Chemical Method for Evaluating Catalytic Turnover Frequencies (TOF) of Moderate to Slow H₂ Oxidation Electrocatalysts

Hydrogen has emerged as a long-term option for storing energy from renewable resources and has led the scientific community to develop enzyme-inspired catalysts for hydrogen oxidation and production. One of the most productive families of catalysts for H₂ oxidation is the pendant amine containing bis­(diphosphine)nickel catalysts. Many derivatives of these nickel complexes have been prepared with varying peripheral functionalities to understand their structure–function relationship. Electrocatalysis is currently utilized as the primary tool to measure the catalytic turnover frequencies (TOFs) of these complexes; however, it does not allow accurate determination of TOFs for slow (<1 s^–1) to moderately active (∼1–10 s^–1) catalysts, and for the slowest catalysts, it can be difficult to determine if they are turning over at all. Further, it does not allow the determination of the TOF in bulk solution. In this work, we demonstrate that a redox chemical procedure involving a change between ferrocenium cation and ferrocene can be utilized to monitor H₂ oxidation catalysts. The ferrocenium cation reoxidizes the reduced nickel catalyst stoichiometrically in the presence of H₂. The change of ferrocenium cation to ferrocene can be monitored in both organic and aqueous solvents via either optical spectroscopy or NMR spectroscopy. Thus, this chemical method complements the existing electrocatalytic procedure and may help to unravel the intricate details of even the slowest H₂ oxidation catalysts.