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

Novel dibasic Schiff bases with three tridentate sites were obtained from the condensation of the triketone 2,4,6-triacetylphloroglucinol (H 3 ptk) with four different hydrazides, benzoyl hydrazide (bhz), furoyl hydrazide (fah), isonicotinoyl hydrazide (inh) and nicotinoyl hydrazide (nah): H 6 ptk(bhz) 3 I , H 6 ptk(fah) 3 II , H 6 ptk(inh) 3 III and H 6 ptk(nah) 3 IV . These ligand precursors I-IV , each being an ONO donor, are tricompartmental building blocks able to form trinuclear complexes having C 3 symmetry. The reaction of I-IV with [V IV O(acac) 2 ] leads to the formation of [{V IV O(H 2 O)} 3 (ptk(bhz) 3 )] 1 , [{V IV O(H 2 O)} 3 (ptk(fah) 3 )] 2 , [{V IV O(H 2 O)} 3 (ptk(inh) 3 )] 3 , and [{V IV O(H 2 O)} 3 (ptk(nah) 3 )] 4 . In methanol/aqueous solutions of M 2 CO 3 (M + = Na + , K + and Cs + ), these complexes are slowly converted into dioxidovanadium( v ) compounds, namely, M 3 [(V V O 2 ) 3 {ptk(bhz) 3 }]·6H 2 O [M + = K + 5 , Na + 9 , Cs + 13 ], M 3 [(V V O 2 ) 3 {ptk(fah) 3 }]·6H 2 O [M + = K + 6 , Na + 10 , Cs + 14 ], M 3 [(V V O 2 ) 3 {ptk(inh) 3 }]·6H 2 O [M + = K + 7 , Na + 11 , Cs + 15 ] and M 3 [(V V O 2 ) 3 {ptk(nah) 3 }]·6H 2 O [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 ( 1 H, 13 C and 51 V), elemental analysis, thermal studies, cyclic voltammetry (CV) and single-crystal X-ray analysis. X-ray diffraction studies of complexes K 2.7 [{(V V O 2 ) 3 ptk(fah) 3 }]·11.5H 2 O·MeOH 6a , Cs 3 [{(V V O 2 ) 3 ptk(bhz) 3 }]·7H 2 O 13a and Cs 3 [{(V V O 2 ) 3 ptk(nah) 3 }]·7.3H 2 O 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 H 2 O 2 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. Solid state and solution studies of trinuclear V IV O- and V V O 2 -complexes and their peroxidase mimicking activity, through oxidation of dopamine to aminochrome, are reported.