Origin of the hydrogen-deuterium kinetic isotope effect for porphyrin metalation

Article abstract of DOI:10.1021/ic50217a053

It has been previously observed that the metalation of a porphyrin is slower in D₂O than H₂O. Discussion of this effect has focused on the role of dissociation of the hydrogen or deuterium atoms bound at the coordination site of the porphyrin. The importance of proton dissociation in determining the overall rate of porphyrin metalation is an essential feature of the intimate reaction mechanism. We have monitored the reaction of tetraphenylporphyrin with Zn(II) using protonated and deuterated forms of the porphyrin free base in dimethylformamide. The rates were found to be identical. The addition of either H₂O or D₂O increases the reaction rate, but the effect of H₂O is greater, leading to a k_H/k_D ratio of 2.3 at high [H₂O] and [D₂O]. These results and the previously reported hydrogen-deuterium kinetic isotope effects found in aqueous solution can be explained by a mechanism involving a hydroxo complex of the metal ion. The isotope effect arises from a difference in the concentrations of reactive metal complexes, not from a difference in the reactivities of protonated and deuterated free-base porphyrins.