Role of solvent in the phosphatase activity of a dinuclear nickel(ii) complex of a Schiff base ligand: mechanistic interpretation by DFT studies

Abstract

A novel dinuclear μ-diphenoxo-μ-acetato metallohydrolase [Ni₂L₂(NCS)(Ac)(H₂O)_0.5(MeOH)_0.5]·1.25H₂O (HL = 2-((E)-(2-(pyridin-2-yl)ethylimino)methyl)-4-chlorophenol) with an intermetallic separation of 3.099 Å has been synthesized and characterized both structurally and spectroscopically. Temperature dependent (2–300 K) magnetic susceptibility measurements show that the compound exhibits a global intramolecular ferromagnetic interaction through the diphenoxido and syn–syn acetate bridges (J = 7.21 cm⁻¹). It is also noticed that a weak antiferromagnetic intermolecular interaction or ZFS effect of the Ni(II) ions is working for lowering of χ_MT at extreme low temperature. The compound’s phosphatase activity, investigated spectrophotometrically on 4-nitrophenylphosphate (4-NPP), demonstrates excellent catalytic efficiency. Additionally, using a DMF medium facilitated a better catalytic pathway (8.18 s⁻¹) than when acetonitrile (MeCN) was used. A plausible 4-NPP hydrolytic reaction mechanism was proposed by utilizing DFT calculations and the results suggest a S_N2-like addition–substitution pathway with two possible catalyst–substrate binding modes (RC1-1 and RC1-2). RC1-2 is regarded as the direct reactant since the water molecule is adjacent to the electron-deficient phosphorus center of the substrate. More importantly, the poor catalytic behavior in the MeCN medium could be ascribed to MeCN molecules having better coordination properties than DMF molecules, since the formation energies of [Ni–MeCN] and [Ni–DMF] are −2.5 and −1.2 kcal mol⁻¹, respectively, according to PCM calculations.