Electrooxidation of alcohols catalyzed by amino alcohol ligated ruthenium complexes

Electrocatalytic formate and alcohol oxidation by hydride transfer at first-row transition metal complexes

The electrocatalytic oxidation of carbon-based liquid fuels, such as formic acid and alcohols, has important applications for our renewable energy transition. Molecular electrocatalysts based on transition metal complexes provide the opportunity to explore the interplay between precise catalyst design and electrocatalytic activity. Recent advances have seen the development of first-row transition metal electrocatalysts for these transformations that operate via hydride transfer between the substrate and catalyst. In this Frontier article, we present the key contributions to this field and discuss the proposed mechanisms for each case. These studies also reveal the remaining challenges for formate and alcohol oxidation with first-row transition metal systems, for which we provide perspectives on future directions for next-generation electrocatalyst design.

Merging electrocatalytic alcohol oxidation with C-N bond formation by electrifying metal-ligand cooperative catalysts

Electrification of thermal chemical processes could play an important role in creating a more energy efficient chemical sector. Here we demonstrate that a range of MLC catalysts can be successfully electrified and used for imine formation from alcohol precursors, thus demonstrating the first example of molecular electrocatalytic C-N bond formation.This novel concept allowed energy efficiency to be increased by an order of magnitude compared to thermal catalysis. Molecular EAO and the electrification of homogeneous catalysts can thus contribute to current efforts for the electrocatalytic generation of C-N bonds from simple building blocks.

Electrification of a Milstein-type catalyst for alcohol reformation

Novel energy and atom efficiency processes will be keys to develop the sustainable chemical industry of the future. Electrification could play an important role, by allowing to fine-tune energy input and using the ideal redox agent: the electron. Here we demonstrate that a commercially available Milstein ruthenium catalyst (1) can be used to promote the electrochemical oxidation of ethanol to ethyl acetate and acetate, thus demonstrating the four electron oxidation under preparative conditions. Cyclic voltammetry and DFT-calculations are used to devise a possible catalytic cycle based on a thermal chemical step generating the key hydride intermediate. Successful electrification of Milstein-type catalysts opens a pathway to use alcohols as a renewable feedstock for the generation of esters and other key building blocks in organic chemistry, thus contributing to increase energy efficiency in organic redox chemistry.

Advances on the Merger of Electrochemistry and Transition Metal Catalysis for Organic Synthesis

Synthetic organic electrosynthesis has grown in the past few decades by achieving many valuable transformations for synthetic chemists. Although electrocatalysis has been popular for improving selectivity and efficiency in a wide variety of energy-related applications, in the last two decades, there has been much interest in electrocatalysis to develop conceptually novel transformations, selective functionalization, and sustainable reactions. This review discusses recent advances in the combination of electrochemistry and homogeneous transition-metal catalysis for organic synthesis. The enabling transformations, synthetic applications, and mechanistic studies are presented alongside advantages as well as future directions to address the challenges of metal-catalyzed electrosynthesis.

Transition Metal Complex Catalyzed Photo- and Electrochemical (De)hydrogenations Involving C=O and C=N Bonds

Herein, we summarize the photo- and electrochemical protocols for dehydrogenation and hydrogenations involving carbonyl and imine functions. The three basic principles that have been explored to interconvert such moieties with transition metal complexes are discussed in detail and the substrate scope is evaluated. Furthermore, we describe some general thermodynamic and kinetic aspects of such electro- and photochemically driven reactions.

1 Introduction

2 Dehydrogenation Reactions

2.1 Electrochemical Dehydrogenations Using High-Valent Metal Species

2.2 Electrochemical Dehydrogenations Involving Metal Hydride species

2.3 Photochemically Driven Dehydrogenation

3 Hydrogenation Reactions

3.1 Electrochemical Protocols

3.2 Photochemical Protocols

4 Conclusion

5 Abbreviations

Real-time mass-spectrometric screening of droplet-scale electrochemical reactions

A droplet-scale, real-time electrochemical reaction screening platform based on droplet spray ionization mass spectrometry (DSI-MS) has been developed. The N,N-dimethylaniline (DMA) radical cation with a half-life of microseconds was readily detected by MS during the electrooxidation of DMA, and the subsequent reactions were followed in real time for minutes.

In Situ Mass Spectrometric Screening and Studying of the Fleeting Chain Propagation of Aniline

A simple and effective approach to studying the mechanism of electrooxidation of aniline (ANI) is reported in this paper. It was accomplished by an innovative electrochemistry (EC)-mass spectrometry (MS) coupling, which can sample directly from a droplet-scale reacting electrolyte for mass spectrometric analysis. With this setup, the polymer chain growth of ANI could be monitored in situ and in real-time. The short-lived radical cations (ANI^•+, m/ z 93.06) as well as the soluble dimer ( m/ z 183.09) and oligomers ( m/ z 274.13, 365.18, ...) were successfully captured. Using the EC-MS and tandem mass spectrometry, the dimers produced by head-to-tail (4-aminodiphenylamine), head-to-head (hydrazobenzene), and tail-to-tail (benzidine) coupling of radical cations were found in the same polymerization process. Moreover, the EC-MS method was also applicable for determining the propagation speed of ANI when applying different electrolyte salts and oxidizing potentials.

High turnover in electro-oxidation of alcohols and ethers with a glassy carbon-supported phenanthroimidazole mediator

Glassy carbon electrodes covalently modified with a phenanthroimidazole mediator promote electrochemical alcohol and ether oxidation: three orders of magnitude increase in TON, to ∼15 000 in each case, was observed compared with homogeneous mediated reactions.

