Proton Relays in Molecular Catalysis for Hydrogen Evolution and Oxidation: Lessons From the Mimicry of Hydrogenases and Electrochemical Kinetic Analyses

The active sites of metalloenzymes involved in small molecules activation often contain pendant bases that act as proton relay promoting proton-coupled electron-transfer processes. Here we focus on hydrogenases and on the reactions they catalyze, i. e. the hydrogen evolution and oxidation reactions. After a short description of these enzymes, we review some of the various biomimetic and bioinspired molecular systems that contain proton relays. We then provide the formal electrochemical framework required to decipher the key role of such proton relay to enhance catalysis in a single direction and discuss the few systems active for H2 evolution for which quantitative kinetic data are available. We finally highlight key parameters required to reach bidirectional catalysis (both hydrogen evolution and hydrogen oxidation catalyzed) and then transition to reversible catalysis (both reactions catalyzed in a narrow potential range) as well as illustrate these features on few systems from the literature.

Electrocatalytic Reduction of CO2 and H2O with Zn(II) Complexes Through Metal-Ligand Cooperation

Air and water-stable zinc (II) complexes of neutral pincer bis(diphenylphosphino)-2,6-di(amino)pyridine ("PN3P") ligands are reported. These compounds, [Zn(κ2-2,6-{Ph2PNR}2(NC5H3))Br2] (R=Me, 1; R=H, 2), were shown to be capable of electrocatalytic reduction of CO2 at -2.3 V vs. Fc+/0 to selectively yield CO in mixed water/acetonitrile solutions. These complexes also electrocatalytically generate H2 from water in acetonitrile solutions, at the same potential, with Faradaic efficiencies of up to 90 %. DFT computations support a proposed mechanism involving the first reduction of 1 or 2 occurring at the PN3P ligand. Furthermore, computational analysis suggested a mechanism involving metal-ligand cooperation of a Lewis acidic Zn(II) and a basic ligand.

A bio-inspired mononuclear manganese catalyst for high-rate electrochemical hydrogen production

[FeFe]-hydrogenase (H₂ase) catalyzes hydrogen evolution reactions (HERs), with an excellent performance that rivals that of platinum, the active site of which is built with crucial structural features required for efficient H-H bond formation. Herein, we report a mononuclear manganese complex (1) that contains a square pyramid coordination sphere and an intramolecular aniline as the proton relay, consistent with the crucial features of the active site in H₂ase. Benefitting from these features, complex 1 electrocatalyzes the HER with a turnover frequency (TOF) exceeding 10 000 s^-1 at -1.45 V (versus the ferrocenium/ferrocene couple) using anilinium tetrafluoroborate as a proton source. This work provides the first Mn-based functional model of H₂ase, serving as a new paradigm for a high performance, low cost, environmentally benign hydrogen production electrocatalyst.

Mononuclear Mn complexes featuring N,S-/N,N-donor and 1,3,5-triaza-7-phosphaadamantane ligands: synthesis and electrocatalytic properties

Mononuclear Mn(i) carbonyl complexes incorporating 2-mercaptobenzothiazole or 2-mercaptobenzimidazole and phosphaadamantane ligands were evaluated as electrocatalysts for the HER both in acetonitrile and acetonitrile/water.

Promoting proton coupled electron transfer in redox catalysts through molecular design

Mini-review on using the secondary coordination sphere to facilitate multi-electron, multi-proton catalysis.

Reaction mechanism of hydrogen evolution catalysed by Co and Fe complexes containing a tetra-dentate phosphine ligand - a DFT study

The reaction mechanism of the electro-catalytic proton reduction in neutral phosphate buffer enabled by mononuclear cobalt and iron complexes containing a tetra-dentate phosphine ligand (MP₄N₂, M = Fe, Co) has been elucidated by density functional calculations. The phosphate from the buffer was found to play a crucial role by coordinating to the metal and delivering a proton to the metal hydride in the H-H bond formation. For the more efficient cobalt catalyst, the starting species is a Co^II complex with a hydrogen phosphate and a water molecule ligated at the two vacant coordination sites. Two sequential proton-coupled electron transfer reductions lead to the formation of a Co^II-H intermediate with a dihydrogen phosphate ligand, and the reduction potentials for these two steps were calculated to be -0.58 V and -0.72 V, respectively. Subsequently, the H-H bond formation takes place via coupling of the Co^II-H and the proton from the dihydrogen phosphate ligand. The total barrier was calculated to be 18.2 kcal mol^-1 with an applied potential of -0.5 V, which can further decrease to only 11.2 kcal mol^-1 with an applied potential of -0.8 V. When the phosphate is displaced by a water molecule, the total barrier for the dihydrogen formation increases by 7.3 kcal mol^-1. For the iron catalyst, the overall mechanism is essentially the same; however, the first reduction (Fe^II/Fe^I, potential of -1.13 V) is likely the rate-limiting step. The calculated results are in good agreement with the experimental data, which showed an onset potential of -0.50 V for the cobalt complex and -1.03 V for the iron complex.