing amine-functionalized Cp^N3 ligands that are protonated via exogenous acid to generate via η⁴-Cp^N3H intermediates (Cp^N3 = 6-amino-1,4-dimethyl-5,7-diphenyl-2,3,4,6-tetrahydrocyclopenta[b]pyrazin-6-yl). Under reducing conditions, state-of-the-art DFT calculations reveal that a coordinated solvent plays a crucial role in mediating stereo- and regioselective proton transfer to generate (endo-Cp^N3H)Fe(CO)₂(NCMe), with other protonation pathways being kinetically insurmountable. To demonstrate regioselective endo-Cp^N3H formation, the isoelectronic model complex (endo-Cp^N3H)Fe(CO)₃ is independently prepared, and kinetic studies with the on-cycle hydride intermediate Cp^N3FeH(CO)₂ under CO cleanly furnish the ring-activated complex (endo-Cp^N3H)Fe(CO)₃ via metal-to-ligand proton migration. The on-cycle complex Cp^N3FeH(CO)₂ reacts with acid to release H₂ and regenerate [Cp^N3Fe(CO)₂(NCMe)]⁺, which was found to be the TOF-determining step via DFT. Collectively, these experimental and computational results underscore the emerging importance of Cp ring activation, inner-sphere solvation, and metal-ligand cooperativity to perform proton-coupled electron transfer catalysis for chemical fuel synthesis.