Nonaqueous Electrocatalytic Oxidation of the Alkylaromatic Ethylbenzene by a Surface Bound RuV(O) Catalyst

Synthetic dioxygenase reactivity by pairing electrochemical oxygen reduction and water oxidation

The reactivity of molecular oxygen is crucial to clean energy technologies and green chemical synthesis, but kinetic barriers complicate both applications. In synthesis, dioxygen should be able to undergo oxygen atom transfer to two organic molecules with perfect atom economy, but such reactivity is rare. Monooxygenase enzymes commonly reductively activate dioxygen by sacrificing one of the oxygen atoms to generate a more reactive oxidant. Here, we used a manganese-tetraphenylporphyrin catalyst to pair electrochemical oxygen reduction and water oxidation, generating a reactive manganese-oxo at both electrodes. This process supports dioxygen atom transfer to two thioether substrate molecules, generating two equivalents of sulfoxide with a single equivalent of dioxygen. This net dioxygenase reactivity consumes no electrons but uses electrochemical energy to overcome kinetic barriers.

Electrocatalytic ethylbenzene valorization using a polyoxometalate@covalent triazine framework with water as the oxygen source

Ethylbenzene (EB) oxidation is an important transformation in the chemical industry. Herein, PMo10V2@CTF, a noble metal free electrocatalyst, was used to promote the oxidative upgrading of EB. Under ambient conditions, 65% of EB was converted to three value-added products using water as the oxygen source yielding a total Faraday efficiency of 90.4%. This excellent performance is ascribed to the homogeneous dispersion of PMo10V2 and its dual role in the electrocatalytic process.

Electrochemical Oxidative Coupling Between Benzylic C(sp3)-H and N-H of Secondary Amines: Rapid Synthesis of α-Amino α-Aryl Esters

An intermolecular electrochemical coupling between the benzylic C(sp³)-H bond and various secondary amines is reported. The electronic behavior of two electronically rich units viz the α-position of α-aryl acetates and amines was engineered electrochemically, thus facilitating their reactivity for the direct access of α-amino esters. A series of acyclic/cyclic secondary amines and α-aryl acetates were tested to furnish the corresponding α-amino esters with high yields (up to 92%) under mild conditions.

Oxoiron(v) mediated selective electrochemical oxygenation of unactivated C-H and C[double bond, length as m-dash]C bonds using water as the oxygen source

An efficient electrochemical method for the selective oxidation of C–H bonds of unactivated alkanes (BDE ≤97 kcal mol⁻¹) and CC bonds of alkenes using a biomimetic iron complex, [(bTAML)Fe^III-OH₂]⁻, as the redox mediator in an undivided electrochemical cell with inexpensive carbon and nickel electrodes is reported. The O-atom of water remains the source of O-incorporation in the product formed after oxidation. The products formed upon oxidation of C–H bonds display very high regioselectivity (75 : 1, 3° : 2° for adamantane) and stereo-retention (RC ∼99% for cyclohexane derivatives). The substrate scope includes natural products such as cedryl acetate and ambroxide. For alkenes, epoxides were obtained as the sole product. Mechanistic studies show the involvement of a high-valent oxoiron(v) species, [(bTAML)Fe^V(O)]⁻ formed via PCET (overall 2H⁺/2e⁻) from [(bTAML)Fe^III-OH₂]⁻ in CPE at 0.80 V (vs. Ag/AgNO₃). Moreover, electrokinetic studies for the oxidation of C–H bonds indicate a second-order reaction with the C–H abstraction by oxoiron(v) being the rate-determining step.

A biomimetic iron complex-mediated selective and efficient electrochemical oxygenation of unactivated C–H bonds and CC bonds using water as an O-atom source.

Selective single-atom electrocatalysts: a review with a focus on metal-doped covalent triazine frameworks

Single-atom electrocatalysts (SACs), which comprise singly isolated metal sites supported on heterogeneous substrates, have attracted considerable recent attention as next-generation electrocatalysts for various key reactions from the viewpoint of the environment and energy. Not only electrocatalytic activity but also selectivity can be precisely tuned via the construction of SACs with a defined coordination structure, such as homogeneous organometallics. Covalent organic frameworks (COFs) are promising supports for single-atom sites with designed coordination environments due to their unique physicochemical properties, which include porous structures, robustness, a wide range of possible designs, and abundant heteroatoms to coordinate single-metal sites. The rigid frameworks of COFs can hold unstable single-metal atoms, such as coordinatively unsaturated sites or easily aggregated Pt-group metals, which exhibit unique electrocatalytic selectivity. This minireview summarizes recent advances in the selective reactions catalysed by SACs, mainly those supported on triazine-based COFs.

