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

Novel Schiff bases with three dibasic tridentate sites, each a ONO donor, 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)3 I, H6ptk(fah)3 II H6ptk(inh)3, III and H6ptk(nah)3 IV. These ligand precursors I-IV are tricompartmental building blocks able to form trinuclear complexes having C3 symmetry. Reaction of I-IV with [VIVO(acac)2] lead to the formation of [{VIVO(H2O)}3(ptk(bhz)3)] 1, [{VIVO(H2O)}3(ptk(fah)3)] 2, [{VIVO(H2O)}3(ptk(inh)3)] 3, [{VIVO(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(VVO2)3{ptk(bhz)3}(H2O)6] [M+ = K+ 5, Na+ 9, Cs+ 13], [M3(VVO2)3{ptk(fah)3}(H2O)6] [M+ = K+ 6, Na+ 10, Cs+ 14], [M3(VVO2)3{ptk(inh)3}(H2O)6] [M+ = K+ (7), Na+ 11, Cs+ 15] and [M3(VVO2)3{ptk(nah)3}(H2O)6] [M+ = K+ 8, Na+ 12, Cs+ 16]. All ligand precursors and complexes are characterized by various techniques: 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 [K3(VVO2)3{ptk(fah)3}(H2O)6)]·4H2O·MeOH 6a, [Cs3(VVO2)3{ptk(bhz)3}(H2O)7] 13a, and [Cs3(VVO2)3{ptk(nah)3}(H3O)Cl(H2O)6] 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 the 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 catalyst was monitored by HPLC showing high activity under mild conditions with good conversions within 1 h. Kinetic studies using compounds 13-16 as catalysts reveal that the reaction follows a Michaelis-Menten like kinetics.