Effects of redox potential, steric configuration, solvent, and alkali metal cations on the binding of carbon dioxide to cobalt(I) and nickel(I) macrocycles

The binding of CO₂ to metal macrocycles has been determined electrochemically by using cyclic voltammetry or differential pulse polarography. The CO₂ binding constants, K_CO2, for a series of Co(I) tetraazamacrocycle complexes in (CH₃)₂SO showed a strong correlation with the Co(II/I) redox potential. Although meso and d,1 stereoisomers of [Co^I(Me₆[14]4,11-diene)]⁺ had identical redox potentials, K_CO2 differed by a factor of 10² for these stereoisomers, suggesting a large steric effect on CO₂ binding. Binding of CO₂ to Ni(I) tetraazamacrocycles in (CH₃)₂SO yielded a redox potential dependence similar to that of Co(I) macrocycles. A weak correlation between K_CO2 and the solvent dielectric constant was found for d,l-[Co(Me₆[14]-4,11-diene)]⁺. In tetrahydrofuran (THF), d,l-[Co(Me₆[14]4,11-diene)]⁺ was found to bind CO₂ more strongly in the presence of 0.1 M Li⁺ than in the presence of 0.1 M tetrabutylammonium (TBA⁺). Electrochemically determined K_CO2's for Co(salen)^- and Co(Me₂salen)^- in THF indicated a strong dependence on redox potential and electrolyte cation.