Single-atom electrocatalysts (SACs) have attracted considerable attention as selective electrocatalysts. Metal-doped covalent triazine frameworks will be a novel platform for selective SACs to solve energy and environmental issues.

Aqueous Electrochemical Partial Oxidation of Gaseous Ethylbenzene by a Ru-Modified Covalent Triazine Framework

Aqueous electrochemical oxidation of hydrocarbons into valuable compounds, such as alcohols and carbonyl compounds, has attracted much attention because these systems can operate under mild conditions without toxic oxidants or flammable solvents. The key requirements to achieve such oxidation reactions are (1) highly reactive species on an electrocatalyst for the activation of C-H bonds and (2) efficient transportation pathway for water-insoluble hydrocarbons to an electrode surface. We have determined that a gas diffusion electrode (GDE) supporting Ru atom-modified covalent triazine frameworks (Ru-CTF) has an activity for the electrooxidation of gaseous ethylbenzene to acetophenone using an aqueous electrolyte. A high-valency Ru═O species was formed in Ru-CTF as an effective active site for O-atom insertion into stable C-H bonds. Furthermore, Ru-CTF showed excellent stability during four consecutive cycles with the replacement of the electrolyte every 12 h, although the reactive Ru═O species is generated. As for the transportation pathway for substrates, the amount of acetophenone generated from gaseous ethylbenzene was much larger than that from ethylbenzene dissolved in an electrolyte. This result indicates that the three-dimensional microstructures in the GDE maximize the transportation of gaseous hydrocarbons and the oxidation reaction occurs at the triple-phase boundary, which enables the use of aqueous electrolytes.

Electrochemical Ruthenium-Catalyzed C-H Hydroxylation of Amine Derivatives in Aqueous Acid

The development of an electrochemically driven, ruthenium-catalyzed C-H hydroxylation reaction of amine-derived substrates bearing tertiary C-H bonds is described. The reaction is performed under constant current electrolysis in a divided cell to afford alcohol products in yields comparable to those of our previously reported process, which requires the use of stoichiometric H₅IO₆ for catalytic turnover. With aqueous acid as solvent, the cathodic electrode reaction simply involves the reduction of protons to evolve hydrogen gas. The optimized protocol offers a convenient, efficient, and atom-economical method for sp³-C-H bond oxidation.

Electron-Withdrawing Boron Dipyrromethene Dyes As Visible Light Absorber/Sensitizers on Semiconductor Oxide Surfaces

The synthesis, characterization, and electrochemical and photophysical properties of the phosphonate-derivatized carbazole (CBZ) and boron dipyrromethene (BODIPY) chromophores in the dyes, BODIPY(CBZ)₂PO₃H₂ (8) and BODIPY(Tol)₂PO₃H₂ (7), are described. The oxide-bound dyes have been explored as light absorbers in dye-sensitized photoelectrosynthesis cell (DSPEC) applications. The BODIPY-CBZ phosphonate ester (6) features a broad, intense UV-visible absorption spectrum with absorptions at 297 and 650 nm that arise from mixed transitions at the CBZ and BODIPY units. Electrochemical measurements on BODIPY(CBZ)₂Br (4) in 0.1 M [nBu₄N][PF₆] in dichloromethane, vs normal hydrogen electrode (NHE), reveal reversible oxidations at 1.19 and 1.41 V and a reversible reduction at -0.59 V. On indium tin oxide (ITO) and TiO₂, a reversible one-electron oxidation appears for 7 at 0.86 and 0.90 V vs NHE in dichloromethane, respectively, which demonstrates the redox stability on metal oxide surfaces. The results of nanosecond transient absorption measurements on SnO₂/TiO₂ electrodes provide direct evidence for excited-state electron injection into the conduction band of TiO₂ following 590 nm excitation. A longer lifetime for 8⁺ compared to 7⁺ is consistent with extensive intramolecular charge separation between the CBZ and BODIPY units on the surface. Photoelectrochemical studies on 8 on a SnO₂/TiO₂ photoanode resulted in sustained photocurrents with current maxima of ∼200 μA/cm² with hydroquinone added as a reductant under 1 sun (AM1.5 100 mW·cm^-2) illumination at pH 4.5 in 0.1 M acetate buffer and 0.4 M LiClO₄. On mixed SnO₂/TiO₂ electrode surfaces, with the added catalyst [Ru(Mebimpy)((4,4'-(OH)₂PO-CH₂)₂bpy)(OH₂)]²⁺ and chromophores 7 and 8, addition of 0.1 M benzyl alcohol resulted in sustained photocurrents of 12 and 35 μA/cm², consistent with oxidation to benzaldehyde.

