Kinetics and mechanism of the catalytic dehydration of HCO₃⁻ by zinc(II) complexes of tripod ligands

Abstract

The kinetics of the catalyzed dehydration of HCO 3 − by zinc(II) containing tripod complexes has been studied at 25 • C using the stopped-flow technique. The direction of reaction curve was changed in aqueous solution when the pH of the solution was greater than 7.5. The pH-profile of rates of the dehydration reactions indicates that only the aqua complex catalyzes the dehydration of HCO 3 − via a ligand substitution process. The second-order rate constants for the dehydration of HCO 3 − catalyzed by complexes Zn 3 L1, Zn 3 L2, Zn 3 L3, and Zn 3 L4 are 0.96, 2.53, 12.05, and 6.99 mol −1 dm 3 s −1 respectively. At the same time, the pK a values 7.60, 7.16, 7.51, and 7.42 for the deprotonation of the Zn(II)-bound water in the four catalysts were obtained, which are consistent with those that resulted from pH titrations, i.e. 7.47, 7.25, 7.52, and 7.38 respectively. The mechanism is proposed and the results are compared with other model complexes of carbonic anhydrase. C

26 Mechanisms of reactions in solution

“…148 In another study, dehydration of HCO 3 2 by Zn(II) complexes having tripod ligands has been found be catalysed solely by the aqua complex. 149…”