Abstract
Methyl-substituted 8-hydroxyquinolines (Hquin) were successfully used to synthetize five-coordinated oxovanadium(IV) complexes: [VO(2,6-(Me)2-quin)2] (1), [VO(2,5-(Me)2-quin)2] (2) and [VO(2-Me-quin)2] (3). Complexes 1–3 demonstrated high catalytic activity in the oxidation of hydrocarbons with H2O2 in acetonitrile at 50 °C, in the presence of 2-pyrazinecarboxylic acid (PCA) as a cocatalyst. The maximum yield of cyclohexane oxidation products attained was 48%, which is high in the case of the oxidation of saturated hydrocarbons. The reaction leads to the formation of a mixture of cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone. When triphenylphosphine is added, cyclohexyl hydroperoxide is completely converted to cyclohexanol. Consideration of the regio- and bond-selectivity in the oxidation of n-heptane and methylcyclohexane, respectively, indicates that the oxidation proceeds with the participation of free hydroxyl radicals. The complexes show moderate activity in the oxidation of alcohols. Complexes 1 and 2 reduce the viability of colorectal (HCT116) and ovarian (A2780) carcinoma cell lines and of normal dermal fibroblasts without showing a specific selectivity for cancer cell lines. Complex 3 on the other hand, shows a higher cytotoxicity in a colorectal carcinoma cell line (HCT116), a lower cytotoxicity towards normal dermal fibroblasts and no effect in an ovarian carcinoma cell line (order of magnitude HCT116 > fibroblasts > A2780).
Highlights
To synthetize the oxidovanadium complexes with methyl-substituted 8-hydroxyquin olines, [VO(2,6-(Me)2 -quin)2 ] (1), [VO(2,5-(Me)2 -quin)2 ] (2), the previously reported procedure based on the reaction of bis(acetylacetonato)oxidovanadium(IV) with the corresponding 8-hydroxyquinoline derivative in open air was employed [62,63]
QuinH and 2,5-(Me)2-quinH resulted in the formation of five-coordinated [VIVO(N∩O)2]. It indicates that the methyl-functionalization of quinH strengthen its coordination capacity to the V(IV) ion, and steric hindrance induced by the methyl group at the 2-position in the to the ion, and hindrance induced by the methyl group at the 2-position in the pyridine ring facilitates thesteric formation of five-coordinated complexes
Our results show that temperature does induce statistically significant alterations in the amount of complex internalized, which suggests that there is no active transport of complexes into cells after 3 or 6 h (Figure 11), which has been described in the literature with oxovanadium complexes [96]
Summary
In the last three decades, vanadium coordination compounds have received increasing interest due to their structural features [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44], catalytic applications [21,22,23,24,25,26,27,28,29,30,31,32,33,45,46] and medicinal importance [33,34,35,36,37,38,39,40,41,42,43,44,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63]. What is important, is that monomeric oxidovanadium(V) complexes [VO(OMe)(N∩O)2 ] with nitroor halogen-substituted quinolin-8-olate ligands were found to be very promising in view of their catalytic properties. These complexes exhibit high catalytic activity toward the oxidation of inert alkanes to alkyl hydroperoxides by H2 O2 in aqueous acetonitrile, with the yield of oxygenate products up to 39% and a TON of 1730 for 1 h [63]. In contrast to the previously reported [VV O(OMe)(N∩O)2 ] with 8-hydroxyquinoline derivatives bearing substituents in the 5or 5,7-positions of the quin backbone, all the complexes here presented contain a vanadium(IV) ion and they are five-coordinated, which was evidenced by the X-ray diffraction analysis, EPR and UV–Vis spectroscopy. To evaluate the antiproliferative effect of complexes 1–3, HCT116 and A2780 cancer cell lines, and normal dermal fibroblasts were used
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