Papers, communications and reviews… our recent published work is here.


Electrochemical Formation and Reactivity of a Mn-Peroxo Complex Bearing an Amido N5 Ligand

A. Massie, N. Kostopoulos, E. N. Grotemeyer, J.-M. Noël, T. A. Jackson, E. Anxolabéhère-Mallart

ChemElectroChem 9 (11), e202200112, 2022

The electrochemical formation of the MnIII(dpaq)(OO) complex supported on a pentadentate ligand featuring an amide moiety, and its reactivity in DMF is probed by low temperature electronic absorption spectro-electrochemistry experiments and structurally characterized using DFT computations. An analysis combining CV and simulations, corroborated by DFT computation supports the formation of a MnII(dpaq)(OOH) complex upon its reduction.
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Phenoxazine Sensitized CO2-to-CO Reduction with an Iron Porphyrin Catalyst: A Redox Properties-Catalytic Performance Study

Martin Kientz, Grace Lowe, Blaine G. McCarthy, Garret M. Miyake, Julien Bonin, Marc Robert

ChemPhotoChem, e202200009, 2022

We have evaluated six phenoxazine derivatives as visible light photosensitizers for the photochemical reduction of CO<sub>2</sub> to CO with an iron porphyrin catalyst in organic media. The phenoxazine core was functionalized with electron-donating or -withdrawing groups to modify the photophysical properties. Both singlet and triplet excited state potentials of the sensitizers spanned several hundred mV range and the ground state oxidation potentials spanned 250 mV. We observed that no correlation can be established between the production of CO and the excited state potential of the phenoxazine, which determines the driving force for electron transfer to the catalyst. On the contrary, a clear correlation can be made between the oxidation potentials of the phenoxazine and the production of CO. This observation indicates that the process was limited by photosensitizer regeneration and highlights the fact that electron transfers not directly related to the catalyst activation could play a key role in homogeneous photocatalytic systems.
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On the existence and role of formaldehyde during aqueous electrochemical reduction of carbon monoxide to methanol by cobalt phthalocyanine

Etienne Boutin, Aude Salamé, Lydia Merakeb, Tamal Chatterjee, Marc Robert

Chem. Eur. J. 28 (27), e202200697, 2022

A long-time challenge in aqueous CO2 electrochemical reduction is to catalyze the formation of products beyond carbon monoxide with selectivity. Formaldehyde is the simplest of these products and one of the most relevant due to its broad use in the industry. Paradoxically it is one of the less reported product. Such scarcity may be in part explained by difficult identification and quantification using conventional chromatography or proton nuclear magnetic resonance techniques. Likewise, indirect detection methods are usually not compatible with labelled study for asserting product origin. Recently, the possible production of formaldehyde during electrochemical reduction of carbon monoxide to methanol at cobalt phthalocyanine molecular catalyst in basic media has been the object of contradictory reports. Applying an analytical procedure based on proton NMR along with labelled studies, we provide definitive evidence for HCHO formation. We have further identified the possible scenarios for methanol formation through formaldehyde and revealed that the formation of the intermediate and its subsequent reduction are taking place at the same single active site. These studies open a new perspective to improve selectivity toward formaldehyde formation and to develop a subsequent chemistry based on reacting it with nucleophiles.
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Highly Efficient Photocatalytic Reduction of CO2 to CO by In Situ Formation of a Hybrid Catalytic System Based on Molecular Iron Quaterpyridine Covalently Linked to Carbon Nitride

Yue Wei, Lingjing Chen, Huan Chen, Lirong Cai, Guiping Tan, Yongfu Qiu, Quanjun Xiang, Gui Chen, Tai-Chu Lau, Marc Robert

Angewandte Chemie Int. Ed., e202116832, 2022

Efficient and selective photocatalytic CO 2 reduction was obtained within a hybrid system that is formed in situ via a Schiff base condensation between a molecular iron quaterpyridine complex bearing an aldehyde function and carbon nitride. Irradiation (blue LED) of an CH 3 CN solution containing 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), triethylamine (TEA), Feqpy-BA (qpy-BA = 4-([2,2':6',2'':6'',2'''-quaterpyridin]-4-yl)benzaldehyde) and C 3 N 4 resulted in CO evolution with a turnover number of 2554 and 95% selectivity. This hybrid catalytic system unlocks covalent linkage of molecular catalysts with semiconductor photosensitizers via Schiff base reaction for high-efficiency photocatalytic reduction of CO 2 , opening a pathway for diverse photocatalysis.
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Advances in Molecular Electrochemical Activation of Dinitrogen

Lydia Merakeb, Marc Robert

Curr. Opin. Electrochem. 29, 100834, 2021

Nitrogen fixation is an important and challenging transformation in chemistry. It is currently achieved by the Haber–Bosch process that reduces N2 to NH3 on large scale but at the cost of a huge environmental footprint. In the search for greener processes, electrochemically driven N2 reduction might become a sustainable answer. The development of molecular homogeneous catalysis has gained momentum over the last decades, and we focus in this review on the emerging electrochemical systems based on molecular catalysts for N2 fixation.
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Molecular Electrocatalytic Hydrogenation of Carbonyls and Dehydrogenation of Alcohols

Niklas von Wolff, Orestes Rivada-Wheelaghan, Damien Tocqueville

ChemElectroChem 8, 4019-4027, 2021

The development of safe and energy efficient redox processes is key for a future sustainable organic chemistry and energy storage/vector applications. Molecular electrocatalysts have demonstrated their potential in the realm of CO2 reduction, however, successful implementations for the reduction of other carbonyl groups remain sporadic. Building on the reversibility of hydrogenation and dehydrogenation of carbonyls and alcohols, an overview of current molecular electrocatalytic systems is presented. Key mechanistic concepts are emphasized to facilitate the link with more mature schemes in transfer hydrogenation, proton- and CO2-reduction. Thus, this work contributes to future catalyst generation development bridging fundamental aspects of electrochemical bond activation with molecular catalytic concepts in the context of societal challenges of today.
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Electrocatalytic and Photocatalytic Reduction of Carbon Dioxide by Earth-abundant Bimetallic Molecular Catalysts

