Applying Nuclear Forward Scattering as In Situ and Operando Tool for the Characterization of FeN4 Moieties in the Hydrogen Evolution Reaction

Nuclear forward scattering (NFS) is a synchrotron-based technique relying on the recoil-free nuclear resonance effect similar to Mössbauer spectroscopy. In this work, we introduce NFS for in situ and operando measurements during electrocatalytic reactions. The technique enables faster data acquisition and better discrimination of certain iron sites in comparison to Mössbauer spectroscopy. It is directly accessible at various synchrotrons to a broad community of researchers and is applicable to multiple metal isotopes. We demonstrate the power of this technique with the hydrogen evolution mechanism of an immobilized iron porphyrin supported on carbon. Such catalysts are often considered as model systems for iron-nitrogen-carbon (FeNC) catalysts. Using in situ and operando NFS in combination with theoretical predictions of spectroscopic data enables the identification of the intermediate that is formed prior to the rate-determining step. The conclusions on the reaction mechanism can be used for future optimization of immobilized molecular catalysts and metal-nitrogen-carbon (MNC) catalysts.

Metal-Tuned Ligand Reactivity Enables CX2 (X = O, S) Homocoupling with Spectator Cu Centers

Ligand non-innocence is ubiquitous in catalysis with ligands in synthetic complexes contributing as electron reservoirs or co-sites for substrate activation. The latter chemical non-innocence is manifested in H+ storage or relay at sites beyond the metal primary coordination sphere. Reaction of a competent CO2-to-oxalate reduction catalyst, namely, [K(THF)3](Cu3SL), where L3- is a tris(β-diketiminate) cyclophane, with CS2 affords tetrathiooxalate at long reaction times or at high CS2 concentrations, where otherwise an equilibrium is established between the starting species and a complex-CS2 adduct in which the CS2 is bound to the C atom on the ligand backbone. X-ray diffraction analysis of this adduct reveals no apparent metal participation, suggesting an entirely ligand-based reaction controlled by the charge state of the cluster. Thermodynamic parameters for the formation of the aforementioned Cligand-CS2 bond were experimentally determined, and trends with cation Lewis acidity were studied, where more acidic cations shift the equilibrium toward the adduct. Relevance of such an adduct in the reduction of CO2 to oxalate by this complex is supported by DFT studies, similar effects of countercation Lewis acidity on product formation, and the homocoupled heterocumulene product speciation as determined by isotopic labeling studies. Taken together, this system extends chemical non-innocence beyond H+ to effect catalytic transformations involving C-C bond formation and represents the rarest example of metal-ligand cooperativity, that is, spectator metal ion(s) and the ligand as the reaction center.

Substituent Effects in Iron Porphyrin Catalysts for the Hydrogen Evolution Reaction

For a future hydrogen economy, non-precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. Metal-nitrogen-carbon (MNC) catalysts with active sites constituting a metal center with fourfold coordination of nitrogen (MN₄ ) show promising performance, but an optimization rooted in structure-property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well-defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphyrin substituents on the electronic and electrocatalytic properties of MN₄ centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [Fe^II (P⋅)]^- radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications.

Syntheses and Properties of Metalated Tetradehydrocorrins

The monoanionic tetrapyrrolic macrocycle B,C-tetradehydrocorrin (TDC) resides chemically between corroles and corrins. This chemical space remains largely unexplored due to a lack of reliable synthetic strategies. We now report the preparation and characterization of Co(II)- and Ni(II)-metalated TDC derivatives ([Co-TDC]⁺ and [Ni-TDC]⁺, respectively) with a combination of crystallographic, electrochemical, computational, and spectroscopic techniques. [Ni-TDC]⁺ was found to undergo primarily ligand-centered electrochemical reduction, leading to hydrogenation of the macrocycle under cathodic electrolysis in the presence of acid. Transient absorption (TA) spectroscopy reveals that [Ni-TDC]⁺ and the two-electron-reduced [Ni-TDC]^- possess long-lived excited states, whereas the excited state of singly reduced [Ni-TDC] exhibits picosecond dynamics. The Co(I) compound [Co-TDC] is air stable, highlighting the notable property of the TDC ligand to stabilize low-valent metal centers in contradistinction to other tetrapyrroles such as corroles, which typically stabilize metals in higher oxidation states.

