The activation of very basic nucleophilic groups, such as –OH, –SH or water, by coordination to a non-redox active metal cation allows hydrolytic metalloenzymes to have large acceleration effects of up to 1010 in ester hydrolysis reactions at physiological pH. Very efficient artificial enzymes employing tridentated oximes can be developed on this basis, as we have recently reported. Amidoximes are basic alpha nucleophiles, showing reactivity in their anionic as well as in their neutral forms, therefore the incorporation of these strong nucleophiles into appropriate ligands and the subsequent coordination with metal cations may also lead to catalytic systems which have not been investigated yet. Here we report the esterolytic reactivity of Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II) metal complexes of a tridentate amidoxime ligand 1 and of a tetradentated bisoxime ligand 2 as catalysts for the hydrolysis of a model carboxylic ester substrate (4-nitrophenyl acetate) and environmentally relevant phosphate triesters (paraoxon and parathion). These ligands reduce their oxime pKa by coordination, increasing the coordinated nucleophile fraction. As a result, esterolytic reactivity is observed from slightly acidic to basic media in water at 25°C. Potentiometric and spectrophotometric titrations were performed in order to determine the acidity and stability constants of the ligands and of their metal complexes. Kinetic studies were conducted varying concentration, metal-to-ligand ratio and pH dependence as to establish the reactivity of the ligands and the effect of the divalent metal cations. Species distribution diagrams were correlated with kinetic data to identify active species in solution, [MHL], [ML] and [ML(OH)] depending on the metal ion. Second order rate constants were determined for each complex and the reactivity of amidoximate in contrast to oximate metal complexes is discussed.

Metal oximate and amidoximate complexes for the catalytic cleavage of carboxylic and phosphate esters

2019 Fall National Meeting

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