Toward Visible-Light Photochemical CO₂‑to-CH₄ Conversion in Aqueous Solutions Using Sensitized Molecular Catalysis

Solar fuels may be generated upon visible light induced catalytic reduction of carbon dioxide. This appealing approach remains highly challenging, especially when earth abundant catalysts, mild conditions, and water as a solvent were used. Employing an iron tetraphenyl porphyrin complex substituted by positively charged trimethylammonio groups at the para position of each phenyl ring and reduction with three electrons by the excited state of an iridium sensitizer (λ > 420 nm) reduce CO₂ to CO and to CH₄ in both acetonitrile and aqueous solutions (acetonitrile/water 3:7 v:v) with good selectivity. Stability of the catalytic system remains a weakness and the reasons were analyzed

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 (<i>t</i> > 50 h) did not lead to catalyst or sensitizer deactivation. A catalytic mechanism is proposed

Cobalt-Bisglyoximato Diphenyl Complex as a Precatalyst for Electrocatalytic H₂ Evolution

Electrochemical investigation of the title Co^II compound in acetonitrile with a strong acid (HClO₄) showed no sign of proton catalysis at the Co^II/Co^I wave, but instead revealed the formation of Co based nanoparticles at the surface of the carbon electrode. Catalytic proton reduction of the resulting nanometer sized cobalt particles at pH 7 was found to occur efficiently. Partial coverage of the carbon substrate by the particles leads to an apparent exchange current density as high as those obtained at a pure cobalt electrode or cobalt films

Dissection of Electronic Substituent Effects in Multielectron–Multistep Molecular Catalysis. Electrochemical CO₂‑to-CO Conversion Catalyzed by Iron Porphyrins

Redox pairs of transition metal complexes are often involved in small molecule activation in response to modern energy challenges as well as in other areas of electrocatalysis. Within such a family of molecular electrocatalysts, ligand substitution is a means of varying catalytic efficiency, best gauged through catalytic Tafel plots relating overpotential and turnover frequency. In practice, efficient molecular catalysis involves multielectron–multistep processes. It is in this framework that we discuss through-structure inductive substituent effects. What the best choice is for the reference thermodynamic index, how the global substituent effect may be expressed as a function of this index, and how it may be dissected into individual effects assigned to each of the reaction steps are challenging questions that are addressed and resolved here for the first time. The discussion is illustrated by the effect of successive phenyl perfluoration and of <i>o</i>,<i>o</i>′-methoxy substitution of the Fe^I/0 tetraphenylporphyrin catalysts of the CO₂-to-CO electrochemical conversion. Consequences on the relative position of the catalytic Tafel plots are also examined. This analysis of through-structure electronic effects is a necessary preliminary to the investigation of substituent through-space effects (electrostatic, H-bonding) because, albeit of different nature, they may occur simultaneously. Investigation of these two aspects of substituent effects and of the rules that emerge thereof pave the way to future imaginative design of catalysts for the CO₂-to-CO-conversion and also for any other molecular catalytic reactions

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 (TOF₀). Foot-of-the-wave analysis of the cyclic voltammetric catalytic responses allows the determination of the TOF, log TOF−η relationship, and TOF₀, 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 CO₂ by iron(0) porphyrins

