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

Abstract

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 pK 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 pK 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 pK a.

Homoconjugated Acids as Low Cyclosiloxane-Producing Silanol Polycondensation Catalysts

“…The contents of the flask were filtered through a disposable 20 μm frit and washed 2 × 10 mL with acetone. 37 To the filtrate was added trifluoroacetic acid (0.77 mL, 1.14 g, 10 mmol), and acetone was removed under reduced pressure. The resultant product was a low (room temperature) melting solid.…”

Electrochemical proton-coupled electron transfer of an anthracene-based azo dye

“…With this in mind, catalysts and reactions can be designed such that changes in pH result in changing the mechanism of PCET between stepwise or concerted PCET. 5,[26][27][28][29] The nature of the proton (either solvated or at chemically accessible sites from an acidic bond or organic acids) can be the driver for tuning the thermodynamics of the proton-transfer step in the chemical reactivity. By tuning the pK a of organic acids, the rate and mechanism of PCET in small-molecule inorganic complexes can be systematically dialled in.…”

Synthesis, characterization and electro-catalytic properties of a water-soluble nickel(II) complex supported by picolinic acid ions

“…Among current technologies for hydrogen production, electrocatalytic water reduction via hydrogen evolution reaction (HER) is considered as one of the most attractive and simple methods [3]. So far, many research groups, including ours, have developed several molecular catalysts for hydrogen generation from organic acid and water based on transition metal complexes, such as nickel [4][5][6][7][8][9][10][11][12][13], cobalt [14][15][16][17], copper [18,19], iron [20] and manganese [21]. Despite much progress in water reduction catalysis, major improvements in several areas, including decreasing structural complexity and increasing solubility in aqueous media, are needed before efficient electrocatalytic water reduction can be realized [22].…”