Bridging the Gap between Electrochemical and Organometallic Activation: Benzyl Chloride Reduction at Silver Cathodes

The fate of nanoparticle surface chemistry during reductive electrosynthesis in aprotic media

Reductive electrochemical coupling of carbon dioxide with organic molecules (electrocarboxylation, EC) represents a green route towards value-added carboxylic acids and serves as a promising strategy for carbon footprint mitigation. Despite the industrial prospects of this synthetic process, little has been done towards the optimization of cathode materials at the nanoscale. Herein, we pave the way towards the use of metal nanoparticles (NPs) as electrocatalysts in EC by demonstrating the effects of NP surface chemistry on electroorganic transformations and the evolution of surface functionalization in the course of reductive electrosynthesis in aprotic media. Using spherical Au NPs capped with citrate or cetylpyridinium chloride (CPC) as our study subjects, we examined the effect of Au NP surface chemistry on the selectivity of EC of benzyl bromide in acetonitrile and determined the fate of the surface adsorbates of Au NPs in the course of the reaction using Raman spectroscopy and X-ray photoelectron spectroscopy. We show that the CPC-stabilized Au NPs outperform the citrate-stabilized NPs at a low applied potential of -1.5 V vs. Ag/Ag+ with the former showing an almost two-fold increase in the faradaic efficiency towards phenylacetic acid. This higher selectivity is attributed to the reaction on the liberated Au surface stemming from the stripping of CPC molecules. In contrast to the CPC-functionalized NPs, the citrate-stabilized Au NPs retain their adsorbates during the reaction, which undergo electrochemical transformations during EC.

Modified Working Electrodes for Organic Electrosynthesis

Organic electrosynthesis has gained much attention over the last few decades as a promising alternative to traditional synthesis methods. Electrochemical approaches offer numerous advantages over traditional organic synthesis procedures. One of the most interesting aspects of electroorganic synthesis is the ability to tune many parameters to affect the outcome of the reaction of interest. One such parameter is the composition of the working electrode. By changing the electrode material, one can influence the selectivity, product distribution, and rate of organic reactions. In this Review, we describe several electrode materials and modifications with applications in organic electrosynthetic transformations. Included in this discussion are modifications of electrodes with nanoparticles, composite materials, polymers, organic frameworks, and surface-bound mediators. We first discuss the important physicochemical and electrochemical properties of each material. Then, we briefly summarize several relevant examples of each class of electrodes, with the goal of providing readers with a catalog of electrode materials for a wide variety of organic syntheses.

Ab Initio Kinetics of Electrochemical Reactions Using the Computational Fc0/Fc+ Electrode

The current state-of-the-art electron-transfer modeling primarily focuses on the kinetics of charge transfer between an electroactive species and an inert electrode. Experimental studies have revealed that the existing Butler-Volmer model fails to satisfactorily replicate experimental voltammetry results for both solution-based and surface-bound redox couples. Consequently, experimentalists lack an accurate tool for predicting electron-transfer kinetics. In response to this challenge, we developed a density functional theory-based approach for accurately predicting current peak potentials by using the Marcus-Hush model. Through extensive cyclic voltammetry simulations, we conducted a thorough exploration that offers valuable insights for conducting well-informed studies in the field of electrochemistry.

Interfacial Science for Electrosynthesis

Interfacial science and electroorganic syntheses are inextricably linked because all electrochemical reactions occur at the interface between the electrode and the solution. Thus, the surface chemistry of the electrode material impacts the organic reaction selectivity. In this short review, we highlight emergent examples of electrode surface chemistries that enable selective electroorganic synthesis in three reaction classes: (1) hydrogenation, (2) oxidation, and (3) reductive C‒C bond formation between two electrophiles. We showcase the breadth of techniques, including materials and in-situ characterization, requisite to establish mechanistic schemes consistent with the observed reactivity patterns. Leveraging an electrode's unique surface chemistry will provide complementary approaches to tune the selectivity of electroorganic syntheses and unlock an electrode's catalytic properties.