Proton-Coupled Electron Transfer: The Engine of Energy Conversion and Storage

Proton-coupled electron transfer (PCET) underpins energy conversion in chemistry and biology. Four energy systems are described whose discoveries are based on PCET: the water splitting chemistry of the Artificial Leaf, the carbon fixation chemistry of the Bionic Leaf-C, the nitrogen fixation chemistry of the Bionic Leaf-N and the Coordination Chemistry Flow Battery (CCFB). Whereas the Artificial Leaf, Bionic Leaf-C, and Bionic Leaf-N require strong coupling between electron and proton to reduce energetic barriers to enable high energy efficiencies, the CCFB requires complete decoupling of the electron and proton so as to avoid parasitic energy-wasting reactions. The proper design of PCET in these systems facilitates their implementation in the areas of (i) centralized large scale grid storage of electricity and (ii) decentralized energy storage/conversion using only sunlight, air and any water source to produce fuel and food within a sustainable cycle for the biogenic elements of C, N and P.

Influence of the proton relay spacer on hydrogen electrocatalysis by cobalt hangman porphyrins

A cobalt hangman porphyrin system with a phenyl spacer between the porphyrin ring and an internal carboxylic acid group as well as its non-hangman analogue were synthesized and utilized for the study of the proton-coupled electron transfer (PCET) kinetics attendant to electrocatalytic hydrogen evolution. Cyclic voltammetry (CV) together with simulations show that a short distance between the proton relay and the redox active cobalt center as well as the increased proton donating strength results in superior catalytic activity. The mechanism of hydrogen generation is at the nexus of proton transfer–electron transfer (PTET) and concerted proton–electron transfer (CPET), as opposed to an ETPT mechanism that is characteristic of hangman systems with longer proton relay networks.

Understanding active sites in molecular (photo)electrocatalysis through complementary vibrational spectroelectrochemistry

Highlighting vibrational spectroelectrochemistry for the investigation of synthetic molecular (photo) electrocatalysts for key energy conversion reactions.

Synthesis of Hangman Chlorins

Two complementary rational synthetic routes have been developed in order to synthesize hangman chlorins, which differ with regard to the order of the installation (pre- and post-formation of the chlorin macrocycle) and position of the xanthene backbone about the chlorin periphery. The versatility of the synthetic method is demonstrated with the preparation of ten new hangman chlorins bearing a xanthene backbone and a pendant carboxylic acid. Cyclic voltammograms of hangman chlorins exhibit a hangman effect derived from intermolecular proton transfer. This hangman effect is manifested in catalytic hydrogen evolution production.

Double Hangman Iron Porphyrin and the Effect of Electrostatic Nonbonding Interactions on Carbon Dioxide Reduction

Hangman porphyrins influence the reaction rates of small molecule activation by positioning a functional group in the secondary coordination sphere of the metal center. Electrocatalysis by hangman porphyrins has examined only one face modification of the macrocycle with a hanging group, thus allowing for circumvention of secondary sphere effects by reaction of the small molecule on the opposite face of the hangman cleft. We now report the synthesis and characterization of a double hangman Fe porphyrin in which both faces of the macrocycle are modified with a hanging group. With this double hangman architecture, we are able to unequivocally examine the role of electrostatic interactions on the carbon dioxide reduction reaction (CO₂RR) and show that CO₂RR rates are significantly attenuated, consistent with the initial reduction of CO₂ to generate the anion, whose binding is diminished within the negatively charged carboxylic groups of the hangman cleft. The results demonstrate the pronounced role that nonbonding electrostatic interactions may play in CO₂RR and highlight the need to manage deleterious electrostatic interactions during catalytic turnover.

Superstructured metallocorroles for electrochemical CO2 reduction

Cobalt and iron complexes of corroles with tyrosine-like proton-transfer-relay moieties in proximity to the metal center have been prepared and fully characterized. The (nitrosyl)iron complex performs very well as an electrocatalyst for the reduction of CO2 to CO.

Proton Relays in Molecular Catalysis of Electrochemical Reactions: Origin and Limitations of the Boosting Effect

Homogeneous catalysis of electrochemical reactions, related to contemporary energy challenges, often involves proton-coupled electron transfer sequences. The idea rapidly emerged that installing the proton donor (for reductions, or acceptor for oxidations) inside the catalyst molecule should be beneficial in terms of efficiency, as it would then be closer to the nerve center of the process (usually the metal in the case of transition metal complex catalysts). If this proton relay has indeed done the job, it has lost its proton at the end of each catalytic loop, and must therefore be reprotonated (for reductions, or deprotonated for oxidations) from acid (or base) from the solution before a new catalytic loop can start. The impression may thus be that there is a zero-sum game. The conditions under which this is not the case may entail, in contrast, a considerable boosting of catalysis. This will also allow explain why the proton is such a specifically appropriate agent for this task.

New class of artificial enzyme composed of Mn-porphyrin, imidazole, and cucurbit[10]uril toward use as a therapeutic antioxidant

In this study, we investigated a new class of artificial enzymes composed of Mn-porphyrin, imidazole, and cucurbit[10]uril (CB[10]) toward therapeutic antioxidants.

