Kinetic Analysis of Competitive Electrocatalytic Pathways: New Insights into Hydrogen Production with Nickel Electrocatalysts

The hydrogen production electrocatalyst Ni­(P^Ph₂N^Ph₂)₂²⁺ (<b>1</b>) is capable of traversing multiple electrocatalytic pathways. When using dimethylformamidium, DMF­(H)⁺, the mechanism of H₂ formation by <b>1</b> changes from an ECEC to an EECC mechanism as the potential approaches the Ni­(I/0) couple. Two electrochemical methods, current–potential analysis and foot-of-the-wave analysis (FOWA), were performed on <b>1</b> to measure detailed kinetics of the competing ECEC and EECC pathways. A sensitivity analysis was performed on the methods using digital simulations to understand their strengths and limitations. Chemical rate constants were significantly underestimated when not accounting for electron-transfer kinetics, even when electron transfer was fast enough to afford a reversible noncatalytic wave. The EECC pathway of <b>1</b> was faster than the ECEC pathway under all conditions studied. Buffered DMF:DMF­(H)⁺ mixtures afforded an increase in the catalytic rate constant (<i>k</i>_obs) of the EECC pathway, but <i>k</i>_obs for the ECEC pathway did not change when using buffered acid. Further kinetic analysis of the ECEC path revealed that base increases the rate of isomerization from exo-protonated Ni(0) isomers to the catalytically active endo-isomers, but decreases the rate of protonation of Ni­(I). FOWA did not provide accurate rate constants, but FOWA was used to estimate the reduction potential of the previously undetected exo-protonated Ni­(I) intermediate. Comparison of catalytic Tafel plots for <b>1</b> under different conditions reveals substantial inaccuracies in the turnover frequency at zero overpotential when the kinetic and thermodynamic effects of the conjugate base are not accounted for properly

Electrocatalytic Hydrogen Production by [Ni(7P^Ph₂N^H)₂]²⁺: Removing the Distinction Between Endo- and Exo-Protonation Sites

A new Ni­(II) complex, [Ni­(7P^Ph₂N^H)₂H]³⁺ (7P^Ph₂N^H = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7P^Ph₂N^H ligand features an NH, ensuring properly positioned protonated amine groups (N–H⁺) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H₂ production in the presence of organic acids (p<i>K</i>_a range 10–13 in CH₃CN), with turnover frequencies ranging from 160 to 780 s^–1 at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni­(P^Ph₂N^{R^′}₂)₂]²⁺ (P^Ph₂N^{R^′}₂ = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni­(7P^Ph₂N^{R^′})₂]²⁺ complexes, catalytic turnover frequencies for H₂ production by [Ni­(7P^Ph₂N^H)₂]²⁺ do not show catalytic rate enhancement upon the addition of H₂O. This finding supports the assertion that [Ni­(7P^Ph₂N^H)₂]²⁺ eliminates the distinction between the endo- and exo-protonation isomers

Electrocatalytic Hydrogen Production by [Ni(7P^Ph₂N^H)₂]²⁺: Removing the Distinction Between Endo- and Exo-Protonation Sites

A new Ni­(II) complex, [Ni­(7P^Ph₂N^H)₂H]³⁺ (7P^Ph₂N^H = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7P^Ph₂N^H ligand features an NH, ensuring properly positioned protonated amine groups (N–H⁺) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H₂ production in the presence of organic acids (p<i>K</i>_a range 10–13 in CH₃CN), with turnover frequencies ranging from 160 to 780 s^–1 at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni­(P^Ph₂N^{R^′}₂)₂]²⁺ (P^Ph₂N^{R^′}₂ = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni­(7P^Ph₂N^{R^′})₂]²⁺ complexes, catalytic turnover frequencies for H₂ production by [Ni­(7P^Ph₂N^H)₂]²⁺ do not show catalytic rate enhancement upon the addition of H₂O. This finding supports the assertion that [Ni­(7P^Ph₂N^H)₂]²⁺ eliminates the distinction between the endo- and exo-protonation isomers