Fe I Intermediates in N 2 O 2 Schiff Base Complexes: Effect of Electronic Character of the Ligand and of the Proton Donor on the Reactivity with Carbon Dioxide

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 Fe I intermediates, capable of reacting with carbon dioxide. In this work, we investigate the electronic and spectroscopic features of Fe I intermediates in five Fe(L N2O2 ) complexes, and evaluate the electrocatalytic reduction of CO 2 in the presence of phenol (PhOH) or trifluoroethanol (TFE) as proton donors. The main findings include: (i) a correlation of the potentials of the Fe II/I couples with the electronic character of the L N2O2 ligand and the energy of the metal-to-ligand charge transfer absorption of Fe I species (determined by spectroelectrochemistry, SEC-UV/Vis); (ii) the reactivity of Fe I species with CO 2 , as proven by cyclic voltammetry and SEC-UV/Vis; (iii) the identification of Fe(salen) as a competent homogeneous electrocatalyst for CO 2 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 × 10 4 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.