Synthesis, Properties, and Reactions of Trinuclear Macrocyclic Nickel(II) and Nickel(I) Complexes: Electrocatalytic Reduction of CO_2 by Nickel(II) Complex

摘要

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~+ (0.01M). 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_2O and its X-ray structure was determined. When [Ni_3(1)](ClO_4)_6 was reduced to a Ni~1 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_2O (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+).