Selective Oxidation of Benzylic C-H Bonds Catalyzed by Cu(II)/{PMo12}

Precise catalytic regulation of carbon radical generation by a highly active oxygen radical to abstract the H atom in a C-H bond is an effective method for the selective activation of C-H synthetic chemistry. Herein, we report a facile catalyst system with commercially available copper(II)/{PMo₁₂} to form a tert-butanol radical intermediate for the selective oxidation of benzylic C-H bonds. The reaction shows a broad range of substrates (benzyl methylene, benzyl alcohols) with good functional group tolerance and chemical selectivity. The corresponding carbonyl compounds were synthesized with good yields under mild conditions. DFT calculations and experimental analysis further demonstrated a reasonable carbon radical mechanism for this type of organic transformation reaction.

Electrochemical C-H oxygenation and alcohol dehydrogenation involving Fe-oxo species using water as the oxygen source

High-valent iron-oxo complexes are key intermediates in C-H functionalization reactions. Herein, we report the generation of a (TAML)Fe-oxo species (TAML = tetraamido macrocyclic ligand) via electrochemical proton-coupled oxidation of the corresponding (TAML)FeIII-OH2 complex. Cyclic voltammetry (CV) and spectroelectrochemical studies are used to elucidate the relevant (TAML)Fe redox processes and determine the predominant (TAML)Fe species present in solution during bulk electrolysis. Evidence for iron(iv) and iron(v) species is presented, and these species are used in the electrochemical oxygenation of benzylic C-H bonds and dehydrogenation of alcohols to ketones.

Crossing the bridge from molecular catalysis to a heterogenous electrode in electrocatalytic water oxidation

Significant progress has been made in designing single-site molecular Ru(II)-polypyridyl-aqua catalysts for homogenous catalytic water oxidation. Surface binding and transfer of the catalytic reactivity onto conductive substrates provides a basis for heterogeneous applications in electrolytic cells and dye-sensitized photoelectrosynthesis cells (DSPECs). Earlier efforts have focused on phosphonic acid (-PO₃H₂) or carboxylic acid (-CO₂H) bindings on oxide surfaces. However, issues remain with limited surface stabilities, especially in aqueous solutions at higher pH under conditions that favor water oxidation by reducing the thermodynamic barrier and accelerating the catalytic rate using atom-proton transfer (APT) pathways. Here, we address the problem by combining silane surface functionalization and surface reductive electropolymerization on mesoporous, nanofilms of indium tin oxide (ITO) on fluorine-doped tin oxide (FTO) substrates (FTO|nanoITO). FTO|nanoITO electrodes were functionalized with vinyltrimethoxysilane (VTMS) to introduce vinyl groups on the electrode surfaces by silane attachment, followed by surface electropolymerization of the vinyl-derivatized complex, [Ru^II(Mebimpy)(dvbpy)(OH₂)]²⁺ (1²⁺; Mebimpy: 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridine; dvbpy: 5,5'-divinyl-2,2'-bipyridine), in a mechanism dominated by a grafting-through method. The surface coverage of catalyst 1²⁺ was controlled by the number of electropolymerization cycles. The combined silane attachment/cross-linked polymer network stabilized 1²⁺ on the electrode surface under a variety of conditions especially at pH > ∼6. Surface-grafted poly1²⁺ was stable toward redox cycling at pH ∼ 7.5 over an ∼4-h period. Sustained heterogeneous electrocatalytic water oxidation by the electrode gave steady-state currents for at least ∼6 h with a Faradaic efficiency of ∼68% for O₂ production.

