Electrophotocatalysis versus Indirect Electrolysis: Electrochemical Selenocyclization of 3-Aza-1,5-dienes Facilitated by Energy Transfer, Direct Photolysis or N-Hydroxyphthalimide

Three hybrid electrochemical protocols, which involve the energy transfer, direct photolysis and N-hydroxyphthalimide catalyst, respectively, are presented for the selenylation/cyclization of the fragile substrates of 3-aza-1,5-dienes with diorganyl diselenides to afford 3-selenomethyl-4-pyrrolin-2-ones. The two electrophotocatalytic reactions and the indirect electrolysis one are both regioselective and external-oxidant- and transition-metal-free, and are associated with a broad substrate scope and high Se-economy, and all three methods are amenable to gram-scale syntheses, late-stage functionalizations, sunlight-induced experiments and all-solar-driven syntheses.

Tuning charge transport by manipulating concentration dependent single-molecule absorption configurations

Understanding and tuning charge transport in molecular junctions is pivotal for crafting molecular devices with tailored functionalities. Here, we report a novel approach to manipulate the absorption configuration within a 4,4'-bipyridine (4,4'-BPY) molecular junction, utilizing the scanning tunneling microscope break junction technique in a concentration-dependent manner. Single-molecule conductance measurements demonstrate that the molecular junctions exhibit a significant concentration dependence, with a transition from high conductance (HC) to low conductance (LC) states as the concentration decreases. Moreover, we identified an additional conductance state in the molecular junctions besides already known HC and LC states. Flicker noise analysis and theoretical calculations provided valuable insights into the underlying charge transport mechanisms and single-molecule absorption configurations concerning varying concentrations. These findings contribute to a fundamental comprehension of charge transport in concentration-dependent molecular junctions. Furthermore, they offer promising prospects for controlling single-molecule adsorption configurations, thereby paving the way for future molecular devices.

Molecular architectures of iron complexes for oxygen reduction catalysis-Activity enhancement by hydroxide ions coupling

Developing cost-effective and high-performance electrocatalysts for oxygen reduction reaction (ORR) is critical for clean energy generation. Here, we propose an approach to the synthesis of iron phthalocyanine nanotubes (FePc NTs) as a highly active and selective electrocatalyst for ORR. The performance is significantly superior to FePc in randomly aggregated and molecularly dispersed states, as well as the commercial Pt/C catalyst. When FePc NTs are anchored on graphene, the resulting architecture shifts the ORR potentials above the redox potentials of Fe2+/3+ sites. This does not obey the redox-mediated mechanism operative on conventional FePc with a Fe2+-N moiety serving as the active sites. Pourbaix analysis shows that the redox of Fe2+/3+ sites couples with HO- ions transfer, forming a HO-Fe3+-N moiety serving as the ORR active sites under the turnover condition. The chemisorption of ORR intermediates is appropriately weakened on the HO-Fe3+-N moiety compared to the Fe2+-N state and thus is intrinsically more ORR active.

Phosphine Oxide-Functionalized Terthiophene Redox Systems

Main group systems capable of undergoing controlled redox events at extreme potentials are elusive yet highly desirable for a range of organic electronics applications including use as energy storage media. Herein we describe phosphine oxide-functionalized terthiophenes that exhibit two reversible 1e- reduction at potentials below -2 V vs Fc/Fc+ (Fc = ferrocene) while retaining high degrees of stability. A phosphine oxide-functionalized terthiophene radical anion was synthesized in which the redox-responsive nature of the platform was established using combined structural, spectroscopic, and computational characterization. Straightforward structural modification led to the identification of a derivative that exhibits exceptional stability during bulk 2e- galvanostatic charge-discharge cycling and enabled characterization of a 2e- redox series. A new multi-electron redox system class is hence disclosed that expands the electrochemical cell potential range achievable with main group electrolytes without compromising stability.

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.