Ligand Noninnocence in β-Diketiminate and β-Diketimine Copper Complexes

Metal-ligand cooperative systems have a long precedent in catalysis, with the classification depending on the site of substrate bond cleavage and formation and on redox state changes. Recently, our group reported the participation of a β-diketiminate ligand in chemical bonding to heterocumulenes such as CO2 and CS2 by tricopper complexes, leading to cooperative catalysis. Herein, we report the reactivity of these copper clusters, [Cu3EL]- (E = S, Se; L = tris(β-diketiminate) cyclophane ligand), toward other electrophiles, viz. alkyl halides and Brønsted acids. We identified a family of ligand-functionalized complexes, Cu3EL (R) (R = primary alkyls), and a series of disubstituted products, Cu3EL (R)2, through single-crystal X-ray diffraction, mass spectrometry, and infrared and UV-visible spectroscopy. As part of mechanistic studies on these alkylation reactions, we evaluated the acid-base reactivity of these complexes and the influence of the backbone substitution on the reduction potential. Implications of these findings for ligand noninnocence and the relevance of the metal core as a cofactor for the ligand's reactivity are discussed.

Radiation-Induced Hydrogel for Water Treatment

Along with serving as drug delivery sensors and flexible devices, hydrogels are playing pioneering roles in water purification. Both chemical and radiation methods can produce hydrogels, with the latter method gaining preference for its pure adducts. The water treatment process entails the removal of heavy and toxic metals (above the threshold amount), dyes, and solid wastes from industrial effluents, seawater, and groundwater, as well as sterilization for microorganism destruction. This review analyzed the different types of hydrogels produced by applying various radiations for water treatment. Particularly, we examined the hydrogels created through the application of varying levels of gamma and electron beam radiation from the electron gun and Co-60 sources. Moreover, we discuss the optimized radiation doses, the compositions (monomers and polymers) of raw materials required for hydrogel preparation, and their performance in water purification. We present and predict the current state and future possibilities of radiation-induced hydrogels. We explain and compare the superiority of one radiation method over other radiation methods (UV-visible, X-ray, microwave, etc.) based on water treatment.

Photoelectrocatalytic Utilization of CO2 : A Big Show of Si-based Photoelectrodes

CO2 is a greenhouse gas that contributes to environmental deterioration; however, it can also be utilized as an abundant C1 resource for the production of valuable chemicals. Solar-driven photoelectrocatalytic (PEC) CO2 utilization represents an advanced technology for the resourcing of CO2 . The key to achieving PEC CO2 utilization lies in high-performance semiconductor photoelectrodes. Si-based photoelectrodes have attracted increasing attention in the field of PEC CO2 utilization due to their suitable band gap (1.1 eV), high carrier mobility, low cost, and abundance on Earth. There are two pathways to PEC CO2 utilization using Si-based photoelectrodes: direct reduction of CO2 into small molecule fuels and chemicals, and fixation of CO2 with organic substrates to generate high-value chemicals. The efficiency and product selectivity of PEC CO2 utilization depends on the structures of the photoelectrodes as well as the composition, morphology, and size of the catalysts. In recent years, significant and influential progress has been made in utilizing Si-based photoelectrodes for PEC CO2 utilization. This review summarizes the latest research achievements in Si-based PEC CO2 utilization, with a particular emphasis on the mechanistic understanding of CO2 reduction and fixation, which will inspire future developments in this field.

Interfacial Tuning of Electrocatalytic Ag Surfaces for Fragment-Based Electrophile Coupling

Construction of C‒C bonds in medicinal chemistry frequently draws on the reductive coupling of organic halides with ketones or aldehydes. Catalytic C(sp3)‒C(sp3) bond formation, however, is constrained by the competitive side reactivity of radical intermediates following sp3 organic halide activation. Here, an alternative paradigm deploys catalytic Ag surfaces for reductive fragment-based electrophile coupling compatible with sp3 organic halides. We use in-situ spectroscopy, electrochemical analyses, and simulation to uncover the catalytic interfacial structure and guide reaction development. Specifically, Mg(OAc)2 outcompetes the interaction between Ag and the aldehyde, thereby tuning the Ag surface for selective product formation. Data are consistent with an increased population of Mg-bound aldehyde facilitating the addition of a carbon-centered radical (product of Ag-electrocatalyzed organic halide reduction) to the carbonyl. Electron transfer from Ag to the resultant alkoxy radical yields the desired alcohol. Molecular interfacial tuning at reusable catalytic electrodes will accelerate development of sustainable organic synthetic methods.

