Carboxy Ester Hydrolysis Promoted by a Zinc(II) 2-[Bis(2-aminoethyl)amino]ethanol Complex: A New Model for Indirect Activation on the Serine Nucleophile by Zinc(II) in Zinc Enzymes

A complexation study on the new Zn(II) complexes of asymmetric tripodal ligand 2-[bis(2-aminoethyl)amino]ethanol (L) has revealed that the alcoholic OH group of complex ZnL exhibits remarkable acidity with a very
low pKa value of 7.7 at 25 °C. Both the monomeric complex [ZnH-0.25L(H2O)](ClO4)1.75 (I) and the dimeric
alkoxide-coordinating complex [Zn2(H-1L)2](ClO4)2 (II) were synthesized, and their structures were determined
by X-ray diffraction. The Zn(II)-bound alkoxide, as the reactive nucleopile toward the hydrolysis of esters, has
shown a second-order rate constant of 0.13 ± 0.01 M-1 s-1 in 10% (v/v) CH3CN at 25 °C in 4-nitrophenyl
acetate (NA) hydrolysis, which is almost the same as the corresponding value for the very reactive alcohol-pendent [12]aneN3−Zn complex. Present work shows for the first time that Zn(II) complexes of the asymmetric
tripodal polyamine bearing an ethoxyl pod can also serve as good models of Zn(II)-containing enzymes.