Synthesis and characterization of phenoxo-bridged dinuclear zinc(II) halide complexes: Role of electron withdrawing group on phosphatase activity

Abstract

Three phenoxo-bridged dinuclear zinc(II) halide complexes $$[\hbox {Zn}(\mathbf{L2})(\hbox {X})]_{2}$$ (X = Cl, Br, I) were isolated from a reduced Schiff base [HL2 = 2-(((2-(dimethylamino)ethyl)amino)methyl)-4-nitrophenol]. Ligand and complexes were characterized using different physico-chemical and spectroscopic methods. Structural study confirms that $$[\hbox {Zn}(\mathbf{L2})(\hbox {Cl})]_{2}$$ contains a discrete dinuclear unit where two zinc(II) ions are bridged by the two phenoxo oxygen atoms of the deprotonated ligand (L2). The complex adopts a rare cis-orientation with respect to chloride ions. The rate of hydrolysis of 4-NPP ((4-nitrophenyl)phosphatehexahydrate) in the presence of zinc(II) complexes was measured by initial rate method giving $$\hbox {k}_{\mathrm{cat}}$$ values in the order $$\hbox {I}>\hbox {Cl}>\hbox {Br}$$ and mechanistic pathway was monitored using ESI-mass spectroscopy. It is worth mentioning that presence of an electron withdrawing $$-\hbox {NO}_{2}$$ group in HL2 eventually increases the Lewis acidity of zinc(II) which in turn reduces the nucleophilic character of metal bound water molecule in the intermediate state causing an overall decrease in the rate of hydrolysis of 4-NPP as compared to our previous work with HL1 (2-((2-(dimethylamino)ethylamino)methyl)phenol). Three new dinuclear zinc(II) halide complexes were synthesized using a tridentate N,N,O-donor ligand and characterized using spectroscopic methods. Structural study of a representative complex reveals phenoxo-bridged dinuclear nature with cis-halide orientation. Complexes show good phosphatase mimicking behaviour in aqueous-DMF. The role of electron withdrawing $$-\hbox {NO}_{2}$$ on phosphatase activity is highlighted.