Tailoring Ag Electron Donating Ability for Organohalide Reduction: A Bilayer Electrode Design

Electrochemical reduction of organohalides provides a green approach in the reduction of environmental pollutants, the synthesis of new organic molecules, and many other applications. The presence of a catalytic electrode can make the process more energetically efficient. Ag is known to be a very good electrode for the reduction of a wide range of organohalides. Herein, we examine the elementary adsorption and reaction steps that occur on Ag and the changes that result from changes in the Ag-coated metal, strain in Ag, solvent, and substrate geometry. The results are used to develop an electrode design strategy that can possibly be used to further increase the catalytic activity of pure Ag electrodes. We have shown how epitaxially depositing one to three layers of Ag on catalytically inert or less active support metal can increase the surface electron donating ability, thus increasing the adsorption of organic halide and the catalytic activity. Many factors, such as molecular geometry, lattice mismatches, work function, and solvents, contribute to the adsorption of organic halide molecules over the bilayer electrode surface. To isolate and rank these factors, we examined three model organic halides, namely, halothane, bromobenzene (BrBz), and benzyl bromide (BzBr) adsorption on Ag/metal (metal = Au, Bi, Pt, and Ti) bilayer electrodes in both vacuum and acetonitrile (ACN) solvent. The different metal supports offer a range of lattice mismatches and work function differences with Ag. Our calculations show that the surface of Ag becomes more electron donating and accessible to adsorption when it forms a bilayer with Ti as it has a lower work function and almost zero lattice mismatch with Ag. We believe this study will help to increase the electron donating ability of the Ag surface by choosing the right metal support, which in turn can improve the catalytic activity of the working electrode.

Three-Component Cross-Electrophile Coupling: Regioselective Electrochemical Dialkylation of Alkenes

The cross-electrophile dialkylation of alkenes enables the formation of two C(sp3)-C(sp3) bonds from readily available starting materials in a single transformation, thereby providing a modular and expedient approach to building structural complexity in organic synthesis. Herein, we exploit the disparate electronic and steric properties of alkyl halides with varying degrees of substitution to accomplish their selective activation and addition to alkenes under electrochemical conditions. This method enables regioselective dialkylation of alkenes without the use of a transition-metal catalyst and provides access to a diverse range of synthetically useful compounds.

In situ Raman monitoring of electroreductive dehalogenation of aryl halides at an Ag/aqueous solution interface

Electroreductive dehalogenation as an efficient and green approach has attracted much attention in pollution remediation. Herein, we have employed a shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) technique to in situ probe the electroreductive dehalogenation process of aryl halides with thiol groups at Ag/aqueous solution interfaces. It is found that 4-bromothiophenol (BTP) and 4-chlorothiophenol (CTP) can turn into mixed products of 4,4'-biphenyldithiol (BPDT) and thiophenol (TP) as the electrode potential decreases. The conversion ratios estimated from the Raman intensity variations of C-Cl and C-Br vibrations are 44% and 58% for CTP and BTP in neutral solution, respectively. Furthermore, the quantitative analysis of benzene ring vibrations reveals a C-C cross coupling between the benzene free radical intermediate and adjacent TP product, which results in increased selectivity for biphenyl products at negative potentials.

Nanogap-Resolved Adsorption-Coupled Electron Transfer by Scanning Electrochemical Microscopy: Implications for Electrocatalysis

Here, we demonstrate for the first time that the mechanism of adsorption-coupled electron-transfer (ACET) reactions can be identified experimentally. The electron transfer (ET) and specific adsorption of redox-active molecules are coupled in many electrode reactions with practical importance and fundamental interest. ACET reactions are often represented by a concerted mechanism. In reductive adsorption, an oxidant is simultaneously reduced and adsorbed as a reductant on the electrode surface through the ACET step. Alternatively, the non-concerted mechanism mediates outer-sphere reduction and adsorption separately when the reductant adsorption is reversible. In electrocatalysis, reversibly adsorbed reductants are ubiquitous and crucial intermediates. Moreover, electrocatalysis is complicated by the mixed mechanism based on simultaneous ACET and outer-sphere ET steps. In this work, we reveal the non-concerted mechanism for ferrocene derivatives adsorbed at highly oriented pyrolytic graphite as simple models. We enable the transient voltammetric mode of nanoscale scanning electrochemical microscopy (SECM) to kinetically control the adsorption step, which is required for the discrimination of non-concerted, concerted, and mixed mechanisms. Experimental voltammograms are compared with each mechanism by employing finite element simulation. The non-concerted mechanism is supported to indicate that the ACET step is intrinsically slower than its outer-sphere counterpart by at least four orders of magnitude. This finding implies that an ACET step is facilitated thermodynamically but may not be necessarily accelerated or catalyzed by the adsorption of the reductant. SECM-based transient voltammetry will become a powerful tool to resolve and understand electrocatalytic ACET reactions at the elementary level.

