Synthesis, Properties, and Reactions of Trinuclear Macrocyclic Nickel(II) and Nickel(I) Complexes: Electrocatalytic Reduction of CO2 by Nickel(II) Complex

The trinuclear NiII complex of trismacrocyclic ligand 1, [Ni3(1)](ClO4)6, where 1 is 8,8′,8"‐(2,2′,2"‐nitrilotriethyl)tris(1,3,6,8,10,13,15‐heptaazatricyclo[11.3.1.13,15]octadecane), has been synthesized by template condensation reactions and shown to have poor axial binding ability. Cyclic voltammetry of the complex shows a three‐electron reversible reduction at −1.59 V and an irreversible oxidation at +1.25 V vs. Ag/Ag+ (0.01 M). Differential pulse voltammetry and Osteryoung square‐wave voltammetry indicate that the three NiII ions are electrochemically independent. A single crystal was obtained as [Ni3(1)](ClO4)3·Cl3·10H2O and its X‐ray structure was determined. When [Ni3(1)](ClO4)6 was reduced to a NiI species by reaction with Na(Hg) in MeCN, [Ni3(1)](ClO4)3 was isolated. [Ni3(1)](ClO4)6 shows catalytic activity for the electrochemical reduction of CO2 to CO in MeCN/H2O (9:1, v/v), as revealed by cyclic voltammetry and controlled‐potential coulometry. The catalytic efficiency of [Ni3(1)]6+ is lower than that of [Ni(cyclam)]2+. It gives a greater ip(CO2)/ip(N2) value but produces less CO per nickel center (3: 8‐methyl‐1,3,6,8,10,13,15‐heptaaza‐tricyclo[11.3.1.13,15]octadecane) compared with monometallic [Ni(3)]2+. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)