The tools, methods and concepts of mechanistic molecular electrochemistry have been established for about four decades. They allowed the exploration of many aspects of organic, inorganic and bio reactivity in various areas ranging from (bio)chemical reactions and catalysis to (bio)analytical sciences. The current and vibrant renewal of electrochemistry, notably driven by contemporary energy challenges, has brought back on stage the use of fundamental molecular electrochemistry and stimulated new developments. The coupling of electrons, protons, and bond breaking or bond forming is a common feature of the catalytic processes that are emblematic of this renewal. These elementary steps, their degree of concertedness as well as their coupling with reactant and coreactant mass transport must be understood to drive the development of efficient processes. This issue illustrates the most prominent aspects of this remarkably active field.

O₂, which is abundant and environmentally benign, is the ideal green oxidant for oxidation reactions, which are key transformations in the chemical industry. Still, O₂ needs to be activated, and this can be achieved through the so-called reductive activation of the O2 paradigm. Taking inspiration from metalloenzymes, where a non-noble redox active metal (iron, copper) controls the partial reduction of O₂ via electron and proton transfers, metal-based synthetic systems can be developed to reproduce oxygenase activity. In the present article, we focus on fundamental aspects that serve as support for the development of 2-electron activation of O₂ and generation of highly oxidizing metal-oxo species, thus paving the road for the development of electrocatalytic systems for organic substrate oxygenation. Scarce examples known in the literature capable of such reactivity and possible future developments are described.

Substituted tetraphenyl Fe porphyrins are versatile molecular catalysts for the activation of small molecules (such as O₂, H⁺ or CO₂), which could lead to renewable energy storage, the direct production of fuels or new catalytic relevant processes. Herein, we review the recent studies of these earth-abundant metal catalysts for the electrochemical activation of dioxygen on the one hand and for the photostimulated reduction of carbon dioxide on the other hand. These two prototype reactions illustrate how mechanistic studies are the only rational approach to gain fundamental insights into the elementary steps that drive the catalysis and for identification of the key intrinsic parameters controlling the reactivity, offering in turn the possibility to rationally tune the structure of the catalysts as well as the catalytic conditions.

A new pentadentate quinoline–pyridine ligand and its iron (1), cobalt (2) and nickel (3) complexes have been synthesized and characterized. The iron complex exhibits excellent photocatalytic activity towards CO₂-to-CO conversion with a TON(CO) of 544 and a selectivity of 99.3% using the commercially available organic dye purpurin as the photosensitiser and BIH as the electron donor. In contrast, the cobalt and nickel complexes result in very low activity for CO production with a TON of only 8 and 15, respectively. On the other hand, all the three complexes show good electrocatalytic activity for CO₂ reduction when using 2,2,2-trifluoroethanol as the proton source with the active intermediate of M0 species. The lack of activity in photocatalytic CO2 reduction by 2 and 3 can be attributed to the redox potential of MI/M0 which is significantly more negative than that of PP⁻/PP²⁻ while in the case of 1 the Fe^I/Fe⁰ redox potential becomes more positive than that of the PP⁻/PP²⁻ couple.

The novel rhenium complexes fac-Re(pdbpy)(CO)₃Cl (pdbpy = 4-phenyl-6-(phenyl-2,6-diol)-2,2′-bipyridine), 1, and fac-Re(ptbpy)(CO)₃Cl (ptbpy = 4-phenyl-6-(phenyl-3,4,5-triol)-2,2′-bipyridine), 2, have been synthesized, and the single crystal X-ray diffraction (SC-XRD) structure of 1 was solved. The electrochemical behaviors of the complexes in acetonitrile under Ar and their catalytic performances for CO₂ reduction with added water and MeOH are discussed. A detailed IR spectroelectrochemical study under Ar and CO₂ atmospheres coupled with DFT calculations allows the identification of reduced species and the interpretation of the reduction mechanisms. Comparison between the rhenium complexes and the corresponding Mn derivatives Mn(pdbpy)(CO)₃Br, 3, and Mn(ptbpy)(CO)₃Br, 4, has been also considered. Finally, photostimulated conversion of the CO₂ was investigated with catalysts (1, 3, and 4) under visible light irradiation (λ > 420 nm) in acetonitrile as solvent. Remarkably, 1 and 3 catalysts were active toward CO₂, producing formate with good selectivity and turnover number (TON). For example, 3 gives 62% selectivity for HCOO^– and a TON of 80, and Re compound 1 gives 74% selectivity for HCOO^– and a TON of 86.