Benzylic C(sp3)-H Bonds Play the Dual Role of Starting Material and Oxidation Inhibitor for Hydrazides in the Electrochemical Synthesis of Hydrazones

The electrooxidation of benzylic C(sp³)-H bonds to produce hydrazones as an alternate for conventional pathways has an enormous dignity. Under the aegis of electricity, instead of hazardous metal catalysts and external oxidants, we unveil an electrochemical process for electrooxidation of various benzylic C(sp³)-H bonds in aqueous media in all pH ranges that subsequently produce hydrazones with further reactions. This electrooxidative reaction strategy provides an acceptable condition for synthesizing hydrazones with various functional groups in good efficiency and amenable to gram-scale synthesis. The electrochemical oxidation condition proves an excellent level of compatibility with super cheap electrolyte NaCl for the oxidation of benzylic C(sp³)-H position despite the highly oxidizable hydrazide group remaining intact in the reaction.

Electrochemically activated carbon-halogen bond cleavage and C-C coupling monitored by in situ shell-isolated nanoparticle-enhanced Raman spectroscopy

The electroreductive cleavage of carbon-halogen bonds has attracted increasing attention in both electrosynthesis and pollution remediation. Herein, by employing the in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) technique, we have successfully investigated the electroreductive dehalogenation process of aryl halides with the thiol group on a smooth Au electrode in aqueous solution at different pH values. The obtained potential-dependent Raman spectra directly reveal a mixture of the reduction products 4,4'-biphenyldithiol (BPDT) and thiophenol (TP). The conversion ratios of the C-Cl and C-Br bonds at pH = 7 are 37% and 55%, respectively. Furthermore, quantitative analysis of the intensity variations of ν(C-Cl), ν(C-Br) and aromatic ν(CC) stretching modes suggests electroreductive dehalogenation via both direct electron transfer reduction and electrocatalytic hydrodehalogenation. Molecular evidence for the C-C cross coupling process through TP reaction with benzene free radical intermediates is found at negative potentials, which leads to the increasing selectivity of biphenyl products.

Direct vs Indirect Grafting of Alkyl and Aryl Halides

The direct and indirect electrochemical grafting of alkyl and aryl halides (RX, ArX) on carbon, metal and polymer surfaces is examined. Their electrochemical reduction occurs at highly negative potential in organic solvents and very often produces carbanions because the reduction potentials of RX and ArX are more negative than those of their corresponding radicals. Therefore, direct electrografting of alkyl and aryl radicals generated from RX and ArX is not easy to perform. This obstacle is overcome using aryl radicals derived from the 2,6-dimethylbenzenediazonium salt (2,6-DMBD), which do not react on the electrode surface due to their steric hindrance but react in solution by abstracting an iodine or bromine atom from RX (X=I, Br) or ArI to give alkyl or aryl radicals. As a consequence, alkyl and aryl radicals are generated at very low driving force by diverting the reactivity of aryl radicals derived from an aryl diazonium salt; they attack the electrode surface and form strongly attached organic layers. This strategy applies to the chemical modification of polymers (polyethylene, polymethylmethacrylate) by alkyl halides under heating.

How to achieve auto-identification in Raman analysis by spectral feature extraction & Adaptive Hypergraph

With the miniaturization of Raman spectrometers, Raman spectroscopy (including Surface-enhanced Raman spectroscopy) has been widely applied to various fields, especially towards rapid detection applications. In order to deal with the accompanied massive databases, large numbers of Raman spectra require to be handled and identified in an effective and automatic manner. This paper proposes an algorithm of material auto-identification, which makes use of machine learning methods to analyze Raman spectra. Firstly, a universal method of spectral feature extraction is designed to automatically process Raman spectra after the background subtraction. Secondly, the extracted feature vectors are used to classify and identify target materials by Adaptive Hypergraph (AH), an efficient classifier in the field of machine learning, in a manner of automation with an accuracy rate of ~99%. Compared with Support Vector Machine (SVM) and Random Forest (RF), two typical methods of classification, the AH classifier provides better performance free of tuning any parameter facing different targets. Thirdly, Cubic Spline Interpolation is introduced to enhance the universal of the proposed algorithm between different databases from different Raman spectrometers with variant vendors. The identification accuracy rate is up to 98% using the high frequency sampling spectra as the learning and the low frequency sampling ones as the testing, respectively.

Surface-enhanced Raman spectroscopy: bottlenecks and future directions

This feature article discusses developmental bottleneck issues in surface Raman spectroscopy in its early stages and surface-enhanced Raman spectroscopy (SERS) in the past four decades and future perspectives.

