pH-Dependent Reduction Potentials and Proton-Coupled Electron Transfer Mechanisms in Hydrogen-Producing Nickel Molecular Electrocatalysts

Abstract

The nickel-based P2(Ph)N2(Bn) electrocatalysts comprised of a nickel atom and two 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane ligands catalyze H2 production in acetonitrile. Recent electrochemical experiments revealed a linear dependence of the Ni(II/I) reduction potential on pH with a slope of 57 mV/pH unit, implicating a proton-coupled electron transfer (PCET) process with the same number of electrons and protons transferred. The combined theoretical and experimental studies herein provide an explanation for this pH dependence in the context of the overall proposed catalytic mechanism. In the proposed mechanisms, the catalytic cycle begins with a series of intermolecular proton transfers from an acid to the pendant amine ligand and electrochemical electron transfers to the nickel center to produce the doubly protonated Ni(0) species, a precursor to H2 evolution. The calculated Ni(II/I) reduction potentials of the doubly protonated species are in excellent agreement with the experimentally observed reduction potential in the presence of strong acid, suggesting that the catalytically active species leading to the peak observed in these cyclic voltammetry (CV) experiments is doubly protonated. The Ni(I/0) reduction potential was found to be slightly more positive than the Ni(II/I) reduction potential, indicating that the Ni(I/0) reduction occurs spontaneously after the Ni(II/I) reduction, as implied by the experimental observation of a single CV peak. These results suggest that the PCET process observed in the CV experiments is a two-electron/two-proton process corresponding to an initial double protonation followed by two reductions. On the basis of the experimental and theoretical data, the complete thermodynamic scheme and the Pourbaix diagram were generated for this catalyst. The Pourbaix diagram, which identifies the most thermodynamically stable species at each reduction potential and pH value, illustrates that this catalyst undergoes different types of PCET processes for various pH ranges. These thermodynamic insights will aid in the design of more effective molecular catalysts for H2 production.

Unravelling the pH-dependence of a molecular photocatalytic system for hydrogen production

“…These amines with a relatively low p K a have been suggested to act as proton relays between the solvent and the metal centre. 13,18,19,43 Although these studies were mainly performed in pure organic solvents or aqueous-organic solvent mixtures in the presence of strong acids, the electrocatalytic proton reduction activity of NiP was observed to increase towards more acidic pH. 17 In this article, we detail the dependence of the catalytic activity of NiP on pH in pure water.…”

“…17 This pH dependence is typical of this type of nickel-based molecular electrocatalysts, and has been attributed to the presence of pendant amines with low p K a , which are thought to act as a proton relay between the solvent and the metal centre. 13,18–20 …”

Ligand‐Based PCET Reduction in a Heteroleptic Ni(bpy)(dithiolene) Electrocatalyst Giving Rise to Higher Metal Basicity Required for Hydrogen Evolution

“…The redox potentials were calculated by employing the experimentally determined reduction potentials of Ni II (bpy)(qdt) (Figure S23) as the benchmark (see Scheme S1 for detail) ,. Moreover, the p K a DMF value of acetic acid (p K a DMF =13.5) was adopted as the benchmark to calculate the p K a values of the possible intermediates (see also Scheme S1 for detail) ,,…”

Hydrogen Tunneling in Enzymes and Biomimetic Models

“…56–57,60,270 These systems have been studied computationally to analyze the dependence of the reduction potentials and p K a ’s on the substituents, to generate Pourbaix diagrams and thermodynamic cycles, and to examine the ring isomerization processes and intramolecular proton transfer reactions. 271–280 Although these theoretical studies do not account for hydrogen tunneling, they have provided useful mechanistic insights.…”