A NiII complex of the tetradentate salen ligand H2LNH2 comprising an anchoring –NH2 group: synthesis, characterization and electrocatalytic CO2 reduction to alcohols

Abstract

A new salen ligand with an anchoring free amine functional group in the ligand backbone and the corresponding Ni^II complex [Ni^IIL^NH₂] (1) were synthesized and characterized. The complex effectively reduced CO₂ to C₁ (HCOOH, CH₃OH) and C₂ (C₂H₅OH) products in a simple H-type reactor upon immobilization on the graphite electrode. Initially HCOOH was formed which was further reduced to CH₃OH and C₂H₅OH in a 6e⁻ and 12e⁻ transfer pathway. An overall current efficiency of 49% was recorded for the formation of alcohols at −1.80 V vs. Ag/AgCl in aqueous media (ERC time 1 h, pH 7.0 and KHCO₃ 0.5 M). Thus, the common limitation of the formation of deeply reduced C₂ products has been overcome. The effect of the anchoring free amine group was further established by examining the CO₂ reduction efficiency of the similar Ni^II complex without having the tethered –NH₂ group. The lack of alcohol production for this case highlighted the importance of the anchoring group with the electrodes during electrochemical reduction.