Glassy carbon electrodes covalently modified with a phenanthroimidazole mediator promote electrochemical alcohol and ether oxidation: three orders of magnitude increase in TON, to ∼15 000 in each case, was observed compared with homogeneous mediated reactions. We propose the deactivation pathways in homogeneous solution are prevented by the immobilization: modified electrode reversibility is increased for a one-electron oxidation reaction. The modified electrodes were used to catalytically oxidize p-anisyl alcohol and 1-((benzyloxy)methyl)-4-methoxybenzene, selectively, to the corresponding benzaldehyde and benzyl ester, respectively.

Electrochemistry-mass spectrometry for mechanism study of oxygen reduction at water/oil interface

Electrochemistry methods have been widely employed in the development of renewable energy, and involved in various processes, e.g. water splitting and oxygen reduction. Remarkable progress notwithstanding, there are still many challenges in further optimization of catalysts to achieve high performance. For this purpose, an in-depth understanding of reaction mechanism is needed. In this study, an electrochemistry-mass spectrometry method based on a Y-shaped dual-channel microchip as electrochemical cell and ionization device was demonstrated. Combined solutions of aqueous phase and oil phase were introduced into mass spectrometer directly when electrochemical reactions were happening to study the reduction of oxygen by decamethylferrocene or tetrathiafulvalene under the catalysis of a metal-free porphyrin, tetraphenylporphyrin, at water/1,2-dichloroethane interfaces. Monoprotonated and diprotonated tetraphenylporphyrin were detected by mass spectrometer, confirming the previously proposed mechanism of the oxygen reduction reaction. This work offers a new approach to study electrochemical reactions at liquid-liquid interface.

Acceleration of reaction in charged microdroplets

Using high-resolution mass spectrometry, we have studied the synthesis of isoquinoline in a charged electrospray droplet and the complexation between cytochrome c and maltose in a fused droplet to investigate the feasibility of droplets to drive reactions (both covalent and noncovalent interactions) at a faster rate than that observed in conventional bulk solution. In both the cases we found marked acceleration of reaction, by a factor of a million or more in the former and a factor of a thousand or more in the latter. We believe that carrying out reactions in microdroplets (about 1–15 μm in diameter corresponding to 0·5 pl – 2 nl) is a general method for increasing reaction rates. The mechanism is not presently established but droplet evaporation and droplet confinement of reagents appear to be two important factors among others. In the case of fused water droplets, evaporation has been shown to be almost negligible during the flight time from where droplet fusion occurs and the droplets enter the heated capillary inlet of the mass spectrometer. This suggests that (1) evaporation is not responsible for the acceleration process in aqueous droplet fusion and (2) the droplet–air interface may play a significant role in accelerating the reaction. We argue that this ‘microdroplet chemistry’ could be a remarkable alternative to accelerate slow and difficult reactions, and in conjunction with mass spectrometry, it may provide a new arena to study chemical and biochemical reactions in a confined environment.

Multistage Reactive Transmission-Mode Desorption Electrospray Ionization Mass Spectrometry

Elucidating reaction mechanisms is important for advancing many areas of science such as catalyst development. It is often difficult to probe fast reactions at ambient conditions with high temporal resolution. In addition, systems involving reagents that cross-react require analytical methods that can minimize interaction time and specify their order of introduction into the reacting system. Here, we explore the utility of transmission mode desorption electrospray ionization (TM-DESI) for reaction monitoring by directing a microdroplet spray towards a series of meshes with micrometer-sized openings coated with reagents, an approach we call multistage reactive TM-DESI (TM (n) -DESI, where n refers to the number of meshes; n = 2 in this report). Various stages of the reaction are initiated at each mesh surface, generating intermediates and products in microdroplet reaction vessels traveling towards the mass spectrometer. Using this method, we investigated the reactivity of iron porphyrin catalytic hydroxylation of propranolol and other substrates. Our experimental results indicate that TM (n) -DESI provides the ability to spatially separate reagents and control their order of introduction into the reacting system, thereby minimizing unwanted reactions that lead to catalyst deactivation and degradation products. In addition, comparison with DESI-MS analyses (the Zare and Latour laboratories published results suggesting accessible reaction times <1 ms) of the reduction of dichlorophenolindophenol by L-ascorbic acid suggest that TM (1) -DESI can access reaction times less than 1 ms. Multiple meshes allow sequential stages of desorption/ionization per MS scan, increasing the number of analytes and reactions that can be characterized in a single experiment.

Nickel phosphine catalysts with pendant amines for electrocatalytic oxidation of alcohols

Nickel phosphine complexes with pendant amines are reported as the first nonprecious metal molecular electrocatalysts for the oxidation of alcohols.

Energy and chemicals from the selective electrooxidation of renewable diols by organometallic fuel cells

Organometallic fuel cells catalyze the selective electrooxidation of renewable diols, simultaneously providing high power densities and chemicals of industrial importance. It is shown that the unique organometallic complex [Rh(OTf)(trop2NH)(PPh3)] employed as molecular active site in an anode of an OMFC selectively oxidizes a number of renewable diols, such as ethylene glycol , 1,2-propanediol (1,2-P), 1,3-propanediol (1,3-P), and 1,4-butanediol (1,4-B) to their corresponding mono-carboxylates. The electrochemical performance of this molecular catalyst is discussed, with the aim to achieve cogeneration of electricity and valuable chemicals in a highly selective electrooxidation from diol precursors.