Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring p K_a Effects and Demonstrating Electrocatalysis

Area of Science:

• Electrochemistry
• Catalysis
• Inorganic Chemistry

Background:

• Achieving substrate selectivity in reductive catalysis (e.g., N2, CO2, O2) is challenging, particularly when the hydrogen evolution reaction (HER) competes.
• Designing molecular catalysts for nitrogen reduction reaction (N2RR) requires understanding factors influencing selectivity over HER.

Purpose of the Study:

• To investigate the influence of acid pKa on the selectivity of a tris(phosphine)borane iron(I) catalyst (P3BFe+) for N2RR versus HER.
• To elucidate the roles of acid-reductant interactions and proton-coupled electron transfer (PCET) in N-H bond formation during N2RR.

Main Methods:

• Utilized a tris(phosphine)borane iron(I) catalyst (P3BFe+) with varying anilinium triflate acids.
• Employed stoichiometric studies, Density Functional Theory (DFT) calculations, and controlled potential electrolysis.
• Investigated the effect of a cobaltocene (Cp*2Co) reductant and its protonated form (Cp*2Co+) as a cocatalyst.

Main Results:

• A strong correlation was observed between acid pKa and N2RR efficiency.
• Stoichiometric studies indicated early-stage N2 reduction intermediates (Fe(NNH), Fe(NNH2)) are compatible with the acid-reductant system.
• DFT studies identified protonated cobaltocene as a key PCET donor and suggested pKa effects stem from cobaltocene protonation kinetics and thermodynamics.
• Inclusion of Cp*2Co+ as a cocatalyst improved ammonia yields.

Conclusions:

• Acid pKa significantly modulates the selectivity of the P3BFe+ catalyst for N2RR over HER.
• The interaction between the acid and the cobaltocene reductant plays a crucial role in N-H bond formation.
• This work presents the first unambiguous demonstration of electrocatalytic nitrogen fixation using a molecular catalyst, achieving up to 6.7 equiv NH3 per Fe.