Carbon Dioxide Electroreduction and Formic Acid Oxidation by Formal Nickel(I) Complexes of Di‐isopropylphenyl Bis‐iminoacenaphthene

Abstract

Processing CO2 into value‐added chemicals and fuels stands as one of the most crucial tasks in addressing the global challenge of the greenhouse effect. In this study, we focused on the complex (dpp‐bian)NiBr2 (where dpp‐bian is di‐isopropylphenyl bis‐iminoacenaphthene) as a precatalyst for the electrochemical reduction of CO2 into CH4 as the sole product. Cyclic voltammetry results indicate that the realization of a catalytically effective pattern requires the three‐electron reduction of (dpp‐bian)NiBr2. The chemically reduced complexes [K(THF)6]+[(dpp‐bian)Ni(COD)]– and [K(THF)6]+[(dpp‐bian)2Ni]– were synthesized and structurally characterized. Analyzing the data from the electron paramagnetic resonance study of the complexes in a solution, along with quantum‐chemical calculations, reveals that the spin density is predominantly localized at their metal centers. The superposition of trajectory maps of the electron density gradient field and the one‐electron electrostatic force field, along with the atomic charges, discloses that, within the first coordination sphere, the interatomic charge transfer occurs from the metal atom to the ligand atoms and that the complex anions can thus be formally described by the general formulas (dpp‐bian)2–Ni+(COD) and (dpp‐bian)2–Ni+. It was shown that the reduced nickel complexes can be oxidized by formic acid; resulting from this reaction, the two‐electron and two‐proton addition product dpp‐bian‐2H is formed.

Proton-assisted seven-electron acceptor properties of di-iso-propylphenyl-bis-iminoacenaphthene

“…For dpp-bian, only the stepwise four-electron electrochemical and chemical reduction is proposed today. 20,24–26 Moreover, on the CV, complex III has an irreversible reduction peak at the potential of −3.17 V, which means that the complex can accept at least one more electron under electrochemical reduction conditions (Fig. 1b, Table 2).…”

“…, the two-electron and two-proton reduction to dpp-bian. 25,26 Thus, it is evident that Ar-bians can accept both electrons and protons.…”