Kinetics and mechanism of ester hydrolysis by metal complexes of 2,6-diacetylpyridine dioxime

The rate constants of the cleavage of 4-nitrophenyl acetate by the 2,6-diacetylpyridine dioxime (H(2)dapd) in pH interval 6-8 increase 10(3)-10(4) times in the presence of Pb(II), Mn(II) and Cd(II), and 2-100 times in the presence of Ni(II), Hg(II), Pr(III) and Zn(II). Complexation of the oxime ligand with Co(II) and Cu(II) does not affect the reaction rate. The complex with Cd(II) operates in a catalytic regime with respect of the oxime ligand. Spectrophotometric study of the coordination of Cd(II) and Zn(II) with H(2)dapd reveals formation of M(H(2)dapd)(2+), M(Hdapd)(+) and M-2(Hdapd)(2)(2+) complexes. Formation constants of these complexes as well as acid dissociation constants of free and complexed H(2)dapd were calculated from spectrophotometric titration results. The reactive species are monomeric complexes of the type M(Hdapd)(+) whereas the dimers are unreactive. The reactivity of M(Hdapd)(+) complexes correlates with basicity of the coordinated oximate anion. The complexes of Cd(II), Mn(II) and Pb(II) unexpectedly possess higher reactivity than much more basic free oximate anion probably because of stabilisation of carbonyl oxygen by chelation with metal cation in the transition state. (C) 1998 Elsevier Science S.A. All rights reserved.