Breaking Bonds with Electrons and Protons. Models and Examples

Besides its theoretical interest, the attention currently aroused by proton-coupled electron transfers (PCET reactions) has two main motives. One is a better understanding of biological processes in which PCET reactions are involved, Photosystem II as well as a myriad of other natural systems. The other is directed toward synthetic processes, many of which are related to global energy challenges. Until recently, the analyses of the mechanism and reactivity of PCET reactions have focused on outersphere transfers, those in which no bond between heavy atoms (all atoms with the exception of H) is concomitantly formed or broken. Conversely, reactions in which electron transfer triggers the breaking of a heavy-atom bond with no proton transfer have been extensively analyzed, both theoretically and experimentally. In both cases, strategies have been developed to distinguish between stepwise and concerted pathways. In each case, kinetic models have been devised, allowing the relation between activation and thermodynamic driving force to be established by means of parameters pertaining to the initial and final state. Although many natural and artificial processes include electron transfer, proton transfer, and heavy-atom bond breaking (/formation), no means were offered until recently to analyze the mechanism of such reactions, notably to establish the degree of concertedness of the three constitutive events. Likewise, no kinetic models were available to describe reactions where the three events are concerted. In this Account, we discuss the strategies to distinguish stepwise, partially concerted (when two of the three events are concerted), and totally concerted pathways in these reactions that include electron transfer, proton transfer, and heavy-atom bond breaking. These mechanism analysis methods are illustrated and validated by three examples. First we describe the electrochemical cleavage of an O–O bond in an aliphatic peroxide molecule with a pendant carboxylic acid group that can serve as proton donor for electron transfer and bond breaking. In the second example, we examine the breaking of one of the C–O bonds of CO₂ within a multistep process where the reduction of CO₂ into CO is catalyzed by an electrogenerated iron(0) porphyrin in the presence of various Brönsted acids. In this case, an intramolecular electron transfer triggers proton transfer and bond cleavage. In the first two examples, all three events are concerted. The third example also involves catalysis. It describes the cleavage of a cobalt–carbon bond in the reduction of chloroacetonitrile catalyzed by an electrogenerated cobalt(I) porphyrin. It illustrates the rather common case where the intermediate formed by the reaction of a transition metal complex with the substrate has to be cleaved to close the catalytic cycle. In the first two examples, all three events are concerted, whereas, in the last case, a partially concerted pathway takes place: proton transfer and bond-breaking (Co–C cleavage) are concerted after an initial electron transfer step. The all-concerted cases require a model that connects the kinetics to the thermodynamic driving force of the reaction. Starting from previous models of outersphere electron transfer, concerted proton-electron transfer, and concerted dissociative electron transfer, we describe a model for all-concerted proton–electron-bond breaking reactions. These pathways skip the high-energy intermediates that occur in stepwise pathways, but could introduce kinetic penalties. The all-concerted model allows one to assess these penalties and the way in which they can be fought by the supplement of driving force offered by concerted proton transfer

Through-Space Charge Interaction Substituent Effects in Molecular Catalysis Leading to the Design of the Most Efficient Catalyst of CO₂‑to-CO Electrochemical Conversion

The starting point of this study of through-space substituent effects on the catalysis of the electrochemical CO₂-to-CO conversion by iron(0) tetraphenyl­porphyrins is the linear free energy correlation between through-structure electronic effects and the iron­(I/0) standard potential that we established separately. The introduction of four positively charged trimethyl­anilinium groups at the para positions of the tetraphenyl­porphyrin (TPP) phenyls results in an important positive deviation from the correlation and a parallel improvement of the catalytic Tafel plot. The assignment of this catalysis boosting effect to the Coulombic interaction of these positive charges with the negative charge borne by the initial Fe⁰–CO₂ adduct is confirmed by the negative deviation observed when the four positive charges are replaced by four negative charges borne by sulfonate groups also installed in the para positions of the TPP phenyls. The climax of this strategy of catalysis boosting by means of Coulombic stabilization of the initial Fe⁰–CO₂ adduct is reached when four positively charged trimethyl­anilinium groups are introduced at the ortho positions of the TPP phenyls. The addition of a large concentration of a weak acidphenolhelps by cleaving one of the C–O bonds of CO₂. The efficiency of the resulting catalyst is unprecedented, as can be judged by the catalytic Tafel plot bench­marking with all presently available catalysts of the electrochemical CO₂-to-CO conversion. The maximal turnover frequency (TOF) is as high as 10⁶ s^–1 and is reached at an overpotential of only 220 mV; the extrapolated TOF at zero overpotential is larger than 300 s^–1. This catalyst leads to a highly selective formation of CO (practically 100%) in spite of the presence of a high concentration of phenol, which could have favored H₂ evolution. It is also very stable, showing no significant alteration after more than 80 h of electrolysis

Proton-Coupled Electron Transfers: pH-Dependent Driving Forces? Fundamentals and Artifacts

Besides its own interest, tryptophan oxidation by photogenerated Ru complexes is one of the several examples where concerted proton–electron transfer (CPET) to water as proton acceptor endowed with a pH-dependent driving force has been invoked to explain the data. Since this notion is contrary to the very basic principles of chemical physics, it was interesting to attempt uncovering the source of this contradiction with an easily accessible substrate. Careful examination of the oxidation of the tryptophan (ethyl ester derivative) bearing a NH₃⁺/NH₂ group showed that there is no trace of such an unconventional H₂O-CPET with a pH-dependent driving force. The reaction mechanism simply consists, with both the NH₃⁺ acid and NH₂ basic forms of the tryptophan derivative, in a rate-determining electron-transfer step followed by deprotonation steps. The same is true with the ethyl ester-methyl amide derivative of tryptophan, whose behavior is even simpler since the molecule does not bear an acid–base group. No such unconventional H₂O-CPET was found with phenol, another easily accessible substrate. It may thus be inferred that the same applies to less easily available systems in which electron transfer occurs intramolecularly. These observations help to rid the road of such artificial obstacles and improve present models of H₂O-CPET reactions, a landmark towards the understanding of the role of water chains in natural systems

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