The binding of CO_2 to metal macrocycles has been determined electrochemically by using cyclic voltammetry or differential pulse polarography. The CO_2 binding constants, K for a series of Co(I) tetraammacrocycle complexes in (CH3)_2SO showed a strong correlation with the Co(II/I) redox potential. Although meso and d,l stereoisomers of [Co^I(Me_6[14]4,11- diene)]^+ had identical redox potentials, K_(CO_)2 differed by a factor of 10^2 for these stereoisomers, suggesting a large steric effect on CO_2 binding. Binding of CO_2 to Ni(I) tetraazamacrocycles in (CH_3)_2SO yielded a redox potential dependence similar to that of Co(I) macrocycles. A weak correlation between K_(CO_2) and the solvent dielectric constant was found for d,l-[Co(Me_6[ 14]-4,11 -diene)]^+. In tetrahydrofuran (THF), d,l-[CO(Me_6[14]4,11-diene)]^+ was found to bind CO_2 more strongly in the presence of 0.1 M Li^+ than in the presence of 0.1 M tetrabutylammonium (TBA^+). Electrochemically determined K_(CO_2)'s for Co(salen)^- and Co(Me_2salen)^- in THF indicated a strong dependence on redox potential and electrolyte cation.
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Schmidt et al. (1990) studied this question.