Phosphatase models: Synthesis, structure and catalytic activity of zinc complexes derived from a phenolic Mannich-base ligand

Abstract

A series of dinuclear [Zn2(L1)2X2] (1–3) and mononuclear [Zn(HL2)X2] complexes (4–6), (X=Cl, Br, I) were synthesised from two Mannich-base compartmental ligands, namely [bis(2-methoxyethyl)aminomethyl]-4-chlorophenol (HL1) and 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-chlorophenol (HL2), respectively. They were characterised by routine physicochemical techniques (CHN, UV, IR, ESI-MS and NMR) and complex 2–5 was further structurally characterised by single crystal X-ray analysis where the Zn…Zn bond-distance is 3.10–3.12Å. All the quintessential complexes exhibit excellent phosphatase activity and the experimental first order rate constant values (kcat) for the hydrolysis of 4-nitrophenyl phosphate ester (PNPP) reaction in methanol are in the range from 1.05 to 214s−1 at 25°C evaluated by monitoring spectrophotometrically the gradual release of p-nitrophenolate (λmax=427nm, ε=18500M−1cm−1). The coordinated X halides affect the phosphatase activity in the order Br>Cl>I (in dinuclear complexes) and Cl>Br>I (in mononuclear) and the trend in the two cases has been well recognised to be due to a different rate determining step. Moreover the influence of chloro atom in para-position of the phenol ring and the role of solvent have been rationalised by comparing the kinetic parameters with those obtained for the corresponding methyl analogues having reasonably close structural resemblance as reported by Sanyal et al. (2014).