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

Abstract

To mimic the phosphate ester hydrolysis behavior of purple acid phosphatases the heterobimetallic complex [(BNPP)Fe IIIL(μ-BNPP)Ni II(H 2O)](ClO 4) ( 1) has been synthesized from the precursor complexes [Fe III(LH 2)(H 2O) 2](ClO 4) 3·3H 2O and [Fe III(LH 2)(H 2O)Cl](ClO 4) 2·2H 2O. In these compounds, L 2− is the anion of the tetraiminodiphenol macrocyclic ligand (H 2L), while LH 2 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 = −2 J S 1· S 2) with S = 3/2 spin ground state. The phosphodiesterase activity of 1 has been studied in 70:30 H 2O–(CH 3) 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: K M = 3.6 × 10 −4 M, V max = 1.83 × 10 −7 M s −1, k cat = 9.15 × 10 −3 s −1. As compared to the uncatalyzed hydrolysis rate of BNPP, the k cat value is 8.3 × 10 8 times higher, showing that 1 behaves as an excellent model for phosphate ester hydrolysis.