Bio-Inspired [NiFe] hydrogenase Catalysts for H2 production. – NiFemim

To reach our objectives, our strategy is to modify the initial NiFe complex and to develop new ligands that should present comparable electronic properties to conserve the predominant active role of the Ni site and also enhance the reactivity. Regarding the Ni site, we envisage to modify the original N2S2 ligand by the addition of electron donating substituents in order to change the redox properties and also to substitute the bipyridine moiety of the N2S2 ligand by C-center motifs with N-heterocyclic carbene functions, leading to original C2S2 ligands. The latter should present favorable electronic properties and should enable the addition of structural elements that can act as potential proton relays. The replacement of the Ni site by an Fe to mimic the FeFe hydrogenase or by a Co should allow to understand the impact of the nature of the metal on the activity. Besides strategies for the immobilization of the most performant catalysts will be developed to prepare robust systems active in aqueous solutions.

During this project, all objectives were not achieved but other were overcome.

(i) To play on the reactivity of the initial LN2S2NiIIFeIICp complex, especially to modify the mechanism to better mimic the hydrogenase, we modified the Cp ligand
Result: The Cp ligand has been modified with the bulkier Cp* ligand. With this ligand, the corresponding LN2S2NiIIFeIICp* complex in collaboration with all partners of this consortium have been isolated and characterized. We have evidenced that this modification has not only a strong effect on the structural properties of the complex but also on its catalytic activity and on the corresponding catalytic mechanism (Publication in ACS Catal. 2018).

(ii) To understand the role of the metal, we have replaced the Ni by Fe to generate the corresponding LN2S2FeIIFeIICp complex.
Result: The LN2S2FeIIFeIICp complex was fully characterized and its reactivity tested in collaboration with all partners. The data revealed that this catalyst present similar performances than its NiFe parent, with a similar mechanism (Publication in ACS Catal. 2020). Besides, during this investigation, we have to characterize the complexes used as building blocks for the synthesis of LN2S2FeIIFeIICp. This work has been performed in collaboration between two partners, the Göttingen univ. and Univ. Grenoble Alpes (3 Publications in J. Am. Chem. Soc. 2019; Wang, Zlatar et al., Chem. Eur J. 2018; Wang, Reinhard et al., Chem. Eur J. 2018).

(iii) To stabilize our molecular catalysts and develop processes in aqueous solution, we have confined and immobilized on electrode our most performant complexes.
Result: We have deposited the NiFe and FeFe catalysts by physiabsorption on graphite. The corresponding modified electrodes display high efficiency for H2 production at low pH in water. This work has been performed via a novel collaboration with A. Dey from India (Publications in Angew. Chem. Int. Ed 2018 and ChemElectroChem 2020). In parallel, we also tested the performance of our catalyst after its confinement in a MOF structure with no significant improvement (Inorg. Chem. 2017).

(iv) To go beyond the reduction of protons, the catalysts have been tested for CO2 reduction.
The same modified electrodes have proven to be efficient for CO2 reduction, still in acidic aqueous solutions. Notably the unique C-based product is CH4, making this NiFe complex unique (Publication in ACS Energy Letters 2020).

The main objectives of this project have been achieved. The strategy proposed to conceive new C2S2 based ligands and the replacement of S-based ligands by Se-based ones has failed, but are currently working on other types of ligands based on the results obtained during this project. Besides, unexpected results that overcome our expectation have been obtained in terms of robustness and systems applicable in aqueous solutions. This project allowed to initiate a new collaboration with an Indian partner that bring us a complementary expertise to develop devices with potential applications.

