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 %.

An ew FeL/Fe 2 L 2 manifold,w ith HL = 2-({[di(2-pyridyl)methyl]-(methyl)amino}methyl)phenol, was preparedi ng ram scale (> 50 %y ield) and characterized in solution and solid state. The monomer/dimer interconversion is controlled in aqueous phase, upon varying the pH conditions. The electrocatalytic hydrogen evolution reaction (HER) occurs through the FeL monomer with added trifluoroacetic acid (TFA)a nd through the Fe 2 L 2 m-oxo dimer in acetate buffer (pH 4.9), with an overpotential of about 1Vand faradaic yield up to 75 %. The resulting i cat /i p valuesi nthe range 15-28a re among the highest reported for Fe-based electrocatalysts (i cat is the catalytic current, whereas i p is the current of an Fe-based redox event).Iron is the most abundant transition metal on earth, and nature employs this element as the active co-factor of several proteins. [1] Following bioinspired guidelines, vis-à-vist heir vast redox chemistry,s ynthetic Fe complexes have been intensively investigated as catalysts in some fundamental oxidation and reduction processes, including selective oxygen transfer, [2] catechol dioxygenation, [3] water oxidation, [4] neutralization of reactive oxygen species( ROS), [5] and proton/water reduction to hydrogen,g enerally described as the hydrogen evolution reaction (HER). [6] In this latter case, recent efforts have been directed towardt he modelling of the dinucleara nd mononuclear active site of [FeFe]-, [NiFe]-,a nd [Fe]-hydrogenase enzymes (H 2 ase mimetics, see Ta ble S1 in the Supporting Information). [7, 8] Most of the resulting complexes display H 2 ase-like activity in organic solvents, with acidic additives,w hereas systems operating in aqueous media are still rare. [7, 8] Interestingly,F e II and Fe III single-site complexes with porphyrins, [9] fluorinated di-glyoxime, [10] and polypyridyl ligands were also reported as active HER catalysts. [11] In particular,a tetradendate N 3 Odonorset provided by apolypyridyl platform was recently exploited to stabilize mononuclear Fe III catalysts for HER in aqueous media. [11] Figures of merit include am oderate overpotential (h,i nt he range 0.66-0.80 V) in CH 3 CN/TFA (TFA: trifluoroacetic acid) solutions as well as in aqueous buffers (pH 3-5) and an i cat /i p (i cat is the catalytic current, whereas i p is the currento fa nF e-basedr edox event) ratio up to 13. [11b] Photo-assisted H 2 evolution has been reported in ethanols olutions with fluoresceina st he photosensitizer,r eaching up to a turnover number of 2100 for the Fe catalyst, with an overall 3% quantum yield. [11c] Therefore, this class of polydentate ligands appears as ap romising one for optimizing the HER performance in water owing to their versatile coordination chemistry and tunable stereoelectronic impact.Here, we address the HERb ym ononuclear and dinuclear Fe III electrocatatalysts, originating from pH-triggered equilibria in the presenceo fl igand HL = 2-({[di(2-pyridyl)methyl](methyl)amino}methyl)phenol (Scheme 1). Thisp olydentate ligand has shown ap rominentb...