Potential-Dependent Electrocatalytic Pathways: Controlling Reactivity with p<i>K</i>_a for Mechanistic Investigation of a Nickel-Based Hydrogen Evolution Catalyst

A detailed mechanistic analysis is presented for the hydrogen evolution catalyst [Ni­(P₂^PhN₂^Ph)₂(CH₂CN)]­[BF₄]₂ in acetonitrile (P₂^PhN₂^Ph = 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane). This complex has a Ni^II/I redox couple at −0.83 V and a Ni^I/0 redox couple at −1.03 V versus Fc^+/0. These two closely spaced redox events both promote proton reduction catalysis, each via a distinct mechanism: an electrochemical ECEC pathway and an EECC route. The EECC mechanism, operative at more negative potentials, was isolated through use of a weak acid (anilinium, p<i>K</i>_a = 10.6 in CH₃CN) to avert protonation of the singly reduced species. Electroanalytical methods and time-resolved spectroscopy were used to analyze the kinetics of the elementary steps of hydrogen evolution catalysis. The rate constant for the formation of a nickel­(II)–hydride intermediate was determined via measurements of peak shift (<i>k</i>₁ = 1.2 × 10⁶ M^–1 s^–1) and through foot-of-the-wave analysis (<i>k</i>₁ = 6.5 × 10⁶ M^–1 s^–1). Reactivity of the isolated hydride with acid to release hydrogen and regenerate the nickel­(II) complex was monitored by stopped-flow spectroscopy. Kinetics obtained from stopped-flow measurements are corroborated by current plateau analysis of the catalytic cyclic voltammograms. These kinetic data suggest the presence of an off-cycle intermediate in the reaction

Reactivity of Proton Sources with a Nickel Hydride Complex in Acetonitrile: Implications for the Study of Fuel-Forming Catalysts

The reactivity of the nickel hydride complex [HNi­(P₂^PhN₂^Ph)₂]⁺ (P₂^PhN₂^Ph = 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane) with a variety of acids to form hydrogen in acetonitrile was evaluated using stopped-flow spectroscopy in order to gain a better understanding of how the proton source influences the reaction kinetics when evaluating fuel-forming catalysts in acetonitrile. This reaction is initiated by the rate-determining step in the catalytic cycle for the hydrogen-evolving catalyst [Ni­(P₂^PhN₂^Ph)₂]²⁺. Proton sources were evaluated with respect to p<i>K</i>_a, homoconjugation, dimerization, heteroconjugation, and aggregation (for water). The effects of water and conjugate base were also studied. A linear free energy relationship between rate constant and p<i>K</i>_a was revealed; rate constants increased with the magnitude of the homoconjugation constant for acids prone to homoconjugation, and second-order reactivity was observed for trifluoroacetic and trichloroacetic acid, suggesting dimerization. Upon the addition of water, an increase in the observed rate constant was seen, in line with the formation of hydronium. When added to trifluoroacetic acid, water was shown to cause a decrease in the observed rate constant, suggesting that water inhibits acid dimerization. Collectively, these findings highlight that the selection of proton sources for the study of molecular electrocatalysts in acetonitrile must account for more than acid p<i>K</i>_a

Linear Free Energy Relationships in the Hydrogen Evolution Reaction: Kinetic Analysis of a Cobaloxime Catalyst

Kinetic analysis of hydrogen production catalyzed by Co­(dmgBF₂)₂(CH₃CN)₂ (dmgBF₂ = difluoroboryl-dimethylglyoxime) was performed in acetonitrile with a series of <i>para</i>-substituted anilinium acids. It was determined that the mechanism of hydrogen evolution is governed by three elementary steps; two are acid concentration and p<i>K</i>_a dependent, whereas the third was shown to be intrinsic to the catalyst, likely reflecting either H–H bond formation or H₂ release. The kinetics of the first proton transfer step, the protonation of the singly reduced catalyst, were evaluated using foot-of-the-wave analysis, as well as current–potential analysis for voltammograms displaying total catalysis behavior. Analysis of the total catalysis peak shift required the empirical determination of a new equation for the ECEC′ catalytic mechanism using digital simulations. The kinetics of the second proton transfer stepassigned to protonation of the doubly reduced, singly protonated speciesand the acid-independent step were determined by analyzing the plateau current of the catalytic wave over a range of acid concentrations. Both proton transfer steps follow linear free energy relationships of log­(<i>k</i>) vs acid p<i>K</i>_a. These linear relationships give slopes of −0.94 and −0.77 for the first and second proton transfers, respectively, indicating that both steps become faster with increasing acid strength

A Practical Beginner’s Guide to Cyclic Voltammetry

Despite the growing popularity of cyclic voltammetry, many students do not receive formalized training in this technique as part of their coursework. Confronted with self-instruction, students can be left wondering where to start. Here, a short introduction to cyclic voltammetry is provided to help the reader with data acquisition and interpretation. Tips and common pitfalls are provided, and the reader is encouraged to apply what is learned in short, simple training modules provided in the Supporting Information. Armed with the basics, the motivated aspiring electrochemist will find existing resources more accessible and will progress much faster in the understanding of cyclic voltammetry

Identification of an Electrode-Adsorbed Intermediate in the Catalytic Hydrogen Evolution Mechanism of a Cobalt Dithiolene Complex

Analysis of a cobalt bis­(dithiolate) complex reported to mediate hydrogen evolution under electrocatalytic conditions in acetonitrile revealed that the cobalt complex transforms into an electrode-adsorbed film upon addition of acid prior to application of a potential. Subsequent application of a reducing potential to the film results in desorption of the film and regeneration of the molecular cobalt complex in solution, suggesting that the adsorbed species is an intermediate in catalytic H₂ evolution. The electroanalytical techniques used to examine the pathway by which H₂ is generated, as well as the methods used to probe the electrode-adsorbed species, are discussed. Tentative mechanisms for catalytic H₂ evolution via an electrode-adsorbed intermediate are proposed

A Practical Beginner’s Guide to Cyclic Voltammetry

Despite the growing popularity of cyclic voltammetry, many students do not receive formalized training in this technique as part of their coursework. Confronted with self-instruction, students can be left wondering where to start. Here, a short introduction to cyclic voltammetry is provided to help the reader with data acquisition and interpretation. Tips and common pitfalls are provided, and the reader is encouraged to apply what is learned in short, simple training modules provided in the Supporting Information. Armed with the basics, the motivated aspiring electrochemist will find existing resources more accessible and will progress much faster in the understanding of cyclic voltammetry