Electrochemical Properties of a Rhodium(III) Mono-Terpyridyl Complex and Use as a Catalyst for Light-Driven Hydrogen Evolution in Water

Molecular hydrogen (H₂) is considered one of the most promising fuels to decarbonize the industrial and transportation sectors, and its photocatalytic production from molecular catalysts is a research field that is still abounding. The search for new molecular catalysts for H₂ production with simple and easily synthesized ligands is still ongoing, and the terpyridine ligand with its particular electronic and coordination properties, is a good candidate to design new catalysts meeting these requirements. Herein, we have isolated the new mono-terpyridyl rhodium complex, [Rh^III(tpy)(CH₃CN)Cl₂](CF₃SO₃) (Rh-tpy), and shown that it can act as a catalyst for the light-induced proton reduction into H₂ in water in the presence of the [Ru(bpy)₃]Cl₂ (Ru) photosensitizer and ascorbate as sacrificial electron donor. Under photocatalytic conditions, in acetate buffer at pH 4.5 with 0.1 M of ascorbate and 530 μM of Ru, the Rh-tpy catalyst produces H₂ with turnover number versus catalyst (TON_Cat*) of 300 at a Rh concentration of 10 μM, and up to 1000 at a concentration of 1 μM. The photocatalytic performance of Ru/Rh-tpy/HA^-/H₂A has been also compared with that obtained with the bis-dimethyl-bipyridyl complex [Rh^III(dmbpy)₂Cl₂]⁺ (Rh2) as a catalyst in the same experimental conditions. The investigation of the electrochemical properties of Rh-tpy in DMF solvent reveals that the two-electrons reduced state of the complex, the square-planar [Rh^I(tpy)Cl] (Rh^I-tpy), is quantitatively electrogenerated by bulk electrolysis. This complex is stable for hours under an inert atmosphere owing to the π-acceptor property of the terpyridine ligand that stabilizes the low oxidation states of the rhodium, making this catalyst less prone to degrade during photocatalysis. The π-acceptor property of terpyridine also confers to the Rh-tpy catalyst a moderately negative reduction potential (Ep_c(Rh^III/Rh^I) = -0.83 V vs. SCE in DMF), making possible its reduction by the reduced state of Ru, [Ru^II(bpy)(bpy^•-)]⁺ (Ru^-) (E_1/2(Ru^II/Ru^-) = -1.50 V vs. SCE) generated by a reductive quenching of the Ru excited state (*Ru) by ascorbate during photocatalysis. A Stern-Volmer plot and transient absorption spectroscopy confirmed that the first step of the photocatalytic process is the reductive quenching of *Ru by ascorbate. The resulting reduced Ru species (Ru^-) were then able to activate the Rh^III-tpy H₂-evolving catalyst by reduction generating Rh^I-tpy, which can react with a proton on a sub-nanosecond time scale to form a Rh^III(H)-tpy hydride, the key intermediate for H₂ evolution.

Exploring the Role of H-Bonding in Organic Electrochemistry - From Supramolecular Applications to Mechanistic Investigations

H-bonds can exert a substantial impact on the course of organic electrode reactions due to their ability to stabilize charged intermediates and products formed during these reactions, as well as facilitate proton-coupled electron transfer (PCET) reactions. This has fundamental implications for the mechanism of organic electrode reactions, but also practical impact in supramolecular chemistry and potentially synthetic electrochemistry. My group's main focus has been on the supramolecular applications, using electron transfer to alter the strength of H-bonds to create highly redox-responsive H-bond dimers. Initially we sought to avoid proton transfer because we feared that would lead to irreversible electrochemistry. However, inevitably proton transfer did show up, but, to our surprise, did not lead to irreversible electrochemistry. To explain this, we developed a new mechanism, the "wedge scheme", that shows how H-bonding can facilitate reversible electron and proton transfer. This insight recently led us to a new PCET-based design strategy for the creation of our most highly redox-responsive H-bond dimers yet.

Chemoselective Electrochemical Hydrogenation of Ketones and Aldehydes with a Well-Defined Base-Metal Catalyst

Hydrogenation reactions are fundamental functional group transformations in chemical synthesis. Here, we introduce an electrochemical method for the hydrogenation of ketones and aldehydes by in situ formation of a Mn-H species. We utilise protons and electric current as surrogate for H2 and a base-metal complex to form selectively the alcohols. The method is chemoselective for the hydrogenation of C=O bonds over C=C bonds. Mechanistic studies revealed initial 3 e- reduction of the catalyst forming the steady state species [Mn2 (H-1 L)(CO)6 ]- . Subsequently, we assume protonation, reduction and internal proton shift forming the hydride species. Finally, the transfer of the hydride and a proton to the ketone yields the alcohol and the steady state species is regenerated via reduction. The interplay of two manganese centres and the internal proton relay represent the key features for ketone and aldehyde reduction as the respective mononuclear complex and the complex without the proton relay are barely active.

