Metallomicelles made of Ni(II) and Zn(II) complexes of 2-pyridinealdoxime-based ligands as catalysts of the cleavage of carboxylic acid esters

Ligands featuring a 6-alkylaminomethyl-2-pyridinealdoxime moiety (alkyl = CH(3), 2a, or n-C(12)H(25), 2b) have been synthesized, and the reactivity of their Ni(II) and Zn(II) complexes in the cleavage of p-nitrophenyl acetate (PNPA) and hexanoate (PNPH) has been investigated in the absence (2a) or in the presence (2b) of CTABr micelles. The micellar complexes are effective in promoting the cleavage of the substrate with accelerations strongly dependent on the pH, being larger in moderately acidic than in neutral solutions. At pH 5 the Ni(II)2b/CTABr micelles increase the rate of the cleavage of PNPH by 3 orders of magnitude as compared to CTABr only and by 2 orders of magnitude as compared to the nonmicellar complex of 2a. Moreover the system is truly catalytic with a turnover rate approaching that of the cleavage. Analysis of the second-order rate constants allows the conclusions that the increased reactivity of the micellar system is due to concentration and local-pH effects and not to the activation of the nucleophile. A comparison of the reactivity of the systems made of complexes of 2 with that of the analogues of the 2-pyridineketoxime ligands (1) previously investigated indicates that the insertion of the new chelating atom in the binding subsite, on one hand, increases the formation constant of the metal ion complexes and, on the other hand, decreases the nucleophilicity of the complexed oximate ion. The balance of these two effects, in the operative conditions, favors ligands 2 over ligands 1 due to the higher fraction of complexed oximes.