A Ni-II complex of the tetradentate salen ligand H2L2NH comprising an anchoring -NH2 group: synthesis, characterization and electrocatalytic CO2 reduction to alcohols

摘要

A new salen ligand with an anchoring free amine functional group in the ligand backbone and the corresponding Ni-II complex [(NiL2NH)-L-II] (1) were synthesized and characterized. The complex effectively reduced CO2 to C-1 (HCOOH, CH3OH) and C-2 (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 C-2 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.