Carol Creutz, Harold A. Schwarz, James F. Wishart, Etsuko Fujita and Norman Sutin
J. Am. Chem. Soc. 113, 3361-3371 (1991)
Abstract:
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 104 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 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 deg.C are CoL(CO2)+, 2.5 x 108 M-1 (prim-N-rac), 6.0 x 106 M-1 (N-meso); CoL(CO)+, 1.6 x 108 M-1 (prim-N-rac), 0.8 x 108 M-1 (N-meso); CoL(H)2+, 2.5 x 1011 M-1 (prim-N-rac), >=7.9 x 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-1s-1) for addition at 25 deg.C: CO2 1.7 x 108 (prim-N-rac), 1.5 x 107 (N-meso); CO, 5.0 X 108 (prim-N-rac), 8.3 x 108 (N-meso); H+, 3.0 x 109 (prim-N-rac), 2.3 x 109 (N-meso).