Effects of ligand structure on the adsorption of nickel tetraazamacrocyclic complexes and electrocatalytic CO2 reduction

N-methyl substituted 1,4,8,11-tetraazacyclotetradecanes were used as ligands for Ni(II) in order to study the relationship between the structure of the complex and its catalytic activity in the electrochemical reduction of carbon dioxide. Systematic increase of N-methyl substitution in [Ni(cyclam)](2+) was found to increase the adsorption of the Ni(II) complex on the mercury electrode and at the same time decreased the stability of the reduced Ni(I) form. Binding of CO2 requires the catalyst to be in the stable, adsorbed and reduced trans I (or RSRS) Ni(I) form. For the tetramethylated derivative the strong adsorption of the Ni(II) complex was found to make its reduction to the catalytically active Ni(I) form impossible. This explains the lack of CO2 catalytic reduction when the tetraazamacrocyclic ligand is fully substituted by methyl groups. (C) 1998 Elsevier Science S.A. All rights reserved.