Computational design of molecular catalysts for hydrogen production – CODEC

CODEC aims at developing a general computational strategy to predict the catalytic performances of bioinspired catalytic systems for the electrochemical conversion of protons into hydrogen. The major objective of the project is to digitally assist the design of new innovative catalysts with the ambitious aim of reproducing the impressive reactivity of the biological system that is the hydrogenase enzyme. Within this project, DFT computations will be used to investigate the elementary steps of the mechanisms involving the selected electrocatalysts to identify the predominant pathway leading to hydrogen production from the nickel and cobalt thiosemicarbazone complexes. The free energy profiles of all possible mechanistic routes will be evaluated along with the corresponding intermediates and transition states resulting from electrochemical and/or chemical events. The calculations of the free energies of the species formed during the protonation and reduction steps will be further used to access thermodynamic quantities like standard redox potentials and pKa values. This will provide the necessary data to generate the corresponding potential-pH diagrams, known as Pourbaix diagrams. This will allow us to evaluate the stability of all possible intermediate species on a broad range of pH values. Using potential-pH diagrams, we will also investigate the influence of the acid acting as the proton source on the catalytic performances of our systems. All the information gathered from the theoretical determination of the Pourbaix diagrams can serve in catalyst design to predict the properties of electrocatalysts that have yet to be synthesized, and to improve catalytic behaviour through the manipulation of redox potentials and pKa values. Eventually, the results from these computational studies will thus serve to develop a new series of nickel and cobalt thiosemicarbazone catalysts with chemical modulation of their ligand skeleton.

As part of the CODEC project, we used DFT calculations to evaluate all possible mechanistic pathways leading to the electrocatalytic production of hydrogen by three nickel complexes with thiosemicarbazone ligands, NiL, in the presence of a proton source. Our results suggest that two possible mechanisms are consistent with published experimental data: (i) one involving two protons and two electrons, and (ii) another with three protons and two electrons. In both cases, the cycle consists of two successive stages of concerted electron and proton transfer (EC-EC). Hydrogen formation occurs via the tunnelling of the two protons to form H2, or involves a third proton with a tunnelling effect of a lower order of magnitude. Our data highlighted the importance of explicitly including the proton source in calculations if experimental data are to be reproduced. We have continued this work and carried out a similar study in an attempt to determine the influence of the metal on the reaction mechanism. We focused on the cobalt complex with thiosemicarbazone ligands, CoL, and demonstrated that a single mechanistic pathway led to the electrocatalytic production of hydrogen by this catalyst. The overall catalytic cycle involves the transfer of three electrons and two protons, where the first electrochemical step corresponds to complex activation. Our analyses indicate that the reactivity is metal-centered, as opposed to ligand-centered as observed with [NiL]. Our study suggests that the rate-limiting step for [CoL] is not related to proton transfer reactions, which was the case for [NiL]. A final study looked at the influence of catalyst nuclearity to determine whether the presence of multiple metal sites could improve catalytic performance. We synthesized a trinuclear nickel-based complex [Ni3L] which was tested in hydrogen production. This complex exhibited higher TON and TOF values, and theoretical calculations showed that the reactivity of the trinuclear complex exclusively involved the central metal ion of [Ni3L], as opposed to the metal-centered, ligand-assisted reactivity of [NiL]. This change in reactivity would explain the improved efficiency of the complex.

Ultimately this computational research project will help to predict the feasibility of the hydrogen evolution reaction as well as the rational design of more effective molecular catalysts. Besides, one can generalize the proposed methodology to be applied to investigate other types of metal-based catalysts and to approach a larger spectrum of reactivities like oxidation reactions.

1. M. Papadakis, A. Barrozo, T. Straistari, N. Queyriaux, A. Putri, J. Fize, M. Giorgi, M. Re´glier, J. Massin, R. Hardre´, M. Orio* «Ligand-Based Electronic Effects on the Electrocatalytic Hydrogen Production by Thiosemicarbazone Nickel Complexes.« Dalton Trans., 2020, 49, 5064-5073.
2. C. Pieri, A. Bhattacharjee, A. Barrozo, B. Faure, M. Giorgi, J. Fize, M. Re´glier, M. Field, M. Orio*, V. Artero, R. Hardre´ «Hydrogen evolution reaction mediated by a trinuclear nickel complex with an all sulfured coordination sphere.« Chem. Comm., 2020, 56, 11106-11109.
3. A. Barrozo, M. Orio* “Unraveling the Catalytic Mechanisms of H2 Production in Thiosemicarbazone Nickel Complexes.” RSC Adv., 2021, 11, 5232-5238.
4. M. Orio*, D. A. Pantazis “Challenges and opportunities for theory in understanding metalloenzymes.« Chem. Comm., 2021, 57, 3952-3974.

In the pursuit of carbon-free fuels, hydrogen can be considered as an apt energy carrier. The design of molecular electrocatalysts for hydrogen production is important for the development of renewable energy sources that are abundant, inexpensive, and environmentally benign. The past years a large number of electrocatalysts was developed and a considerable effort has been thus directed toward the design of earth-abundant, first-row transition-metal complexes. Considering the possibility to enhance catalytic activity using non-innocent ligands, we have associated the electroactive thiosemicarbazone ligand with transition metal ions to generate a series of bio-inspired complexes as hydrogen evolution electrocatalysts. We have shown that the complexes exhibit high electrocatalytic activity for proton reduction which render these systems competitive to the most efficient nickel- and cobalt-based catalysts reported in the literature. Yet, their reaction mechanism as well as the key elements to understand, rationalize and enhance the reactivity of these catalysts are still unknown. The project CODEC aims at addressing these issues by developing a general computational strategy to predict the catalytic performances of bio-inspired complexes for hydrogen production. Our goal is to understand the reaction mechanisms of our electrocatalysts by identifying the electronic parameters that govern their reactivity and determine the crucial structural elements to achieve efficient hydrogen production. Our strategy will lead to the determination of computational potential-pH diagrams which provide a rich array of information about the redox-active species. The Pourbaix diagrams will permit similar analyses of other electrocatalysts for direct comparison with the most efficient systems reported in the literature. These numerical investigations will provide a proof-of-concept that quantum chemistry can be used as a predictive tool to rationalize reactivity, enhance catalytic activity and guide the design of new synthetic targets. Ultimately it will help to predict the feasibility of the hydrogen evolution reaction as well as the rational design of more effective molecular catalysts. Besides, one can generalize the proposed methodology to be applied to investigate other types of metal-based catalysts and to approach a larger spectrum of reactivities like oxidation reactions.