The Essential Roles of Cp Ring Activation and Coordinated Solvent During Electrocatalytic H2 Production with Amine-Rich Fe Complexes

Cyclopentadienyl (Cp), a classic ancillary ligand platform, can be chemically non-innocent 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 underexplored. We report in-depth kinetic details for electrocatalytic H2 production using Fe complexes [CpFe(CO)2(NCMe)]+ with amine-rich cyclopentadienyl ligands (Cp = enCpN; en = ethylenediamine, N = NHiPr, Pyrrolidinyl), which generate H2 with a maximum turnover frequency of 266 s-1 via η4-CpH intermediates. Under reducing conditions, state-of-the-art DFT calculations reveal that coordinated solvent plays a crucial role in mediating stereo- and regioselective proton transfer to generate (endo-CpH)Fe(CO)2(NCMe), followed by rapid solvent release and ligand-to-metal proton transfer to generate CpFeH(CO)2. The isoelectronic model complex (endo-CpH)Fe(CO)3 is prepared and structurally characterized, and the replacement of CO with NCMe dramatically increases the ligand dissociation barrier from ΔG‡ ≅ 5 kcal/mol to ΔG‡ ≅ 34 kcal/mol. The on-cycle intermediate CpFeH(CO)2 was prepared and cleanly reacts to release H2 and regenerate [CpFe(CO)2(NCMe)]+, which is rate-limiting during electrocatalysis. The solvent-free complex CpFe(CO)2 was found to be catalytically inactive and reversibly dimerizes in solution to form the crystallographically characterized dimer [CpFe(CO)2]2, further supporting the important role of coordinated solvent during H2 production electrocatalysis. 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.