(1) Balestri, D.; ...; Artero, V.; Duboc, C.; Pelagatti, P.; Marchio, L.; Gennari, M. Heterogenization of a NiFe Hydrogenase Mimic through Simple and Efficient Encapsulation into a Mesoporous MOF. Inorg. Chem. 2017, 56, 14801-14808.
(2) Ahmed, M. E.; ...; Gennari, M.; Duboc, C.; Dey, A.; Artero, V. Hydrogen Evolution from Aqueous Solutions Mediated by a Heterogenized NiFe -Hydrogenase Model: Low pH Enables Catalysis through an Enzyme-Relevant Mechanism. Angew. Chem. Int. Ed. 2018, 57, 16001-16004.
(3) Brazzolotto, D.; ... S.; Meyer, F.; Orio, M.; Artero, V.; Hall, M. B.; Duboc, C. Tuning Reactivity of Bioinspired NiFe -Hydrogenase Models by Ligand Design and Modeling the CO Inhibition Process. ACS Catal. 2018, 8, 10658-10667.
(4) Wang, ... Meyer, F.; Gennari, M.; Duboc, C. Solvent- and Halide-Induced (Inter)conversion between Iron(II)-Disulfide and Iron(III)-Thiolate Complexes. Chem. Eur. J. 2018, 24, 11973-11982.
(5) Wang, L. K.; ... F.; Gennari, M.; Meyer, F.; Duboc, C.; Gruden, M. Experimental and Theoretical Identification of the Origin of Magnetic Anisotropy in Intermediate Spin Iron(III) Complexes. Chem. Eur. J. 2018, 24, 5091-5094.
(6) Wang, L.; Gennari, ...S.; Artero, V.; Meyer, F.; de Visser, S. P.; Duboc, C. A Non-Heme Diiron Complex for (Electro)catalytic Reduction of Dioxygen: Tuning the Selectivity through Electron Delivery. J. Am. Chem. Soc. 2019, 141, 8244-8253.
(7) Barrozo, A. M. Orio, M.; Molecular electrocatalysts for Hydrogen Evolution Reaction: The input from quantum chemistry. Chem. Sus. Chem., 2019, 12, 4905-4915.
(8) Ahmed, M. E.; ...; Artero, V.; Dey, A.; Duboc, C. Repurposing a Bio-Inspired NiFe Hydrogenase Model for CO2 Reduction with Selective Production of Methane as the Unique C-Based Product. ACS Energy Letters 2020, 5, 3837-3842.
(9) Wang, L.; Gennari, ....; Meyer, F.; Orio, M.; Artero, V.; Duboc, C. Role of the Metal Ion in Bio-Inspired Hydrogenase Models: Investigation of a Homodinuclear FeFe Complex vs Its Heterodinuclear NiFe Analogue. ACS Catal. 2020, 10, 177-186.
(10) Ahmed, M. E.; ...Gennari, M.; Ghosh Dey, S.; Artero, V.; Dey, A.; Duboc, C. An [FeFe]-Hydrogenase Mimic Immobilized through Simple Physiadsorption and Active for Aqueous H2 Production. ChemElectroChem 2021, 8, 1674-1677.
(11) Gennari, M.; Duboc, C. Bio-inspired, Multifunctional Metal-Thiolate Motif: From Electron Transfer to Sulfur Reactivity and Small-Molecule Activation. Acc. Chem. Res. 2020, 53, 2753-2761.
(12) Ghosh, A. C.; Duboc, C.; Gennari, M. Synergy between metals for small molecule activation: Enzymes and bio-inspired complexes. Coord. Chem. Rev. 2021, 428.
(13) Orio M., Pantazis D. A.; Successes, challenges and opportunities for quantum chemistry in understanding métalloenzymes for solar fuel research. Chem. Comm., 2021, 57, 3952-3974.

Hydrogen production through water splitting appears to be among the preferred solutions in the long run for the storage of renewable energy. Nature offers efficient H2 evolution catalysts in the form of hydrogenases, organometallic enzymes containing nickel and/or iron sites whose catalytic performances rival commonly used platinum catalysts for hydrogen production. Hydrogenases thus offer fascinating blueprints for the design of new molecular catalysts based on earth-abundant metals, to be implemented in technological devices such as electrolysers or photo-electrochemical cells. However, all heterodinuclear NiFe model systems reported so far do not reproduce the Ni-centered chemistry that occurs at the active site of [NiFe] hydrogenases, and their efficiency is still very low compared to the enzyme.
By means of the development of innovative bio-inspired H2 evolution catalysts, our major objectives are (i) to contribute to the full understanding of the catalytic mechanism of the [NiFe] hydrogenase and (ii) to develop efficient mimics of this enzyme that model not only the structure and function of the active site but also its impressive reactivity. This requires that the H+/H2 reactivity and redox events occur at the Ni site of a dithiolato-bridged heterobimetallic NiFe complex. Our preliminary results with the characterization of an unprecedented Ni-centered H+ reduction catalyst, which accurately models multiple states of the [NiFe]-hydrogenase, represent a real breakthrough in this field and will be the foundation of this project. More specifically, we plan to focus on the following points: (i) isolation and characterization of active metal hydride intermediates relevant to the [NiFe]-hydrogenase mechanism, (ii) determination of the crucial structural elements to achieve efficient catalysis with Ni-centered hydrogenase chemistry, (iii) definition of the factors that control the electron and proton transfer sequence during the catalytic cycle, (iv) importance of a proton relay for the performance of the catalysts and role of the second coordination sphere, and (v) effect of replacing a sulfur by a selenium donor atom on the redox and electronic properties of the NiFe complexes, and on their reactivity for H2 evolution and toward O2. The latter will provide important insight into key functional differences between [NiFe] and [NiFeSe] hydrogenases.
The French-German consortium involved in this fundamental research project brings together partners who combine all expertise required to reach the ambitious multidisciplinary goals, namely expertise from synthetic coordination chemistry, spectroscopy and kinetics, electrochemistry, catalysis and quantum chemistry.