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

Abstract

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,l stereoisomers of [Coᴵ(Me₆[14]4,l 1-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.

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