Metal Ion Catalysis in the Hydrolysis of Esters of 2-Hydroxy-1,10-phenanthroline: The Effects of Metal Ions on Intramolecular Carboxyl Group Participation.
Rate constants have been determined for hydrolysis of the acetate, glutarate, and phthalate monoesters of 2-hydroxy-1,10-phenanthroline in water at 30 degrees C and &mgr; = 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 approximately 5-fold faster than that of the phthalate monoester. The plots of log k(obsd) 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, Cu(2+), Ni(2+), Zn(2+), and Co(2+), 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 10(5) to 10(8) 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 10(2) - to 10(3) -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 degrees C are approximately 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. Copyright 2000 Academic Press.
Fife
Thomas H.
TH
Department of Biochemistry, University of Southern California, Los Angeles, California, 90033
Pujari
M. P.
MP