Intermediate stages of electrochemical oxidation of single-crystalline platinum revealed by in situ Raman spectroscopy

Understanding the atomistic details of how platinum surfaces are oxidized under electrochemical conditions is of importance for many electrochemical devices such as fuel cells and electrolysers. Here we use in situ shell-isolated nanoparticle-enhanced Raman spectroscopy to identify the intermediate stages of the electrochemical oxidation of Pt(111) and Pt(100) single crystals in perchloric acid. Density functional theory calculations were carried out to assist in assigning the experimental Raman bands by simulating the vibrational frequencies of possible intermediates and products. The perchlorate anion is suggested to interact with hydroxyl phase formed on the surface. Peroxo-like and superoxo-like two-dimensional (2D) surface oxides and amorphous 3D α-PtO₂ are sequentially formed during the anodic polarization. Our measurements elucidate the process of the electrochemical oxidation of platinum single crystals by providing evidence for the structure-sensitive formation of a 2D platinum-(su)peroxide phase. These results may contribute towards a fundamental understanding of the mechanism of degradation of platinum electrocatalysts.

Understanding degradation of platinum catalysts during oxygen reduction is vital for improving proton-exchange membrane fuel cells. Here, the authors identify intermediate stages in the oxidation of Pt(111) and Pt(100) in perchloric acid using in situ shell-isolated nanoparticle-enhanced Raman spectroscopy.

Powerful Surface Chemistry Approach for the Grafting of Alkyl Multilayers on Aluminum Nanoparticles

The synthesis of aluminum nanoparticles (Alnp) has raised promising perspectives these past few years for applications in energetic materials. However, because of their high reactivity, it is crucial to functionalize them before their use. In this work, we propose an original and simple chemical approach to graft spontaneously alkyl layers derived from alkyl halides at the surface of Alnp, by relying on the highly reductive character of these nanoparticles, when they are in the unoxidized form. Alnp were prepared in a glovebox and reacted with alkyl halides (RI and RBr) to give modified Alnp-R, as shown by infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction, thermogravimetric analysis (TGA), and microscopy. The coating is made of alkyl multilayers, which were found to be strongly anchored at the Alnp surface, as it resisted 2 h of rinsing in toluene. An electrocatalytic electron transfer promoted by Alnp is proposed to describe the mechanism of this grafting reaction.

Electrocatalytic reduction of PhCH2Br on a Ag–Y zeolite modified electrode

A Ag-exchanged Y zeolite was prepared and modified on a glass carbon electrode, which displayed excellent catalytic activity towards electrochemical reduction and carboxylation of PhCH₂Br.

Electrografting of alkyl films at low driving force by diverting the reactivity of aryl radicals derived from diazonium salts

Alkyl and partial perfluoroalkyl groups are strongly attached to carbon surfaces through (i) the abstraction of the iodine atom from an iodoalkane by the sterically hindered 2,6-dimethylphenyl radical and (ii) the reaction of the ensuing alkyl radical with the carbon surface. Since the 2,6-dimethylphenyl radical is obtained at -0.25 V/Ag/AgCl by reducing the corresponding diazonium salt, the electrografting reaction is facilitated by ∼1.7 V by comparison with the direct electrografting of the iodo compounds. Layers of various thicknesses, including monolayers, are obtained by controlling the time duration of the electrolysis. The grafted films are characterized by electrochemistry, IR, XPS, ellipsometry, and water contact angles.

Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes for Monitoring and Understanding Heterogeneous Catalysis

ABSTRACT

Surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) were until recently limited in their applicability to the majority of heterogeneous catalytic reactions. Recent developments begin to resolve the conflicting experimental requirements for SERS and TERS on the one hand, and heterogeneous catalysis on the other hand. This article discusses the development and use of SERS and TERS to study heterogeneous catalytic reactions, and the exciting possibilities that may now be within reach thanks to the latest technical developments. This will be illustrated with showcase examples from photo- and electrocatalysis.

GRAPHICAL ABSTRACT

Computational electrochemistry: prediction of liquid-phase reduction potentials

This article reviews recent developments and applications in the area of computational electrochemistry. Our focus is on predicting the reduction potentials of electron transfer and other electrochemical reactions and half-reactions in both aqueous and nonaqueous solutions. Topics covered include various computational protocols that combine quantum mechanical electronic structure methods (such as density functional theory) with implicit-solvent models, explicit-solvent protocols that employ Monte Carlo or molecular dynamics simulations (for example, Car-Parrinello molecular dynamics using the grand canonical ensemble formalism), and the Marcus theory of electronic charge transfer. We also review computational approaches based on empirical relationships between molecular and electronic structure and electron transfer reactivity. The scope of the implicit-solvent protocols is emphasized, and the present status of the theory and future directions are outlined.