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Relationship between Antioxidant Activity and Ligand Basicity in the Dipicolinate Series of Oxovanadium(IV) and Dioxovanadium(V) Complexes

Oxidative stress plays an important role in the pathogenesis of many serious diseases, including cancer, atherosclerosis, coronary artery disease, Parkinson’s disease, Alzheimer’s disease, stroke and myocardial infarction. In the body’s natural biochemical processes, harmful free radicals are formed, which can be removed with the help of appropriate enzymes, a balanced diet or the supply of synthetic antioxidant substances such as flavonoids, vitamins or anthocyanins to the body. Due to the growing demand for antioxidant substances, new complex compounds of transition metal ions with potential antioxidant activity are constantly being sought. In this study, four oxovanadium(IV) and dioxovanadium(V) dipicolinate (dipic) complexes with 1,10-phenanthroline (phen), 2,2′-bipyridyl (bipy) and the protonated form of 2-phenylpyridine (2-phephyH): (1) [VO(dipic)(H2O)2]·2 H2O, (2) [VO(dipic)(phen)]·3 H2O, (3) [VO(dipic)(bipy)]·H2O and (4) [VOO(dipic)](2-phepyH)·H2O were synthesized including one new complex, so far unknown and not described in the literature, i.e., [VOO(dipic)](2-phepyH)·H2O. The oxovanadium(IV) dipicolinate complexes with 1,10-phenanthroline and 2,2′-bipyridyl have been characterized by several physicochemical methods: NMR, MALDI-TOF-MS, IR, but new complex [VOO(dipic)](2-phepyH)·H2O has been examined by XRD to confirm its structure. The antioxidant activities of four complexes have been examined by the nitrotetrazolium blue (NBT) method towards superoxide anion. All complexes exhibit high reactivity with superoxide anion and [VOO(dipic)](2-phepyH)·H2O has higher antioxidant activity than L-ascorbic acid. Our studies confirmed that high basicity of the auxiliary ligand increases the reactivity of the complex with the superoxide radical.


Introduction
The human body has many defense mechanisms that neutralize the harmful effects of reactive oxygen species. Antioxidants play an important role in reducing oxidative damage in the human body [1][2][3][4][5]. These are compounds which, even at a very low concentration, compared to the substrate, can delay or prevent its oxidation. Antioxidants can be divided into two groups. The first are antioxidants that interrupt radical reactions by donating hydrogen atoms or electrons to radicals, which leads to the formation of compounds with greater stability. Such compounds include: tocopherols [6,7], phenols [8], hydroquinones [9]. The second group includes substances whose action is synergistic. They are capable of scavenging oxygen and chelating ions involved in the formation of radicals [10].
According to the reaction mechanism, methods for measuring antioxidant capacity can be divided into methods based on hydrogen atom transfer (HAT), electron transfer (ET), and both. In the methods based on the transfer of the hydrogen atom, the result depends on the dissociation energy of the bond and the ionization potential in the group that is the donor of the hydrogen atom. HAT reactions are usually fast and independent of the solvent and the pH of the environment. Electron transfer reactions depend on the ionization potential of the active functional group in the antioxidant molecule, and therefore also depend on the pH. The value of the ionization potential decreases with increasing pH, as the electron-donor capacity increases [11][12][13][14]. Antioxidants have multiple effects: inter alia, they prevent the formation of oxidants, i.e., free radicals, inhibit the initiation of the oxidation process of metals such as cadmium, mercury, copper and lead, which is associated with supporting the immune system, or they intercept the reactive oxidants-oxidants and inhibit reactions chain-which threaten the formation of radicals [15].
Recently, there has been increasing interest in complex compounds containing d-block metal ions with organic ligands exhibiting antioxidant properties.  [16]. Therefore, our interests focused on the antioxidant properties of oxovanadium(IV) and dioxovanadium(V) complex compounds. The methods of synthesis, characterization and biological properties of dioxovanadium(V) OH-substituted dipicolinate complexes have been previously described in the literature [17]. These complexes exhibit insulin-like activity. The Crans group conducted research on Cl-substituted dipicolinate complexes of vanadium(III, IV, V) and they confirmed that these compounds cause anti-diabetic effect on rats [18]. Studies have confirmed that the oxygenation state of vanadium has an impact on the insulin-like properties of complexes [18]. The antioxidant properties of the vanadium complexes can be directed towards the two-electron transfer reactions then their toxicity is lowered [19,20].
In this study, four oxovanadium(IV) and dioxovanadium(V) dipicolinate (dipic) complexes with 1,10-phenanthroline (phen), 2,2 -bipyridyl (bipy) and the protonated form The oxovanadium(IV) dipicolinate complexes with 1,10-phenanthroline and 2,2 -bipyridyl have been characterized by several physicochemical methods: NMR, MALDI-TOF-MS, IR, but new complex [VOO(dipic)](2-phepyH)·H 2 O has been examined by XRD to confirm its structure. All four complexes have been tested towards antioxidant activities by the nitrotetrazolium blue (NBT) method against the superoxide anion. The purpose of the studies was the examination of the impact of heterocyclic ligands on the reactivity of the complex with superoxide anion.

Materials and Methods
All materials used in this work have been purchased from Merck.
Full crystallographic details for the title compound have been deposited in the Cambridge Crystallographic Data Center (deposition No. CCDC 2087422) and they may be obtained from http://www.ccdc.cam.ac.uk (accessed on 6 June 2021), e-mail: de-posit@ccdc.cam.ac.uk or The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK.
Elemental analysis of the complex compounds was performed using the Vario EL Cube analyzer and the percentage of carbon, nitrogen and hydrogen in the tested compound was determined.
The IR spectra were recorded on a KBr tablet using a BRUKER IFS 66 spectrophotometer. Measurements were carried out in the range from 4000 cm −1 to 650 cm −1 .
The NMR spectra were recorded on a Bruker Avance III 500 instrument. DMSO-d6 was used as a solvent.
The NBT test: a solution containing the superoxide anion radical was prepared. Initially, 6.5 mg of KO 2 and 90 mg of 18-crown-6 were dissolved in 50 mL of DMSO. The solution was then placed under ultrasound for 7 min. In the next step, an NBT solution with a concentration of 1 mg/mL was prepared. The next step was to prepare solutions of complex compounds in DMSO. Depending on the solubility of a given complex compound, a 1mM stock solution was prepared, and then subsequent solutions of the complex compounds were prepared by diluting the stock solution. The measurement samples were prepared by mixing 1.5 mL of the radical solution in DMSO, 0.5 mL of the complex compound solution, and 0.1 mL of NBT. Control samples contained 1.5 mL of superoxide anion solution, 0.5 mL of DMSO and 0.1 mL of NBT solution. The reaction of the superoxide anion radical with NBT was monitored spectrophotometrically at a wavelength of 560 nm. The reference substance was L-ascorbic acid. Measurement of absorbance was carried out approximately 30 min after mixing the reactants. Measurement of absorbance during NBT testing was carried out on a Perkin-Elmer Lambda 45 spectrophotometer. The apparatus used is characterized by a Peltier system with a reading accuracy of 1 nm and a gap width of 1 nm at a scanning speed of 120.00 nm min −1 combined with a thermostatic system.

Conclusions
The structure of the new complex [VOO(dipic)](2-phepyH)·H 2 O was described for the first time. Moreover, the performed spectroscopic analyzes and elemental analysis confirmed the composition of the obtained series of dipicolinate oxovanadium(IV) coordination compounds. The conducted research confirmed that the synthesized complex compounds