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

Cyclic voltammetry and bulk electrolysis showed that [Ni(ii)(salphen)] [1], [Ni(ii)(Bu-t-salphen)] [2], and a binuclear Ni(ii) compound combining salphen and Bu-t-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 O-2-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.