Probing the Activity of Iron Peroxo Porphyrin Intermediates in the Reaction Layer during the Electrochemical Reductive Activation of O2

Herein we report the first example of using scanning electrochemical microscopy (SECM) to quantitatively analyze O₂ reductive activation in organic media catalyzed by three different Fe porphyrins. For each porphyrin, SECM can provide in one single experiment the redox potential of various intermediates, the association constant of Fe^II with O₂ , and the pK_a of the Fe^III (OOH^- )/ Fe^III (OO^2- ) couple. The results obtained can contribute to a further understanding of the parameters controlling the catalytic efficiency of the Fe porphyrin towards O₂ activation and reduction.

Electron Transfer and Electron Excitation Processes in 2,5-Diaminoterephthalate Derivatives with Broad Scope for Functionalization

Derivatives of 2,5-diaminoterephthalate (DAT) are efficient fluorescence dyes that are also redox-active, thus allowing for the electrochemical manipulation of spectral properties. The electrochemical behaviour of seven DAT derivatives was studied by cyclic voltammetry in dichloromethane. In the absence of a proton donor, DATs should be oxidized in two one-electron steps. The first step is usually quasi-reversible while the second step is either quasi-reversible or irreversible. Some electrochemical properties such as the formal potentials and the ratio between the anodic and the cathodic current were determined from the cyclic voltammograms. Correlation between the formal potential of first oxidation and the absorption or the fluorescence emission wavelengths are established for this specific type of dyes. These correlations were confirmed with density functional theory calculations.

Metal-Bonded Redox-Active Triarylamines and Their Interactions: Synthesis, Structure, and Redox Properties of Paddle-Wheel Copper Complexes

Four new triphenylamine ligands with different substituents in the para position and their corresponding copper(II) complexes are reported. This study includes their structural, spectroscopic, magnetic, and electrochemical properties. The complexes possess a dinuclear copper(II) paddle‐wheel core, a building unit that is also common in metal‐organic frameworks. Electrochemical measurements demonstrate that the triphenylamine ligands and the corresponding complexes are susceptible to oxidation, resulting in the formation of stable radical cations. The square‐wave voltammograms observed for the complexes are similar to those of the ligands, except for a slight shift in potential. Square‐wave voltammetry data show that, in the complexes, these oxidations can be described as individual one‐electron processes centered on the coordinated ligands. Spectroelectrochemistry reveals that, during the oxidation of the complexes, no difference can be detected for the spectra of successively oxidized species. For the absorption bands of the oxidized species of the ligands and complexes, only a slight shift is observed. ESR spectra for the chemically oxidized complexes indicate ligand‐centered radicals. The copper ions of the paddle‐wheel core are strongly antiferromagnetic coupled. DFT calculations for the fully oxidized complexes indicate a very weak ferromagnetic coupling between the copper ions and the ligand radicals, whereas a very weak antiferromagnetic coupling is found among the ligand radicals.

La5 Ti2 Cu0.9 Ag0.1 S5 O7 Modified with a Molecular Ni Catalyst for Photoelectrochemical H2 Generation

The stable and efficient integration of molecular catalysts into p-type semiconductor materials is a contemporary challenge in photoelectrochemical fuel synthesis. Here, we report the combination of a phosphonated molecular Ni catalyst with a TiO2 -coated La5 Ti2 Cu0.9 Ag0.1 S5 O7 photocathode for visible light driven H2 production. This hybrid assembly provides a positive onset potential, large photocurrents, and high Faradaic yield for more than three hours. A decisive feature of the hybrid electrode is the TiO2 interlayer, which stabilizes the oxysulfide semiconductor and allows for robust attachment of the phosphonated molecular catalyst. This demonstration of an oxysulfide-molecular catalyst photocathode provides a novel platform for integrating molecular catalysts into photocathodes and the large photovoltage of the presented system makes it ideal for pairing with photoanodes.

