We present monometallic H2 production electrocatalysts containing electron-rich triamine-cyclopentadienyl (Cp) ligands coordinated to iron. After selective CO extrusion from the iron tricarbonyl precursors, electrocatalysis is observed via cyclic voltammetry in the presence of an exogenous acid. Contrary to the fact that amines in the secondary coordination sphere are often protonated during electrocatalysis, comprehensive quantum-chemical calculations indicate that the amines likely do not function as proton relays; instead, endo-Cp ring protonation is most favorable after 1e– reduction. This unusual mechanistic pathway emphasizes the need to consider a broad domain of H+/e– addition products by synergistically combining experimental and theoretical resources.
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.
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.
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