Mechanistic insights into the electrochemical reduction of CO2 to CO on Ni(salphen) complexes

Abstract

Cyclic voltammetry and bulk electrolysis showed that [Ni(II)(salphen)] [1], [Ni(II)(^tBu-salphen)] [2], and a binuclear Ni(II) compound combining salphen and ^tBu-salphen [3] react with CO₂ to yield a metal–carbonyl species that is stable under an oxygen free atmosphere. Upon exposure to air, a stoichiometric amount of CO is released (detected by gas chromatography) and protonation regenerates the initial complex. To shed light on the mechanism of CO₂ reduction and O₂-dependent CO release by [1], UV-vis, EPR and SEC-IR spectroscopy studies complemented with DFT calculations were performed. It is proposed that the mono reduced [Ni(I)(salphen)]⁻, ²[1]⁻, formed a CO₂ complex, ²[1(CO₂)]⁻, which was then further reduced to ³[1(CO₂)]²⁻. After addition of two protons, the coordinated CO₂ was reduced to CO and released, regenerating ¹[1]. Alternatively, ²[1(CO₂)]⁻ is protonated and then reduced to the same intermediate as before, continuing the same way. In the second cycle, the CO released competed with CO₂ and coordinated to ²[1]⁻ much more strongly, thereby deactivating the system. The new ²[1(CO)]⁻ was reduced to ³[1(CO)]²⁻ which was identified by comparison of experimental spectroscopic (UV-vis, EPR, SEC-IR) data with DFT calculated parameters.