Synthesis, Properties, and Reactions of Trinuclear Macrocyclic Nickel( II ) and Nickel( I ) Complexes: Electrocatalytic Reduction of CO 2 by Nickel( II ) Complex

The trinuclear Ni II complex of trismacrocyclic ligand 1 , [Ni 3 ( 1 )](ClO 4 ) 6 , where 1 is 8,8′,8"‐(2,2′,2"‐nitrilotriethyl)tris(1,3,6,8,10,13,15‐heptaazatricyclo[11.3.1.1 3,15 ]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 Ni II ions are electrochemically independent. A single crystal was obtained as [Ni 3 ( 1 )](ClO 4 ) 3 ·Cl 3 ·10H 2 O and its X‐ray structure was determined. When [Ni 3 ( 1 )](ClO 4 ) 6 was reduced to a Ni I species by reaction with Na(Hg) in MeCN, [Ni 3 ( 1 )](ClO 4 ) 3 was isolated. [Ni 3 ( 1 )](ClO 4 ) 6 shows catalytic activity for the electrochemical reduction of CO 2 to CO in MeCN/H 2 O (9:1, v/v), as revealed by cyclic voltammetry and controlled‐potential coulometry. The catalytic efficiency of [Ni 3 ( 1 )] 6+ is lower than that of [Ni(cyclam)] 2+ . It gives a greater i p (CO 2 )/ i p (N 2 ) value but produces less CO per nickel center ( 3 : 8‐methyl‐1,3,6,8,10,13,15‐heptaaza‐tricyclo[11.3.1.1 3,15 ]octadecane) compared with monometallic [Ni( 3 )] 2+ . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)