Metal Ion Catalysis in the Hydrolysis of Esters of 2-Hydroxy-1,10-phenanthroline: The Effects of Metal Ions on Intramolecular Carboxyl Group Participation

Abstract

Rate constants have been determined for hydrolysis of the acetate, glutarate, and phthalate monoesters of 2-hydroxy-1,10-phenanthroline in water at 30°C and μ = 0.1 M with KCl. The hydrolysis reactions of the esters are hydroxide ion catalyzed at pH > 9. The phthalate and glutarate monoesters have in addition pH-independent reactions from pH 5.5 to 9 that involve intramolecular participation by the neighboring carboxylate anion. The pH-independent reaction of the glutarate monoester is ∼5-fold faster than that of the phthalate monoester. The plots of log kobsd vs pH for hydrolysis of the carboxyl substituted esters are bell shaped at pH < 5, which indicates a rapid reaction of the zwitterionic species (carboxyl anion and protonated phenanthroline nitrogen). The divalent metal ions, Cu2+, Ni2+, Zn2+, and Co2+, complex strongly with the esters; saturation occurs at metal ion concentrations less than 0.01 M. The 1:1 metal ion complexes have greatly enhanced rates of hydrolysis; the second-order rate constants for the OH− reactions are increased by factors of 105 to 108 by the metal ion. The pH-rate constant profiles for the phthalate and glutarate ester metal ion complexes have a sigmoidal region below pH 6 that can be attributed to a metal ion-promoted carboxylate anion nucleophilic reaction. The carboxyl group reactions are enhanced 102 - to 103 -fold by the metal ions, which allows the neighboring group reaction to be competitive with the favorable metal ion-promoted OH− reaction at pH < 6, but not at pH > 6. The half-lives of the pH-independent neighboring carboxyl group reactions of the Cu(II) complexes at 30°C are ∼l2 s. The other metal ion complexes are only slightly less reactive (half-lives vary from 2.5 to 40 s). These are the most rapid neighboring carboxyl group reactions that have been observed in ester hydrolysis.