KINETICS AND MECHANISM OF DECARBOXYLATION OF HYDROGEN CARBONATE COORDINATED TO A MACROCYCLIC CHROMIUM(III) COMPLEX - A COMPARISON WITH THE CARBONIC-ANHYDRASE ENZYMES

Decarboxylation of the cis-[Cr(cycb)F(OCOOH)]+ complex, cycb = rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, has been investigated at a range of temperatures and hydrogen ion concentrations in 1 M (Na,H)(Br,OH). The relative robustness of hydrogen carbonate complexes towards decarboxylation has been correlated with the acid dissociation constants of the coordinated hydrogen carbonate ligand. It is suggested that the rate-determining step for the decarboxylation reaction is proton transfer from an uncoordinated oxygen atom to the coordinated oxygen atom of the carbonate ligand. A unified mechanism for the decarboxylation path of both the carbonic anhydrase enzymes and simple carbonate complexes is presented.