Facile Separation, Spectroscopic Identification, and Electrochemical Properties of Higher Trifluoromethylated Derivatives of [70]Fullerene

We survey the structure and electronic properties of the family of higher trifluoromethylated C70 (CF3 )n molecules with n=14, 16, 18, and 20. Twenty-two available compounds, of which thirteen are newly obtained and characterized, demonstrate the broad diversity of π-system topologies, which enabled us to study the interplay between the CF3 addition pattern and the electronic properties. UV/Vis spectroscopic and cyclic voltammetric studies demonstrate the importance of the exact addition pattern rather than the plain number of addends. Of particular interest is the skew pentagonal pyramid (SPP) addition pattern, which enables formation of closed-shell cyclopentadienyl anions C70 (CF3 )n-1- through CF3 detachment upon electron transfer. A detailed study of the process is presented for a SPP-C70 (CF3 )16 where potentiostatic electrolysis at the second reduction potential gives C70 (CF3 )15- oxidizable to a persistent C70 (CF3 )15. radical. Together with the literature data for the lower C70 (CF3 )n compounds with n=2-12, the present results show good correlation between the experimental boundary level positions and the DFT predictions. The compounds turn out to be electron acceptor molecular semiconductors with experimental LUMO energies and HOMO-LUMO gaps within the ranges of -4.3 to -3.7 eV and 1.6 to 3.3 eV, respectively, depending on the shape of the conjugated fragments. The HOMO levels fall within the range of -5.6 to -6.9 eV and show linear correlation with the number of addends.

Ligand Substituents Govern the Efficiency and Mechanistic Path of Hydrogen Production with [Cp*Rh] Catalysts

We demonstrate that [Cp*Rh] complexes bearing substituted 2,2'-bipyridyl ligands are effective hydrogen evolution catalysts (Cp*=η⁵ -pentamethylcyclopentadienyl). Disubstitution (at the 4 and 4' positions) of the bipyridyl ligand (namely -tBu, -H, and -CF₃ ) modulates the catalytic overpotential, in part due to involvement of the reduced ligand character in formally rhodium(I) intermediates. These reduced species are synthesized and isolated here; protonation results in formation of complexes bearing the unusual η⁴ -pentamethylcyclopentadiene ligand, and the properties of these protonated intermediates further govern the catalytic performance. Electrochemical studies suggest that multiple mechanistic pathways are accessible, and that the operative pathway depends on the applied potential and solution conditions. Taken together, these results suggest synergy in metal-ligand cooperation that modulates the mechanisms of fuel-forming catalysis with organometallic compounds bearing multiple non-innocent ligands.

Steep pH Gradients and Directed Colloid Transport in a Microfluidic Alkaline Hydrothermal Pore

All life on earth depends on the generation and exploitation of ionic and pH gradients across membranes. One theory for the origin of life proposes that geological pH gradients were the prebiotic ancestors of these cellular disequilibria. With an alkaline interior and acidic exterior, alkaline vents match the topology of modern cells, but it remains unknown whether the steep pH gradients persist at the microscopic scale. Herein, we demonstrate the existence of 6 pH-unit gradients across micrometer scales in a microfluidic vent replicate. Precipitation of metal sulfides at the interface strengthens the gradients, but even in the absence of precipitates laminar flow sustains the disequilibria. The gradients drive directed transport at the fluid interface, leading to colloid accumulation or depletion. Our results confirm that alkaline vents can provide an exploitable pH gradient, supporting their potential role at the emergence of chemiosmosis and the origin of life.

Ligand modification transforms a catalase mimic into a water oxidation catalyst

The catalytic reactivity of the high-spin Mn(II) pyridinophane complexes [(Py2NR2)Mn(H2O)2](2+) (R=H, Me, tBu) toward O2 formation is reported. With small macrocycle N-substituents (R=H, Me), the complexes catalytically disproportionate H2O2 in aqueous solution; with a bulky substituent (R=tBu), this catalytic reaction is shut down, but the complex becomes active for aqueous electrocatalytic H2O oxidation. Control experiments are in support of a homogeneous molecular catalyst and preliminary mechanistic studies suggest that the catalyst is mononuclear. This ligand-controlled switch in catalytic reactivity has implications for the design of new manganese-based water oxidation catalysts.

Covalent grafting of silatranes to carbon interfaces

Covalent Si-C grafting of a silatrane cage to a carbon-based interface provides a truly conjugated benzyl-type system in which the 3 c-4 e orbital of the silatrane interacts with the macroscopic π-type substituent (graphite Csp2 network) through hyperconjugation. This process, studied by voltammetry, EIS, FTIR, SEM and DFT modeling, allows one to build carbon-based conducting interfaces with electronically conjugated molecular extensions. Non-conjugated covalent grafting of an alkyl silatrane moiety provides chemically stable functional interfaces that have good promise for electrochemically-driven applications, for example, electrochemical spin-writing.