Thermodynamics and kinetics of carbon dioxide binding to two stereoisomers of a cobalt(I) macrocycle in aqueous solution

The thermodynamics and kinetics of binding of CO₂, CO, and H⁺ to N-racemic and N-meso stereoisomers of the cobalt(I) macrocycle CoL⁺ (L = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) have been determined in aqueous media with use of the pulse radiolysis technique and transient ultraviolet-visible spectroscopy. N-rac- or N-meso-CoL⁺ was produced by the hydrated electron reduction of N-rac- or N-meso-CoL²⁺, with tert-butyl alcohol generally added to scavenge hydroxyl radicals. Reactions of both N-rac- and N-meso CoL⁺ are readily followed by the disappearance of intense (ε 1 × 10⁴ M^-1 cm^-1) absorption bands at 630 and 635 nm, respectively. The product absorptions are much less intense (ε 200-500 M^-1 cm^-1) and shifted to higher energy (400-500 nm). Carbon dioxide adducts were also produced via the reactions of N-rac- or N-meso-CoL²⁺ with formate radical ^.CO₂ ^- in solutions containing sodium formate and CO₂ and hydrogen ion adducts (hydrides) by the reactions with ^.H atom in acid solution. The known stereochemistry of the methyl radical additions to N-rac- and N-meso-CoL²⁺ was used to assign the different isomers of the adducts. Thus, reaction of ^•H and ^•CO²⁺ with N-rac-CoL²⁺ yields sec-N-rac-CoL(H)²⁺ and sec-N-rac-CoL(CO₂)⁺, respectively, and the reaction of N-rac-CoL⁺ with H⁺ and CO₂ yields prim-N-rac-CoL(H)²⁺ and prim-N-rac-CoL(CO₂)⁺, respectively. As expected, only N-meso adducts are obtained from either N-meso-CoL⁺ or N-meso-CoL²⁺. Formation constants for the complexes (determined by analysis of equilibrium absorptions or equilibration kinetics) at 25°C are CoL(CO₂)⁺, 2.5 × 10⁸ M^-1 (prim-N-rac), 6.0 × 10⁶ M^-1 (N-meso); CoL(CO)⁺, 1.6 × 10⁸ M^-1 (prim-N-rac), 0.8 × 10⁸ M^-1 (N-meso); CoL(H)²⁺, 2.5 × 10¹¹ M^-1 (prim-N-rac), ≥7.9 × 10¹³ M^-1 (N-meso). The rate constants are large and generally parallel the stability of the adduct. The following are values of k (M^-1 s^-1) for addition at 25°C: CO₂, 1.7 × 10⁸ (prim-N-rac), 1.5 × 10⁷ (N-meso); CO, 5.0 × 10⁸ (prim-N-rac), 8.3 × 10⁸ (N-meso); H⁺, 3.0 × 10⁹ (prim-N-rac), 2.3 × 10⁹ (N-meso).