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Because of the significance of energy-related small molecule activation, this review covers recent progress in hydrogen evolution, oxygen evolution, and oxygen reduction reactions catalyzed by porphyrins and corroles.

2nd coordination sphere controlled electron transfer of iron hangman complexes on electrodes probed by surface enhanced vibrational spectroscopy

Surface enhanced vibrational spectroscopy shows the correlation between electron transfer kinetics and protonation degree of Fe Hangman complexes on electrodes.

Iron hangman complexes exhibit improved catalytic properties regarding O₂ and H₂O₂ reduction, which are attributed to the presence of a proton donating group in defined vicinity of the catalytic metal centre. Surface enhanced resonance Raman (SERR) and IR (SEIRA) spectro-electrochemistry has been applied concomitantly for the first time to analyse such iron hangman porphyrin complexes attached to electrodes in aqueous solution. While the SERR spectra yield information about the redox state of the central iron, the SEIRA spectra show protonation and deprotonation events of the 2^nd coordination sphere. To investigate the influence of a proton active hanging group on the heterogeneous electron transfer between the iron porphyrin and the electrode, two hangman complexes with either an acid or ester functional group were compared. Using time resolved SERR spectroscopy the electron transfer rates of both complexes were determined. Complexes with an acid group showed a slow electron transfer rate at neutral pH that increased significantly at pH 4, while complexes with an ester group exhibited a much faster, but pH independent rate. SEIRA measurements were able to determine directly for the first time a pK_a value of 3.4 of a carboxylic hanging group in the immobilized state that shifted to 5.2 in D₂O buffer solution. The kinetic data showed an increase of the heterogeneous electron transfer rate with the protonation degree of the acid groups. From these results, we propose a PCET which is strongly modulated by the protonation state of the acid hanging group via hydrogen bond interactions.

A bioinspired polymer-bound Mn-porphyrin as an artificial active center of catalase

The complex comprising a cationic Mn-porphyrin and carboxymethyl poly(1-vinylimidazole) (CM-PVIm) was prepared as an artificial active center of catalase. Interestingly, the catalase activity of the complex depends on the chain length of the polymer and the chemical structure of Mn-porphyrin. This study is one step forward in the development of a new class of water-soluble catalase mimics.

Corrole and nucleophilic aromatic substitution are not incompatible: a novel route to 2,3-difunctionalized copper corrolates

The insertion of a -NO2 group onto the corrole framework represents a key step for subsequent synthetic manipulation of the macrocycle based on the chemical versatility of such a functionality. Here we report results of the investigation of a copper 3-NO2-triarylcorrolate in nucleophilic aromatic substitution reactions with "active" methylene carbanions, namely diethyl malonate and diethyl 2-chloromalonate. Although similar reactions on nitroporphyrins afford chlorin derivatives, nucleophilic attack on carbon-2 of corrole produces 2,3-difunctionalized Cu corrolates in acceptable yields (ca. 30%), evidencing once again the erratic chemistry of this contracted porphyrinoid.

Proton-coupled electron transfer chemistry of hangman macrocycles: Hydrogen and oxygen evolution reactions

The splitting of water into its constituent elements is an important solar fuels conversion reaction for the storage of renewable energy. For each of the half reactions of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), multiple protons and electrons must be coupled to avoid high-energy intermediates. To understand the mechanistic details of the PCET chemistry that underpins HER and OER, we have designed hangman porphyrin and corrole catalysts. In these hangman constructs, a pendant acid/base functionality within the secondary coordination sphere is "hung" above the macrocyclic redox platform on which substrate binds. The two critical thermodynamic properties of a PCET event, the redox potential and pKa may be tuned with the macrocycle and hanging group, respectively. This review outlines the synthesis of these catalysts, as well as the examination of the PCET kinetics of hydrogen and oxygen evolution by the hangman catalysts. The insights provided by these systems provide a guide for the design of future HER and OER catalysts that use a secondary coordination sphere to manage PCET.

Post-synthetic modification of hangman porphyrins synthesized on the gram scale

We report a multi-gram scale synthesis of methyl 6-formyl-4-dibenzofurancarboxylate and its subsequent use in the gram scale synthesis of a dibenzofuran-functionalized hangman porphyrin containing a pendant carboxylic acid (HPD-CO2H). HPD-CO2H can be isolated as a free carboxylic acid in high purity with minimal purification. Post-synthetic modification of HPD-CO2H allows for the introduction of any desired pendant group in good yields, resulting in a practical amount of hangman porphyrin ligand with an easily customizable second coordination sphere. The cobalt complexes of these hangman porphyrins are shown to be active proton reduction electrocatalysts.