THERMODYNAMICS AND KINETICS OF CARBON-DIOXIDE BINDING TO 2 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 (epsilon-1 x 10(4) M-1 cm-1) absorption bands at 630 and 635 nm, respectively. The product absorptions are much less intense (epsilon-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 asign 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 absorption or equilibration kinetics) at 25-degrees-C are CoL(CO2)+, 2.5 x 10(8) M-1 (prim-N-rac), 6.0 x 10(6) M-1 (N-meso); CoL(CO)+, 1.6 x 10(8) M-1 (prim-N-rac), 0.8 x 10(8) M-1 (N-meso); CoL(H)2+, 2.5 x 10(11) M-1 (prim-N-rac), greater-than-or-equal-to 7.9 x 10(13) 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-degrees-C: CO2, 1.7 x 10(8) (prim-N-rac), 1.5 x 10(7) (N-meso); CO, 5.0 x 10(8) (prim-N-rac), 8.3 x 10(8) (N-meso); H+, 3.0 x 10(9) (prim-N-rac), 2.3 x 10(9) (N-meso).