Solid–Liquid Phase Structural Studies of Bis(2-Picolyl)Amine-Based Zinc(II) Complexes as Functional Hydrolase Models: The Detoxification of Fenitrothion

The reaction of the tridentate ligand bis(2-picolyl)amine (BPA) with the chloride or perchlorate salts of zinc(II) ions yielded two zinc(II) complexes of the types: [(BPA)ZnCl2]·2H2O (1) and [(BPA)2Zn](ClO4)2 (2). The intermediate complex species, namely [(BPA)Zn(OH)(H2O)](ClO4) (3) and [(BPA)Zn-OP(= S)(CH3)2] (4) were successfully isolated separately in good yields and pure forms. Complexes 1–4 were characterized in the solid state by using elemental analyses, thermal analyses, FT-IR, and X-ray single crystal diffraction as well as in solution by using 1H NMR titration and cyclic voltammetry (CV). From X-ray structural and spectral features it can be concluded that the coordination environment around Zn(II) ion in 1 is a distorted square pyramidal geometry with τ5 value of 0.148. Whereas in 2, two picolyl chelates in the equatorial plane and two slightly longer axial pyridines are coordinated in a distorted octahedral trans-facial arrangement. Kinetic studies were performed on these complexes to evaluate their efficiency as functional hydrolase models for the hydrolysis of the toxic organophosphate insecticide, Fenitrothion. The pH-dependence of the catalytic rate gives a bell-shaped curve, which gives a strong proof that the hydroxoaqua complex [(BPA)Zn(H2O)(OH)]− 3 is the catalytically active species in the hydrolysis reactions than that of the octahedral [(BPA)2Zn]2+ with fully saturated coordination sphere.