The kinetics of the substitution of coordinated H2O on Co(III) by cyanide in aquacobalamin (vitamin B12a) and in a corrole analogue

Abstract

The kinetics of the substitution of H2O on Co(III) in aquacobalamin (H2OCbl+, vitamin B12a) and in a Co(III) corrole analogue of B12a, aqua-[10-(2-[[4-(1H-imidazol-1-yl-methyl)benzoyl]amino]phenyl)-5,15-diphenylcorrolato]-cobalt(III), [H2O-DPTC-Co], were determined in 80:20 MeOH/H2O and low ionic strength. The second order rate constant kII(25°C) for the reaction of H2OCbl+ is 2720M−1s−1 (ΔH‡=82(5) kJmol−1 and ΔS‡=96(19) JK−1mol−1) while for [H2O-DPTC-Co], kII(25°C)=990M−1s−1 (ΔH‡=41(1) kJmol−1 and ΔS‡=−50(3) JK−1mol−1). It is argued that differences in ΔS‡ are due to differential solvent electrostriction, arising from the different charges at the metal center in the two complexes. A smaller ΔH‡ for the reaction with [H2O-DPTC-Co] suggests that the transition state occurs earlier along the reaction coordinate, consistent with the higher affinity of Co(III) for CN− in the corrole. On this basis only, Co(III) is more labile towards CN− in the corrole than in the corrin. However, because of differences in ΔS‡, H2OCbl+ is more labile towards CN− than [H2O-DPTC-Co] above the isokinetic temperature of 8°C. The nature of the equatorial ligand in Co(III) macrocyclic complexes significantly affects the lability of the metal ion.