Deactivation of catecholase-like activity of a dinuclear Ni(II) complex by incorporation of an additional Ni(II)

Abstract

A new dinuclear Ni(II) complex [Ni₂L₂ (PhCOO)(H₂O) ₂]ClO₄ (1) has been synthesized by reacting Ni(ClO₄)₂•6H₂O with a reduced Schiff base ligand [(3-dimethylamino-propylamino)-methyl]-phenol (HL) in presence of benzoic acid in 1:1:1 ratios. Structural analysis reveals that complex 1 is a di-μ₂-phenoxo-bridged dinuclear Ni(II) complex and has an additional syn-syn benzoate bridge. Enzyme kinetic studies reflect that it (1) is effective in mimicking catecholase like activity with 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate. Kinetic measurements suggest that the rate of catechol oxidation follows first order kinetics with respect to the catalyst. The possible intermediates during the course of reaction are identified from ESI–MS spectra. The characteristic EPR spectra of the complex in the presence of 3,5-DTBC supports the formation of nickel(II)-semiquinonate as an intermediate species during the catalytic oxidation. When the synthesis is carried out with Ni(II):HL ratio of 3:2, a different compound [Ni₃L₂ (PhCOO)₄] (2) is produced incorporating an additional Ni(II)-benzoate. 2 possesses a linear trinuclear structure with the tridentate reduced Schiff base ligand coordinated to the terminal nickel atoms which are linked to the central Ni(II) by phenoxo and carboxylate bridges. Unlike 1, it does not show any catalytic activity towards catechol oxidation. Variable temperature magnetic susceptibility measurements have also been performed for 1 and 2. Complex 1 indicates the presence of intradimer antiferromagnetic interactions (J = −5.65 cm⁻¹). In contrast to complex 1, ferromagnetic coupling (J = 3.55 cm⁻¹) is observed in complex 2.