Marc Robert

A Local Proton Source Enhances CO2 Electroreduction to CO by a Molecular Fe Catalyst

Lending a Hand to CO2 Reduction Although plants and microbes have been reducing CO2 with ease for millennia, people still find it extremely challenging. A cost-effective synthetic scheme for transforming CO2 into fuels and commodity chemicals would be a double boon, lowering atmospheric concentration of the greenhouse gas while supplementing (and perhaps ultimately replacing) dwindling petroleum feedstocks. Toward this end, Costentin et al. (p. 90) show that an iron catalyst for the electrochemical reduction of CO2 to CO gets an efficiency boost from phenol substituents appended to the ligand framework. Phenol groups in an iron complex appear to facilitate catalysis of carbon dioxide reduction by supplying protons. Electrochemical conversion of carbon dioxide (CO2) to carbon monoxide (CO) is a potentially useful step in the desirable transformation of the greenhouse gas to fuels and commodity chemicals. We have found that modification of iron tetraphenylporphyrin through the introduction of phenolic groups in all ortho and ortho′ positions of the phenyl groups considerably speeds up catalysis of this reaction by the electrogenerated iron(0) complex. The catalyst, which uses one of the most earth-abundant metals, manifests a CO faradaic yield above 90% through 50 million turnovers over 4 hours of electrolysis at low overpotential (0.465 volt), with no observed degradation. The basis for the enhanced activity appears to be the high local concentration of protons associated with the phenolic hydroxyl substituents.

Turnover Numbers, Turnover Frequencies, and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and Preparative-Scale Electrolysis

The search for efficient catalysts to face modern energy challenges requires evaluation and comparison through reliable methods. Catalytic current efficiencies may be the combination of many factors besides the intrinsic chemical properties of the catalyst. Defining turnover number and turnover frequency (TOF) as reflecting these intrinsic chemical properties, it is shown that catalysts are not characterized by their TOF and their overpotential (η) as separate parameters but rather that the parameters are linked together by a definite relationship. The log TOF-η relationship can often be linearized, giving rise to a Tafel law, which allows the characterization of the catalyst by the value of the TOF at zero overpotential (TOF(0)). Foot-of-the-wave analysis of the cyclic voltammetric catalytic responses allows the determination of the TOF, log TOF-η relationship, and TOF(0), regardless of the side-phenomena that interfere at high current densities, preventing the expected catalytic current plateau from being reached. Strategies for optimized preparative-scale electrolyses may then be devised on these bases. The validity of this methodology is established on theoretical grounds and checked experimentally with examples taken from the catalytic reduction of CO(2) by iron(0) porphyrins.

Concerted Proton−Electron Transfers: Electrochemical and Related Approaches

Proton-coupled electron transfers (PCETs) are omnipresent in natural and artificial chemical processes. Given the contemporary challenges associated with energy conversion, pollution abatement, and the development of high-performance sensors, a greater understanding of the mechanisms that underlie the practical efficiency of PCETs is a timely research topic. In contrast to hydrogen-atom transfers, proton and electron transfers involve different centers in PCET reactions. The reaction may go through an electron- or proton-transfer intermediate, giving rise to the electron-proton transfer (EPT) and the proton-electron transfer (PET) pathways. When the proton and electron transfers are concerted (the CPET pathway), the high-energy intermediates of the stepwise pathways are bypassed, although this thermodynamic benefit may have a kinetic cost. The primary task of kinetics-based mechanism analysis is therefore to distinguish the three pathways, quantifying the factors that govern the competition between them, which requires modeling of CPET reactivity. CPET models of varying sophistication have appeared, but the large number of parameters involved and the uncertainty of the quantum chemical calculations they may have to resort to make experimental confrontation and inspiration a necessary component of model testing and refinement. Electrochemical PCETs are worthy of particular attention, if only because most applications in which PCET mechanisms are operative involve collection or injection of electricity through electrodes. More fundamentally, changing the electrode potential is an easy and continuous means of varying the driving force of the reaction, whereas the current flowing through the electrode is a straightforward measure of its rate. Consequently, the current-potential response in nondestructive techniques (such as cyclic voltammetry) can be read as an activation-driving force relationship, provided the contribution of diffusion has been taken into account. Intrinsic properties (properties at zero driving force) are consequently a natural outcome of the electrochemical approach. In this Account, we begin by examining the modeling of CPET reactions and then describe illustrating experimental examples inspired by two biological systems, photosystem II and superoxide dismutase. One series of studies examined the oxidation of phenols with, as proton acceptor, either an attached nitrogen base or water (in water as solvent). Another addressed interconversion of aquo-hydroxo-oxo couples of transition metal complexes, using osmium complexes as prototypes. Finally, the reduction of superoxide ion, which is closely related to its dismutation, allowed the observation and rationalization of the remarkable properties of water as a proton donor. Water is also an exceptional proton acceptor in the oxidation of phenols, requiring very small reorganization energies, both in the electrochemical and homogeneous cases. These varied examples reveal general features of PCET reactions that may serve as guidelines for future studies, suggesting that research emphasis might be profitably directed toward new biological systems on the one hand and on the role of hydrogen bonding and hydrogen-bonded environments (such as water or proteins) on the other.

