Structural, spectroscopic, and magnetic properties of a diphenolate-bridged FeIIINiII complex showing excellent phosphodiester cleavage activity

To mimic the phosphate ester hydrolysis behavior of purple acid phosphatases the heterobimetallic complex [(BNPP)FeIIIL(μ-BNPP)NiII(H2O)](ClO4) (1) has been synthesized from the precursor complexes [FeIII(LH2)(H2O)2](ClO4)3·3H2O and [FeIII(LH2)(H2O)Cl](ClO4)2·2H2O. In these compounds, L2− is the anion of the tetraiminodiphenol macrocyclic ligand (H2L), while LH2 is the zwitterionic form in which the phenolic protons are shifted to the two metal-uncoordinated imine nitrogens, and BNPP is bis(4-nitrophenyl)phosphate. The X-ray crystal structure of compound 1 has been determined. The structure of 1 comprises of two edge-shared distorted octahedrons whose metal centers are bridged by two equatorial phenolate oxygens and two axially disposed oxygens of a BNPP ligand. The internuclear Fe⋯Ni distance is 3.083 Å. The high-spin iron(III) and nickel(II) in 1 are antiferromagnetically coupled (J = −7.1 cm−1; H = −2JS1·S2) with S = 3/2 spin ground state. The phosphodiesterase activity of 1 has been studied in 70:30 H2O–(CH3)2SO medium with NaBNPP as the substrate. The reaction rates have been measured by varying pH (3–10), temperature (25–50 °C), and with different concentrations of the substrate and complex at a fixed pH and temperature. Treatment of the rate data, obtained at pH 6.0 and at 35 °C, by the Michaelis–Menten approach have provided the following parameters: KM = 3.6 × 10−4 M, Vmax = 1.83 × 10−7 M s−1, kcat = 9.15 × 10−3 s−1. As compared to the uncatalyzed hydrolysis rate of BNPP, the kcat value is 8.3 × 108 times higher, showing that 1 behaves as an excellent model for phosphate ester hydrolysis.

The complex [(BNPP)FeL(μ-BNPP)Ni(H2O)](ClO4) behaves as a functional model of purple acid phosphatase by catalyzing the hydrolysis of BNPP to NPP and NP in water–DMSO medium, where the reaction rate is maximum at pH 6.0 and is 8 × 108 times faster compared to the uncatalyzed reaction.Figure optionsDownload as PowerPoint slideHighlights
► Synthesis of heterobinuclear Fe(III)–Ni(II) macrocyclic complex.
► Variable temperature magnetic study.
► Electrochemical study.
► Phosphodiester cleavage activity.