Spectroscopic, thermodynamic, kinetic studies and oxidase/antioxidant biomimetic catalytic activities of tris(3,5-dimethylpyrazolyl)borate Cu(ii) complexes

Abstract

A series of copper(II) complexes, viz. [Tp^MeMeCu(Cl)(H₂O)] (1), [Tp^MeMeCu(OAc)(H₂O)] (2), [Tp^MeMeCu(NO₃)] (3) and [Tp^MeMeCu(ClO₄)] (4) containing tris(3,5-dimethylpyrazolyl)borate (KTp^MeMe), have been synthesized and fully characterized. The substitution reaction of 1 with thiourea was studied under pseudo-first-order conditions as a function of concentration, temperature and pressure in methanol and acetonitrile as solvents. Two reaction steps that both depended on the nucleophile concentration were observed for both solvents. Substitution of coordinated methanol is about 40 times faster than the substitution of chloride. In acetonitrile, the rate constant for the displacement of coordinated acetonitrile was more than 20 times faster than the substitution of chloride. The reported activation parameters indicate that both reaction steps follow a dissociative mechanism in both solvents. On going from methanol to acetonitrile, the rate constant for the displacement of the solvent becomes more than 200 times faster due to the more labile acetonitrile, but the substitution mechanism remained to have a dissociative character. The antioxidant activities of 1–4 were evaluated for superoxide dismutase (SOD), glutathione-s-transferase (GST0 and glutathione reduced (GSH-Rd) activity. 1 and 2 were found to show (p < 0.05) the highest antioxidant activity in comparison to 3 and 4, which can be ascribed to the geometric configuration as well as the nature of the co-ligand. 1 showed catechol oxidase activity with turnover numbers of 20 min⁻¹ and a coordination affinity for 3,5-DTBC of K₁, = 31 mM⁻¹. K₁ is rather large and seems to be typical for faster biomimetic models, and also for the enzyme itself (25 mM⁻¹). The reaction rate depended linearly on the complex concentration, indicating a first-order dependence on the catalyst concentration.