Trinuclear vanadium(iv) and vanadium(v) complexes derived from 2,4,6-triacetylphloroglucinol and study of their peroxidase mimicking activity

Abstract

Novel dibasic Schiff bases with three tridentate sites were obtained from the condensation of the triketone 2,4,6-triacetylphloroglucinol (H₃ptk) with four different hydrazides, benzoyl hydrazide (bhz), furoyl hydrazide (fah), isonicotinoyl hydrazide (inh) and nicotinoyl hydrazide (nah): H₆ptk(bhz)₃I, H₆ptk(fah)₃II, H₆ptk(inh)₃III and H₆ptk(nah)₃IV. These ligand precursors I–IV, each being an ONO donor, are tricompartmental building blocks able to form trinuclear complexes having C₃ symmetry. The reaction of I–IV with [V^IVO(acac)₂] leads to the formation of [{V^IVO(H₂O)}₃(ptk(bhz)₃)] 1, [{V^IVO(H₂O)}₃(ptk(fah)₃)] 2, [{V^IVO(H₂O)}₃(ptk(inh)₃)] 3, and [{V^IVO(H₂O)}₃(ptk(nah)₃)] 4. In methanol/aqueous solutions of M₂CO₃ (M⁺ = Na⁺, K⁺ and Cs⁺), these complexes are slowly converted into dioxidovanadium(V) compounds, namely, M₃[(V^VO₂)₃{ptk(bhz)₃}]·6H₂O [M⁺ = K⁺5, Na⁺9, Cs⁺13], M₃[(V^VO₂)₃{ptk(fah)₃}]·6H₂O [M⁺ = K⁺6, Na⁺10, Cs⁺14], M₃[(V^VO₂)₃{ptk(inh)₃}]·6H₂O [M⁺ = K⁺7, Na⁺11, Cs⁺15] and M₃[(V^VO₂)₃{ptk(nah)₃}]·6H₂O [M⁺ = K⁺8, Na⁺12, Cs⁺16]. All ligand precursors and complexes are characterized by various techniques such as FT-IR, UV/Visible, EPR, NMR (¹H, ¹³C and ⁵¹V), elemental analysis, thermal studies, cyclic voltammetry (CV) and single-crystal X-ray analysis. X-ray diffraction studies of complexes K_2.7[{(V^VO₂)₃ptk(fah)₃}]·11.5H₂O·MeOH 6a, Cs₃[{(V^VO₂)₃ptk(bhz)₃}]·7H₂O 13a and Cs₃[{(V^VO₂)₃ptk(nah)₃}]·7.3H₂O 16a reveal their distorted square pyramidal geometry by coordinating through phenolate oxygen (of ptk), azomethine nitrogen and enolate oxygen (of hydrazide) atoms. The reactivity of complexes 5–16 and their catalytic potential were screened towards their peroxidase mimetic activity in the oxidation of dopamine to aminochrome driven by H₂O₂ as an oxidant. The conversion of dopamine to aminochrome with different catalysts was monitored by HPLC showing high activity under mild conditions with good conversions within 1 h. Kinetic studies using compounds 13–16 as catalyst precursors reveal that the reaction follows a Michaelis–Menten-like kinetics.