DIVALENT METAL-ION CATALYZED-HYDROLYSIS OF N-METHYL-4'-NITRO-6-CARBOXYPICOLINANILIDE - PH-RATE PROFILE TRANSITIONS AND BUFFER INHIBITION

The hydrolysis of N-methyl-4'-nitro-6-carboxypicolinanilide (1) was st udied in water at 40.0 OC in the absence and presence of Cu(II), Zn(II ), Ni(II), and Co(II), and the results are compared to those obtained for the des-6-carboxy compound (2). The ''spontaneous'' hydrolysis exh ibits a linear unit slope pH-rate profile between pH 9 and 11, indicat ive of a straightforward HO--dependent reaction. Parallel, leftward-sh ifted lines in the pH-rate profile are seen in the presence of divalen t metal ions between pH 5 and 7, implicating the dominance of a M(II)- catalyzed HO- reaction in this pH range. The dimensionless rate enhanc ement factors for 1 at saturating concentrations of M(II) are about 2 x 10(4) for Ni(II), 4 X 10(4) for Co(II), 7 X 10(5) for Zn(II), and 1 X 10(6) for Cu(II). There is little difference between the kinetic beh aviors of 1 and 2 except that the carboxylate ligand in 1 results in s aturation at lower M(II) concentrations and permits the kinetics to be studied over a wider pH range, though the intrinsic catalytic effecti veness of the bound metal ion is actually somewhat reduced. The produc t pyridine-2,6-dicarboxylic acid binds M(II) more strongly than does t he reactant 1, but not by a sufficient margin to preclude the observat ion of true catalytic reactions (to 100% hydrolysis) using as little a s 0.1 equiv of Cu(II) and Zn(II), though there is a progressive slowin g in rate at high percent conversion as the amount of M(II) used decre ases. Inhibition of the M(II)-catalyzed hydrolysis by so-called ''biol ogical buffers'' is seen above pH 7.5, to a degree which increases wit h decreasing Lewis acidity of M(II). The switchover from pH-dependent to pH-independent behavior for Cu(II) catalysis at pH similar to 7 app ears to reflect titration of the metal-bound water, producing 1-Cu(II) -OH or a kinetically equivalent form as the species undergoing hydroly sis.