The reduction of carbon dioxide represents an ambitious target, with potential impact on several of the United Nations’ sustainable development goals including climate action, renewable energy, sustainable cities, and communities. This process shares a common issue with other redox reactions involved in energy-related schemes (i.e., proton reduction to hydrogen and water oxidation to oxygen), that is, the need for a catalyst in order to proceed at sustainable rates. Moreover, the reduction of CO 2 faces an additional selectivity complication, since several products can be formed, including carbon monoxide, formic acid/formate, methanol, and methane. In this Mini-Review, we will discuss iron-based molecular catalysts that catalyze the reduction of CO 2 , focusing in particular on the selectivity of the processes, which is rationalized and guided on the basis of the reaction mechanism. Inspired by the active sites of carbon monoxide dehydrogenases, several synthetic systems have been proposed for the reduction of CO 2 ; these are discussed in terms of key intermediates such as iron hydrides or Fe-CO 2 adducts, where the ligand coordination motif, together with the presence of co-additives such as Brønsted acids, nucleophiles, or CO 2 trapping moieties, can guide the selectivity of the reaction. A mechanistic comparison is traced with heterogeneous iron single-atom catalysts. Perspectives on the use of molecular catalysts in devices for sustainable reduction of CO 2 are finally given.

Iron complexes with a N2O2‐type N,N′‐bis(salicylaldehyde)‐1,2‐phenylenediamine salophen ligand catalyze the electrochemical reduction of CO2 to CO in acetonitrile with phenol as the proton donor, giving rise to 90–99 % selectivity, faradaic efficiency up to 58 %, and turnover frequency up to 103 s−1 at an overpotential of 0.65 V. This novel class of molecular catalyst for CO2 reduction operate through a mononuclear FeI intermediate, with phenol being involved in the process with first‐order kinetics. The molecular nature of the catalyst and the low cost, easy synthesis and functionalization of the salophen ligand paves the way for catalyst engineering and optimization. Competitive electrodeposition of the coordination complex at the electrode surface results in the formation of iron‐based nanoparticles, which are active towards heterogeneous electrocatalytic processes mainly leading to proton reduction to hydrogen (faradaic efficiency up to 80 %) but also to the direct reduction of CO2 to methane with a faradaic efficiency of 1–2 %.

The utilization of carbon dioxide as a raw material represents nowadays an appealing strategy in the renewable energy, organic synthesis, and green chemistry fields. Besides reduction strategies, carbon dioxide can be exploited as a single-carbon-atom building block through its fixation into organic scaffolds with the formation of new C-C bonds (carboxylation processes). In this case, activation of the organic substrate is commonly required, upon formation of a carbanion C−, being sufficiently reactive toward the addition of CO2. However, the prediction of the reactivity of C− with CO2 is often problematic with the process being possibly associated with unfavorable thermodynamics. In this contribution, we present a thermodynamic analysis combined with density functional theory calculations on 50 organic molecules enabling the achievement of a linear correlation of the standard free energy (ΔG0) of the carboxylation reaction with the basicity of the carbanion C−, expressed as the pKa of the CH/C− couple. The analysis identifies a threshold pKa of ca 36 (in CH3CN) for the CH/C− couple, above which the ΔG0 of the carboxylation reaction is negative and indicative of a favorable process. We then apply the model to a real case involving electrochemical carboxylation of flavone and chalcone as model compounds of α,β-unsaturated ketones. Carboxylation occurs in the β-position from the doubly reduced dianion intermediates of flavone and chalcone (calculated ΔG0 of carboxylation in β = −12.8 and −20.0 Kcalmol-1 for flavone and chalcone, respectively, associated with pKa values for the conjugate acids of 50.6 and 51.8, respectively). Conversely, the one-electron reduced radical anions are not reactive toward carboxylation (ΔG0 > +20 Kcalmol-1 for both substrates, in either α or β position, consistent with pKa of the conjugate acids < 18.5). For all the possible intermediates, the plot of calculated ΔG0 of carboxylation vs. pKa is consistent with the linear correlation model developed. The application of the ΔG0 vs. pKa correlation is finally discussed for alternative reaction mechanisms and for carboxylation of other C=C and C=O double bonds. These results offer a new mechanistic tool for the interpretation of the reactivity of CO2 with organic intermediates.

Dye‐sensitized photoanodes for C−H activation in organic substrates are assembled by vacuum sublimation of a commercially available quinacridone (QNC) dye in the form of nanosized rods onto fluorine‐doped tin oxide (FTO), TiO2, and SnO2 slides. The photoanodes display extended absorption in the visible range (450–600 nm) and ultrafast photoinduced electron injection (<1 ps, as revealed by transient absorption spectroscopy) of the QNC dye into the semiconductor. The proton‐coupled electron‐transfer reactivity of QNC is exploited for generating a nitrogen‐based radical as its oxidized form, which is competent in C−H bond activation. The key reactivity parameter is the bond‐dissociation free energy (BDFE) associated with the N⋅/N−H couple in QNC of 80.5±2.3 kcal mol−1, which enables hydrogen atom abstraction from allylic or benzylic C−H moieties. A photoelectrochemical response is indeed observed for organic substrates characterized by C−H bonds with BDFE below the 80.5 kcal mol−1 threshold, such as γ‐terpinene, xanthene, or dihydroanthracene. This work provides a rational, mechanistically oriented route to the design of dye‐sensitized photoelectrodes for selective organic transformations.

The characterization of competent intermediates of metal complexes, involved in catalytic transformations for the activation of small molecules, is an important target for mechanistic comprehension and catalyst design. Iron complexes deserve particular attention, due to the rich chemistry of iron that allows their application both in oxidation and reduction processes. In particular, iron complexes with tetradentate Schiff base ligands show the possibility to electrochemically generate FeI intermediates, capable of reacting with carbon dioxide. In this work, we investigate the electronic and spectroscopic features of FeI intermediates in five Fe(LN2O2) complexes, and evaluate the electrocatalytic reduction of CO2 in the presence of phenol (PhOH) or trifluoroethanol (TFE) as proton donors. The main findings include: (i) a correlation of the potentials of the FeII/I couples with the electronic character of the LN2O2 ligand and the energy of the metal-to-ligand charge transfer absorption of FeI species (determined by spectroelectrochemistry, SEC-UV/Vis); (ii) the reactivity of FeI species with CO2, as proven by cyclic voltammetry and SEC-UV/Vis; (iii) the identification of Fe(salen) as a competent homogeneous electrocatalyst for CO2 reduction to CO, in the presence of phenol or trifluoroethanol proton donors (an overpotential of 0.91 V, a catalytic rate constant estimated at 5 × 104 s−1, and a turnover number of 4); and (iv) the identification of sudden, ligand-assisted decomposition routes for complexes bearing a ketylacetoneimine pendant, likely associated with the protonation under cathodic conditions of the ligands.