Huan Chen, Lingjing Chen, Gui Chen, Marc Robert, Tai-Chu Lau

ChemPhysChem 22 (18), 1835-43, 2021

Converting CO2 into useful resources by electrocatalysis and photocatalysis is a promising strategy for recycling of the gas and electrification of the industry. Numerous studies have shown that multinuclear metal catalysts have higher selectivity and catalytic activity than monometallic catalysts due to the synergistic effects between the metal sites. In this review, we summarize some of the recent progress on the electrocatalytic and photocatalytic reduction of CO2 by earth-abundant bimetallic molecular catalysts.
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Taming Electron Transfers: From Breaking Bonds to Creating Molecules

Niklas von Wolff, Marc Robert

The Chemical Record 21 (9), 2095-2106, 2021

The electron is the ultimate redox reagent to build and reshape molecular structures. Understanding and controlling the parameters underlying dissociative electron transfer (DET) reactivity and its coupling with proton transfer is crucial for combining selectivity, kinetics and energy efficiency in molecular chemistry. Reactivity understanding and mechanistic elements in DET processes are traced back and key examples of current research efforts are presented, demonstrating a large variety of applications. The involvement of DET pathways indeed encompasses a broad range of processes such as photoredox catalysis, CO2 reduction and alcohol oxidation. Interplay between these experimental examples and fundamental mechanistic study provides a powerful path to the understanding of driving force-rate relationships, which is crucial for the development of future generations of energy efficient catalytic schemes in redox organic chemistry.
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Highlights and Challenges in the Selective Reduction of Carbon Dioxide to Methanol

Sara Navarro-Jaén, Mirella Virginie, Julien Bonin, Marc Robert, Robert Wojcieszak and Andrei Y. Khodakov

Nat. Rev. Chem. 5, 564-579, 2021

Carbon dioxide (CO2) is the iconic greenhouse gas and the major factor driving present global climate change, incentivizing its capture and recycling into valuable products and fuels. The 6H+/6e− reduction of CO2 affords CH3OH, a key compound that is a fuel and a platform molecule. In this Review, we compare different routes for CO2 reduction to CH3OH, namely, heterogeneous and homogeneous catalytic hydrogenation, as well as enzymatic catalysis, photocatalysis and electrocatalysis. We describe the leading catalysts and the conditions under which they operate, and then consider their advantages and drawbacks in terms of selectivity, productivity, stability, operating conditions, cost and technical readiness. At present, heterogeneous hydrogenation catalysis and electrocatalysis have the greatest promise for large-scale CO2 reduction to CH3OH. The availability and price of sustainable electricity appear to be essential prerequisites for efficient CH3OH synthesis.
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Molecule/Semiconductor Hybrid Materials for Visible-Light CO2 Reduction: Design Principles and Interfacial Engineering

Akinobu Nakada, Hiromu Kumagai, Marc Robert, Osamu Ishitani and Kazuhiko Maeda

Acc. Mater. Res. 2 (6), 458–470, 2021

Because of increasing concerns over the depletion of energy sources and the concomitant increase in CO2 emissions, much attention has been devoted to carbon capture and utilization technologies. Among the various methods and schemes proposed, visible-light-driven CO2 reduction in combination with water oxidation, one of the representative models of artificial photosynthesis, is an attractive solution because it enables abundant water and inexhaustible solar energy to be used to produce value-added chemicals. Molecular metal complexes and semiconductors are promising candidates for photocatalysts that can reduce CO2 to CO, formate, formaldehyde, or other hydrocarbons. Although both molecular metal complexes and semiconductors have strengths and weaknesses, their weaknesses (low oxidation ability and low selectivity for reduction reactions) can be overcome via the construction of a suitable molecule/semiconductor hybrid material. However, facilitating electron transfer at the molecule/semiconductor junction while suppressing unfavorable back electron transfer events is challenging. Consequently, the number of molecule/semiconductor hybrid systems that show a reasonable level of visible-light photocatalytic activity is limited, despite the development of a large number of visible-light-driven semiconductors and molecular photocatalysts (or catalysts). In this Account, we describe our approaches to developing hybrid photocatalysts and photoelectrodes for CO2 reduction. We have been developing both molecular (photo)catalysts and semiconductor photocatalysts individually, the latter of which are also designed for visible-light water splitting. For example, supramolecular photocatalysts that possess both photosensitizer and catalyst units in a single molecule can reduce CO2 to formate or CO in a homogeneous system, with high selectivity toward the desired product and high quantum yields of several tens of percent. However, nonoxide semiconductors such as C/N-based polymers and mixed-anion compounds exhibit a strong photooxidation ability under visible light. Carefully designed molecule/semiconductor hybrid materials achieve CO2 reduction under visible light with high product selectivity and stability even in an aqueous environment, where the concentration of CO2 is low but that of protons is high. Visible-light CO2 reduction combined with H2O oxidation is possible via the construction of a photoelectrochemical cell that comprises a molecular photocathode and an n-type semiconductor photoanode. Although our photosystems can be regarded as model systems for artificial photosynthesis, their light-energy conversion efficiencies are still unsatisfactory. To improve the efficiency, materials design, including interfacial engineering at the molecule/semiconductor junction, is important and is the general theme of the results highlighted in this Account.
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