Electrocatalysis with Molecular Transition-Metal Complexes for Reductive Organic Synthesis

Abstract

Electrocatalysis enables the formation or cleavage of chemical bonds by a genuine use of electrons or holes from an electrical energy input. As such, electrocatalysis offers resource-economical alternative pathways that bypass sacrificial, waste-generating reagents often required in classical thermal redox reactions. In this Perspective, we showcase the exploitation of molecular electrocatalysts for electrosynthesis, in particular for reductive conversion of organic substrates. Selected case studies illustrate that efficient molecular electrocatalysts not only are appropriate redox shuttles but also embrace the features of organometallic catalysis to facilitate and control chemical steps. From these examples, guidelines are proposed for the design of molecular electrocatalysts suited to the reduction of organic substrates. We finally expose opportunities brought by catalyzed electrosynthesis to functionalize organic backbones, namely using sustainable building blocks.

TEMPO‐Modified Polymethacrylates as Mediators in Electrosynthesis: Influence of the Molecular Weight on Redox Properties and Electrocatalytic Activity

“…The profile shown is well explained when considering the relationship between D and Chemistry-A European Journal the hydrodynamic radius r of a diffusing species according to the Stokes-Einstein equation (D ~rÀ 1 ), and substituting r by M n a using scaling theory with a being the Flory exponent (a = 0.5 for ideal random polymer coils in a theta solvent and a = 0.6 for swollen coils in a good solvent). [27] In practice, the following empirical relationship has proven useful, (2) which relates D to M n , the (hypothetical) diffusion coefficient, and the molecular weight of the monomer unit (D mon and M mon , respectively). [16][17] In the context of the abovementioned Stokes-Einstein equation and scaling relation, the exponent 0.55 would represent a scenario in which the polymer chain is randomly coiled in a good solvent (0.5 < a < 0.6).…”

“…Indirect electrolysis using redox mediators is a frequently used approach toward controlling selectivity and reducing energy consumption of electro-organic transformations. [1] To address a multitude of synthetic problems, a well-established portfolio of mediators is available including organometallic compounds, [2] metal ions, [3] halides, [4] triarylamines, [5] iodoarenes, [6] and N-oxyl radicals. [7] Although the benefits of mediators are undisputed, they may be offset by more difficult separation procedures, increasing waste generation, and additional expenses, which is why concepts to facilitate separation and recycling deserve more attention.…”

Electrochemical Epoxidation Catalyzed by Manganese Salen Complex and Carbonate with Boron-Doped Diamond Electrode

“…Electrocatalysis employing metal complexes has attracted a great deal of attention in the field of organic synthesis and has shown significant progress in recent years [ 42 , 43 , 44 , 45 ]. In this study, inspired by Tanaka’s work [ 17 ], we have examined electrochemical epoxidation via in situ electrochemical generation of percarbonate by using various metal complexes, including manganese salen complexes and manganese and vanadium porphyrin complexes as a mediator in an organic solvent–aqueous carbonate two-phase system in a simple undivided cell.…”

The Use of Redox Mediators in Electrocatalysis and Electrosynthesis

“…Redox mediators, which are a series of molecules, ions, or compounds, can be reversibly oxidized and reduced during electrochemical reactions. For their applications in electrocatalysis and electrosynthesis reactions, they were used as electron‐hole transportation shuttles to accelerate the electrochemical reactions [6] . Moreover, redox mediators can act as electron‐hole mediators to decouple one electrochemical reaction into multi electrochemical steps or electrochemical‐chemical steps to overcome the higher energy barriers.…”

“…For their applications in electrocatalysis and electrosynthesis reactions, they were used as electron-hole transportation shuttles to accelerate the electrochemical reactions. [6] Moreover, redox mediators can act as electron-hole mediators to decouple one electrochemical reaction into multi electrochemical steps or electrochemicalchemical steps to overcome the higher energy barriers. Furthermore, both of the electrocatalysis and electrosynthesis are usually operated in a two-electrodes cell, the reaction rate is dependent on the electrode or electrochemical step with sluggish kinetics.…”