Deciphering Reversible Homogeneous Catalysis of the Electrochemical H₂ Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects**

Abstract

Nickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so‐called DuBois’ catalysts) capable of bidirectional/reversible electrocatalytic oxidation and production of dihydrogen. This unique behaviour is directly linked to the presence of proton relays installed close to the metal center. We report here for the arginine derivative [Ni(P2CyN2Arg)2]6+ on a mechanistic model and its kinetic treatment that may apply to all DuBois’ catalysts and show that it allows for a good fit of experimental data measured at different pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from balanced equilibria related to hydrogen uptake/evolution on one side and (metal)‐hydride installation/capture on the other side, both controlled by concentration effects resulting from the presence of proton relays and connected by two square schemes corresponding to proton‐coupled electron transfer processes. We show that the catalytic bias is controlled by the kinetic of the H2 uptake/evolution step. Reversibility does not require that the energy landscape be flat, with redox transitions occurring at potentials up to 250 mV away for the equilibrium potential, although such large deviations from a flat energy landscape can negatively impacts the rate of catalysis when coupled with slow interfacial electron transfer kinetics.

Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions

“…Additional work by Artero, Costentin and coworkers focuses on the bidirectionality (H 2 evolution/H 2 oxidation) of the system, where all of the kinetic constants were extracted for the proposed reversible reaction scheme. 26 Notably, this study shows the importance of the ligand scaffold not only for its conformational flexibility and proper positioning of the pendant amine for metal hydride formation, but also for the effective deprotonation of the nickel-hydride species.…”

Kinetic Modeling of the Reversible or Irreversible Electrochemical Responses of FeFe-Hydrogenases

“…An experimental comparison between Cr HydA1 and Tam HydS in this respect is difficult because the Michaelis constants are difficult to measure accurately and impossible to measure and compare at the same temperature. However, eq is another illustration of the idea that flat energy landscapes are not required to obtain a reversible catalytic response: , that the catalytic cycle that includes high or low energy intermediates may favor reversibility, sometimes at the expense of turnover frequency.…”

“…51,52 The recent investigation of the [Ni(P 2 Cy N 2 Arg ) 2 ] 6+ Dubois complex that reversibly converts H + and H 2 30,53 also showed that the reduction potential of the Ni II /Ni I H + couple is much lower than that of the more reduced Ni III H − /Ni II H − couple, which makes the half-reduced tautomers Ni I H + and Ni III H − unstable. 39 In both cases, therefore, potential inversion 54 destabilizes the half-reduced form of the active site and thus decreases E cat ox − E cat red and contributes to make the response reversible.…”

“…Section S2.1). Since the proton is transferred to the H-cluster from the final proton relay (the side chain of a conserved cysteine residue in group A hydrogenases) rather than directly from bulk water or from the buffer, the rate constants of the protonation and deprotonation steps, k 1 and k –1 , respectively, depend on the pH according to k 1 = k 1 max 1 + K r e l a y false[ H + false] k − 1 = k − 1 max 1 + false[ H + false] K r e l a y where K relay is the acidity constant of the proton relay. , …”

“…Here, we follow up on recent advances in the kinetic modeling of bidirectional catalytic voltammetry − to propose an original point of view on the catalytic cycle of prototypical and atypical FeFe-hydrogenases. We aim at giving a full description of how the voltammogram shapes (catalytic potentials and limiting currents) depend on pH and H 2 concentration.…”