Investigation of Phenols Activity in Early Stage Oxidation of Edible Oils by Electron Paramagnetic Resonance and ¹⁹F NMR Spectroscopies Using Novel Lipid Vanadium Complexes As Radical Initiators

A novel dynamic method for the investigation of the phenols activity in early stage oxidation of edible oils based on the formation of α-tocopheryl radicals initiated by oil-soluble vanadium complexes is developed. Two new vanadium complexes in oxidation states V and IV were synthesized by reacting 2,2′-((2-hydroxyoctadecyl)­azanediyl)­bis­(ethan-1-ol) (C18DEA) with [VO­(acac)₂] and 1-(bis­(pyridin-2-ylmethyl)­amino)­octadecan-2-ol (C18DPA) with VOCl₂. Addition of a solution of either complex in edible oils resulted in the formation of α-tocopheryl radical, which was monitored by electron paramagnetic resonance (EPR) spectroscopy. The intensity of the α-tocopheryl signal in the EPR spectra was measured versus time. It was found that the profile of the intensity of the α-tocopheryl signal versus time depends on the type of oil, the phenolic content, and the storage time of the oil. The time interval until the occurrence of maximum peak intensity be reached (<i>t</i>_m), the height of the maximum intensity, and the rate of the quenching of the α-tocopheryl radical were used for the investigation of the mechanism of the edible oils oxidation. ¹⁹F NMR of the ¹⁹F labeled phenolic compounds (through trifluoroacetate esters) and radical trap experiments showed that the vanadium complexes in edible oil activate the one electron reduction of dioxygen to superperoxide radical. Superperoxide reacts with the lipids to form alkoperoxyl and alkoxyl lipid radicals, and all these radicals react with the phenols contained in oils

Aerial Oxidation of a V^IV–Iminopyridine Hydroquinonate Complex: A Trap for the V^IV–Semiquinonate Radical Intermediate

The reaction of 2,5-bis­[<i>N</i>,<i>N</i>′-bis­(2-pyridyl-aminomethyl)­aminomethyl]-<i>p</i>-hydroquinone (H₂bpymah) with VO²⁺ salts in acetonitrile or water at a low pH (2.2–3.5) results in the isolation of [{V^IV(O)­(Cl)}₂(μ-bpymah)], the <i>p</i>-semiquinonate complex [{V^IV(O)­(Cl)}₂(μ-bpymas)]­(OH), the cyclic mixed-valent hexanuclear compound [{V^V(O)­(μ-O)­V^IV(O)}­(μ-bpymah)]₃, and [(V^VO₂)₂(μ-bpymah)]. [{V^IV(O)­(Cl)}₂(μ-bpymas)]­(OH) is an intermediate of the radical-mediated oxidation of [{V^IV(O)­(Cl)}₂(μ-bpymah)] from O₂. At lower pH values (2.2), a reversible intramolecular electron transfer from the metal to the ligand of [{V^IV(O)­(Cl)}₂(μ-bpymas)]­(OH) is induced with the concurrent substitution of chlorine atoms by the oxygen-bridging atoms, resulting in the formation of [{V^V(O)­(μ-O)­V^IV(O)}­(μ-bpymah)]₃. The metal complexes were fully characterized by X-ray crystallography, infrared (IR) spectroscopy, and magnetic measurements in the solid state, as well as by conductivity measurements, UV–vis spectroscopy, and electrochemical measurements in solution. The oxidation states of the metal ions and ligands were determined by the crystallographic data. The [{V^IV(O)­(Cl)}₂(μ-bpymah)]–[{V^IV(O)­(Cl)}₂(μ-bpymas)]­(OH) redox process is electrochemically reversible. The V^IV ion in the semiquinonate compound exhibits a surprisingly low oxophilicity, resulting in the stabilization of OH^– counterions at acidic pH values. An investigation of the mechanism of this reaction reveals that these complexes induce the reduction of O₂ to H₂O₂, mimicking the activity of enzymes incorporating two redox-active centers (metal–organic) in the active site

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H₂bihyat. Water-Assisted Activation of the Mo^VIO Bond and Reversible Dimerization of <i>cis</i>-[Mo^VIO₂(bihyat)] to [Mo^VI₂O₄(bihyat)₂(H₂O)₂]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat) with <i>cis</i>-[Mo^VIO₂(acac)₂] in tetrahydrofuran resulted in isolation of the mononuclear compound <i>cis</i>-[Mo^VIO₂(bihyat)] (<b>1</b>). The treatment of Na₂Mo^VIO₄·2H₂O with the ligand H₂bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>2</b>) and <i>trans</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>3</b>) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum­(VI) compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat^2– ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo₂^VIO₂(μ₂-O₂)}⁴⁺ moiety. The potentiometry revealed that the Mo^VIbihyat^2– species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3–5. A subtle change in the coordination environment of the five-coordinate compound <b>1</b> with ligation of a weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic than that in <b>1</b>, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (<b>1</b> → <b>2</b>/<b>3</b>). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum­(VI) species, vibrational spectra, and energetics of the metal–ligand interaction for the three molybdenum­(VI) compounds <b>1</b>–<b>3</b> have been studied by means of density functional theory calculations

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H₂bihyat. Water-Assisted Activation of the Mo^VIO Bond and Reversible Dimerization of <i>cis</i>-[Mo^VIO₂(bihyat)] to [Mo^VI₂O₄(bihyat)₂(H₂O)₂]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat) with <i>cis</i>-[Mo^VIO₂(acac)₂] in tetrahydrofuran resulted in isolation of the mononuclear compound <i>cis</i>-[Mo^VIO₂(bihyat)] (<b>1</b>). The treatment of Na₂Mo^VIO₄·2H₂O with the ligand H₂bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>2</b>) and <i>trans</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>3</b>) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum­(VI) compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat^2– ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo₂^VIO₂(μ₂-O₂)}⁴⁺ moiety. The potentiometry revealed that the Mo^VIbihyat^2– species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3–5. A subtle change in the coordination environment of the five-coordinate compound <b>1</b> with ligation of a weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic than that in <b>1</b>, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (<b>1</b> → <b>2</b>/<b>3</b>). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum­(VI) species, vibrational spectra, and energetics of the metal–ligand interaction for the three molybdenum­(VI) compounds <b>1</b>–<b>3</b> have been studied by means of density functional theory calculations

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H₂bihyat. Water-Assisted Activation of the Mo^VIO Bond and Reversible Dimerization of <i>cis</i>-[Mo^VIO₂(bihyat)] to [Mo^VI₂O₄(bihyat)₂(H₂O)₂]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat) with <i>cis</i>-[Mo^VIO₂(acac)₂] in tetrahydrofuran resulted in isolation of the mononuclear compound <i>cis</i>-[Mo^VIO₂(bihyat)] (<b>1</b>). The treatment of Na₂Mo^VIO₄·2H₂O with the ligand H₂bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>2</b>) and <i>trans</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>3</b>) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum­(VI) compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat^2– ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo₂^VIO₂(μ₂-O₂)}⁴⁺ moiety. The potentiometry revealed that the Mo^VIbihyat^2– species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3–5. A subtle change in the coordination environment of the five-coordinate compound <b>1</b> with ligation of a weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic than that in <b>1</b>, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (<b>1</b> → <b>2</b>/<b>3</b>). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum­(VI) species, vibrational spectra, and energetics of the metal–ligand interaction for the three molybdenum­(VI) compounds <b>1</b>–<b>3</b> have been studied by means of density functional theory calculations