Boron-capped tris(glyoximato) cobalt clathrochelate as a precursor for the electrodeposition of nanoparticles catalyzing H2 evolution in water.

Electrochemical investigation of a boron-capped tris(glyoximato)cobalt clathrochelate complex in the presence of acid reveals that the catalytic activity toward hydrogen evolution results from an electrodeposition of cobalt-containing nanoparticles on the electrode surface at a modest cathodic potential. The deposited particles act as remarkably active catalysts for H(2) production in water at pH 7.

Generation and Reactions of an Unsubstituted N‐Heterocyclic Carbene Boryl Anion

Lying low: A lithiated unsubstituted N-heterocyclic carbene (NHC) boryl anion can be generated by reduction, and trapped by electrophiles (see scheme; dipp=2,6-diisopropylphenyl) to provide new substituted NHC boranes. It is yet another example of a low-valent boron compound or boron-containing reactive intermediate stabilized by an NHC, thereby extending the scope of NHC borane chemistry. Copyright

Selective and Efficient Photocatalytic CO2 Reduction to CO Using Visible Light and an Iron-Based Homogeneous Catalyst

Converting CO2 into valuable compounds using sunlight as the energy input and an earth-abundant metal complex as the catalyst is an exciting challenge related to contemporary energy issues as well as to climate change. By using an inexpensive organic photosensitizer under visible-light excitation (λ > 400 nm) and a substituted iron(0) tetraphenylporphyrin as a homogeneous catalyst, we have been able to generate carbon monoxide from CO2 selectively with high turnover numbers. Sustained catalytic activity over a long time period (t > 50 h) did not lead to catalyst or sensitizer deactivation. A catalytic mechanism is proposed.

The electrochemical approach to concerted proton—electron transfers in the oxidation of phenols in water

Establishing mechanisms and intrinsic reactivity in the oxidation of phenol with water as the proton acceptor is a fundamental task relevant to many reactions occurring in natural systems. Thanks to the easy measure of the reaction kinetics by the current and the setting of the driving force by the electrode potential, the electrochemical approach is particularly suited to this endeavor. Despite challenging difficulties related to self-inhibition blocking the electrode surface, experimental conditions were established that allowed a reliable analysis of the thermodynamics and mechanisms of the proton-coupled electron-transfer oxidation of phenol to be carried out by means of cyclic voltammetry. The thermodynamic characterization was conducted in buffer media whereas the mechanisms were revealed in unbuffered water. Unambiguous evidence of a concerted proton–electron transfer mechanism, with water as proton acceptor, was thus gathered by simulation of the experimental data with appropriately derived theoretical relationships, leading to the determination of a remarkably large intrinsic rate constant. The same strategy also allowed the quantitative analysis of the competition between the concerted proton–electron transfer pathway and an OH−-triggered stepwise pathway (proton transfer followed by electron transfer) at high pHs. Investigation of the passage between unbuffered and buffered media with the example of the PO4H2−/PO4H2− couple revealed the prevalence of a mechanism involving a proton transfer preceding an electron transfer over a PO4H2−-triggered concerted process.