Electrochemical Hydroxylation of Arenes Catalyzed by a Keggin Polyoxometalate with a Cobalt(IV) Heteroatom

The sustainable, selective direct hydroxylation of arenes, such as benzene to phenol, is an important research challenge. An electrocatalytic transformation using formic acid to oxidize benzene and its halogenated derivatives to selectively yield aryl formates, which are easily hydrolyzed by water to yield the corresponding phenols, is presented. The formylation reaction occurs on a Pt anode in the presence of [Co^III W₁₂ O₄₀ ]^5- as a catalyst and lithium formate as an electrolyte via formation of a formyloxyl radical as the reactive species, which was trapped by a BMPO spin trap and identified by EPR. Hydrogen was formed at the Pt cathode. The sum transformation is ArH+H₂ O→ArOH+H₂ . Non-optimized reaction conditions showed a Faradaic efficiency of 75 % and selective formation of the mono-oxidized product in a 35 % yield. Decomposition of formic acid into CO₂ and H₂ is a side-reaction.

Mechanisms of molecular water oxidation in solution and on oxide surfaces

Initial experiments on water oxidation by well-defined molecular catalysts were initiated with the goal of finding solutions to solar energy conversion.

Efficient Light-Driven Oxidation of Alcohols Using an Organic Chromophore-Catalyst Assembly Anchored to TiO2

The ligand 5-PO3H2-2,2':5',2″-terthiophene-5-trpy, T3 (trpy = 2,2':6',2″-terpyridine), was prepared and studied in aqueous solutions along with its metal complex assembly [Ru(T3)(bpy)(OH2)](2+) (T3-Ru-OH2, bpy = 2,2'-bipyridine). T3 contains a phosphonic acid group for anchoring to a TiO2 photoanode under aqueous conditions, a terthiophene fragment for light absorption and electron injection into TiO2, and a terminal trpy ligand for the construction of assemblies comprising a molecular oxidation catalyst. At a TiO2 photoanode, T3 displays efficient injection at pH 4.35 as evidenced by the high photocurrents (∼350 uA/cm(2)) arising from hydroquinone oxidation. Addition of [Ru(bpy)(OTf)][OTf]2 (bpy = 2,2'-bipyridine, OTf(-) = triflate) to T3 at the free trpy ligand forms the molecular assembly, T3-Ru-OH2, with the oxidative catalyst fragment: [Ru(trpy)(bpy)(OH2)](2+). The new assembly, T3-Ru-OH2, was used to perform efficient light-driven oxidation of phenol (230 μA/cm(2)) and benzyl alcohol (25 μA/cm(2)) in a dye-sensitized photoelectrosynthesis cell.

Nonaqueous electrocatalytic water oxidation by a surface-bound Ru(bda)(L)2 complex

Electrocatalytic water oxidation by a heterogeneous Ru(bda) catalyst is enhanced in a non-aqueous environment relative to water as the solvent.

Structural heterogeneity and dynamics of dyes on TiO2: implications for charge transfer across organic–inorganic interfaces

Structural heterogeneity, solvation, and thermal fluctuations all contribute to multiple dye–semiconductor charge injection rates.

Starch-coated maghemite nanoparticles functionalized by a novel cobalt Schiff base complex catalyzes selective aerobic benzylic C–H oxidation

The CoL₂@SMNP nanocatalyst catalyzed heterogeneous aerobic oxidation of benzylic C–H bonds of saturated hydrocarbons and alcohols to the related carbonyl compounds.

Oxidation of alkylaromatics to aromatic ketones catalyzed by metalloporphyrins under the special temperature control method

The aerobic oxidation of alkylaromatics to aromatic ketones catalyzed by metalloporphyrins under the special temperature control method was systematically investigated. Three novel and key points were found to have significant functions in this process, that is, the special temperature control method (initiation at higher temperature and reaction at lower temperature), the synergistic effect of the composite catalysts comprising cobalt and manganese porphyrins, and the amount of catalysts in the reaction. Subsequently, the effects of substitutes on alkylaromatics were also explored under the same conditions. Results showed that alkylaromatic conversions gradually increased from 8.5% to 54.3% with the para-substituents shifting from the electron-donating group to the electron-withdrawing group (i.e., methoxy < hydro < bromo < acyl < nitro). A possible mechanism for this reaction was also proposed.