LiBH₄ is often employed as a reducing agent for metal nanoparticle (NP) preparation but is inherently a solid-state H2 hydrogen storage agent. Herein it is shown, through a combination of electron/optical microscopies and single entity electrochemical study, that LiBH₄ is stored in the solid state within an ionic liquid (IL) as nanocrystals (NCs). The electrochemical monitoring of an immiscible water|IL (w|IL) micro-liquid|liquid interface (LLI) shows interfacial charge exchange associated with the stochastic impacts of single NCs. Meanwhile, in situ optical monitoring of a w|metal or w|IL interface shows that such impacts are associated with the development of a H₂-in-IL micro/nano-foam related to the poor solubility of H₂. Both the presence of solid NCs and the latter H₂-in-IL foam suggest that H₂ release from LiBH₄-in-IL is a slow, but likely controlled process. The rate of H₂ production at a macroscopic LLI is further confirmed by gas chromatographic measurements, in very good agreement with microscopic observations. The electrochemical LLI provides unique investigative access to LiBH₄ NCs and offers insight into H₂ storage in ILs, or for direct borohydride fuel cells, as well as NP synthesis.

Using a phenoxazine-based organic photosensitizer and an iron porphyrin molecular catalyst, we demonstrated photochemical reduction of CO₂ to CO and CH₄ with turnover numbers (TONs) of 149 and 29, respectively, under visible-light irradiation (λ > 435 nm) with a tertiary amine as sacrificial electron donor. This work is the first example of a molecular system using an earth-abundant metal catalyst and an organic dye to effect complete 8e–/8H+ reduction of CO₂ to CH₄, as opposed to typical 2e^-/2H⁺ products of CO or formic acid. The catalytic system continuously produced methane even after prolonged irradiation up to 4 days. Using CO as the feedstock, the same reactive system was able to produce CH4 with 85% selectivity, 80 TON and a quantum yield of 0.47%. The redox properties of the organic photosensitizer and acidity of the proton source were shown to play a key role in driving the 8e^-/8H⁺ processes.

The electrochemical catalytic reduction of CO2 into CO could be achieved with excellent selectivity and rate in acetonitrile in the presence of phenol with cobalt 2,2′:6′,2″:6″,2‴-quaterpyridine complex [CoII(qpy)(H2O)2]2+ (Co) acting as a molecular catalyst. Upon using cyclic voltammetry at low and high scan rate (up to 500 V/s) two catalytic pathways have been identified. At a low concentration of phenol (<1 M), catalysis mainly occurs after the reduction of Co with three electrons. In that case, the selectivity for CO production is ca. 80% with 20% of H2 as by product, along with a turnover frequency of 1.2 × 104 s–1 for CO production at an overpotential η of ca. 0.6 V. The triply reduced active species binds to CO2 and the C–O bond is cleaved thanks to the acid. At very large concentration of phenol (3 M), another pathway becomes predominant: the doubly reduced species binds to CO2, while its reductive protonation leads to CO formation. As already shown, this later process is endowed with fast rate at low overpotential (turnover frequency of 3 × 104 s–1 at η = 0.3 V) and 95% selectivity for CO production. By varying the phenol concentration and the scan rate in voltammetry experiments, it was thus possible to identify, activate, and characterize several pathways for the CO2-to-CO conversion and to decipher Co electrochemical reactivity toward CO2.

Associating a metal‐based catalyst to a carbon‐based nanomaterial is a promising approach for the production of solar fuels from CO₂. Upon appending a CoII quaterpyridine complex [Co(qpy)]² at the surface of multi‐walled carbon nanotubes, CO2 conversion into CO was realized in water at pH 7.3 with 100 % catalytic selectivity and 100 % Faradaic efficiency, at low catalyst loading and reduced overpotential. A current density of 0.94 mA cm⁻² was reached at −0.35 V vs. RHE (240 mV overpotential), and 9.3 mA cm⁻² could be sustained for hours at only 340 mV overpotential with excellent catalyst stability (89 095 catalytic cycles in 4.5 h), while 19.9 mA cm⁻² was met at 440 mV overpotential. Such a hybrid material combines the high selectivity of a homogeneous molecular catalyst to the robustness of a heterogeneous material. Catalytic performances compare well with those of noble‐metal‐based nano‐electrocatalysts and atomically dispersed metal atoms in carbon matrices.

Efficient and selective photostimulated CO₂-to-CO reduction by a photocatalytic system consisting of an iron-complex catalyst and a mesoporous graphitic carbon nitride (mpg-C₃N₄) redox photosensitizer is reported for the first time. Irradiation in the visible region (λ ≥ 400 nm) of an CH₃CN/triethanolamine (4:1, v/v) solution containing [Fe(qpy)(H₂O)₂]²⁺ (qpy = 2,2′:6′,2′′:6′′,2′′-quaterpyridine) and mpg-C₃N₄ resulted in CO evolution with 97% selectivity, a turnover number of 155, and an apparent quantum yield of ca. 4.2%. This hybrid catalytic system, comprising only earth abundant elements, opens new perspectives for solar fuels production using CO₂ as a renewable feedstock.