Pendant Acid–Base Groups in Molecular Catalysts: H-Bond Promoters or Proton Relays? Mechanisms of the Conversion of CO2 to CO by Electrogenerated Iron(0)Porphyrins Bearing Prepositioned Phenol Functionalities

Two derivatives of iron tetraphenylporphyrin bearing prepositioned phenolic functionalities on two of the opposed phenyl groups prove to be remarkable catalysts for the reduction of CO2 to CO when generated electrochemically at the Fe(0) oxidation state. In one case, the same substituents are present on the two other phenyls, whereas in the other the two other phenyls are perfluorinated. They are taken as examples of the possible role of pendant acid-base groups in molecular catalysis. The prepositioned phenol groups incorporated into the catalyst molecule induce strong stabilization of the initial Fe(0)CO2 adduct through H-bonding, confirmed by DFT calculations. This positive factor is partly counterbalanced by the necessity, resulting from the same stabilization, to inject an additional electron to trigger catalysis. Thanks to the preprotonation of the initial Fe(0)CO2 adduct, the potential required for this second electron transfer is not very distant from the potential at which the adduct is generated by addition of CO2 to the Fe(0) complex. The protonation step involves an internal phenolic group and the reprotonation of the phenoxide ion thus generated by added phenol. The prepositioned phenol groups thus play both the role of H-bonding stabilizers and high-concentration proton donors. They play the same role in the second electron transfer step which closes the catalytic loop concertedly with the breaking of one of the two C-O bonds of CO2 and with proton transfer. It is also remarkable that reprotonation by added phenol is concerted with the three other events.

Proton-coupled electron transfer cleavage of heavy-atom bonds in electrocatalytic processes. Cleavage of a C-O bond in the catalyzed electrochemical reduction of CO2.

Most of the electrocatalytic processes of interest in the resolution of modern energy challenges are associated with proton transfer. In the cases where heavy atom bond cleavage occurs concomitantly, the question arises of the exact nature of its coupling with proton-electron transfer within the catalytic cycle. The cleavage of a C-O bond in the catalyzed electrochemical conversion of CO2 to CO offers the opportunity to address this question. Electrochemically generated iron(0) porphyrins are efficient, specific, and durable catalysts provided they are coupled with Lewis or Brönsted acids. The cocatalyst properties of four Brönsted acids of increasing strength, water, trifluoroethanol, phenol, and acetic acid, have been systematically investigated. Preparative-scale electrolyses showed that carbon monoxide is the only product of the catalytic reaction. Methodic application of a nondestructive technique, cyclic voltammetry, with catalyst and CO2 concentrations, as well as H/D isotope effect, as diagnostic parameters allowed the dissection of the reaction mechanism. It appears that the key step of the reaction sequence consists of an electron transfer from the catalyst concerted with the cleavage of a C-O bond and the transfer of one proton. This is the second example, and an intermolecular version of such a concerted proton-electron bond-breaking reaction after a similar electrochemical process involving the cleavage of O-O bonds has been identified. It is the first time that a proton-electron transfer concerted with bond breaking has been uncovered as the crucial step in a catalytic multistep reaction.

Efficient and selective molecular catalyst for the CO2-to-CO electrochemical conversion in water

Significance CO2-to-CO electrochemical conversion is a key step in the production of liquid fuels through dihydrogen-reductive Fischer–Tropsch chemistry. Among molecular catalysts, iron porphyrins reduced electrochemically to the Fe(0) state are particularly efficient and led to a deeper understanding of mechanisms involving coupled bond-breaking proton–electron transfer processes. The replacement of nonaqueous solvents by water should make the CO2-to-CO half-cell reaction much more attractive for applications, particularly because it would allow association with a water-oxidation anode through a proton-exchange membrane. Here it is demonstrated that electrochemical CO production catalyzed by a water-soluble iron porphyrin can occur with high catalytic efficiency. Manipulation of pH and buffering then allows conversions from those involving complete CO selectivity to ones with prescribed CO–H2 mixtures. Substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a water-soluble molecule able to catalyze the electrochemical conversion of carbon dioxide into carbon monoxide. The reaction, performed in pH-neutral water, forms quasi-exclusively carbon monoxide with very little production of hydrogen, despite partial equilibration of CO2 with carbonic acid—a low pKa acid. This selective molecular catalyst is endowed with a good stability and a high turnover frequency. On this basis, prescribed composition of CO–H2 mixtures can be obtained by adjusting the pH of the solution, optionally adding an electroinactive buffer. The development of these strategies will be greatly facilitated by the fact that one operates in water. The same applies for the association of the cathode compartment with a proton-producing anode by means of a suitable separator.