Aerial Oxidation of a V(IV)-Iminopyridine Hydroquinonate Complex: A Trap for the V(IV)-Semiquinonate Radical Intermediate

Experimental and Theoretical Investigation of the Mechanism of the Reduction of O2 from Air to O22- by VIVO2+-N,N,N-Amidate Compounds and Their Potential Use in Fuel Cells

The two-electron reductive activation of O2 to O22- is of particular interest to the scientific community mainly due to the use of peroxides as green oxidants and in powerful fuel cells. Despite of the great importance of vanadium(IV) species to activate the two-electron reductive activation of O2, the mechanism is still unclear. Reaction of VIVO2+ species with the tridentate-planar N,N,N-carboxamide (ΗL) ligands in solution (CH3OH:H2O) under atmospheric O2, at room temperature, resulted in the quick formation of [VV(═O)(η2-O2)(κ3-L)(H2O)] and cis-[VV(═O)2(κ3-L)] compounds. Oxidation of the VIVO2+ complexes with the sterically hindered tridentate-planar N,N,N-carboxamide ligands by atmospheric O2 gave only cis-[VV(═O)2(κ3-L)] compounds. The mechanism of formation of [VV(═O)(η2-O2)(κ3-L)(H2O)] (I) and cis-[VV(═O)2(κ3-L)] (II) complexes vs time, from the interaction of [VIV(═O)(κ3-L)(Η2Ο)2]+ with atmospheric O2, was investigated with 51V, 1H NMR, UV-vis, cw-X-band EPR, and 18O2 labeling IR and resonance Raman spectroscopies revealing the formation of a stable intermediate (Id). EPR, MS, and theoretical calculations of the mechanism of the formation of I and II revealed a pathway, through a binuclear [VIV(═O)(κ3-L)(H2O)(η1,η1-O2)VIV(═O)(κ3-L)(H2O)]2+ intermediate. The results from cw-EPR, 1H NMR spectroscopies, cyclic voltammetry, and the reactivity of the complexes [VIV(═O)(κ3-L)(Η2Ο)2]+ toward O2 reduction fit better to an intermediate with a binuclear nature. Dynamic experiments in combination with computational calculations were undertaken to fully elucidate the mechanism of the O2 reduction to O22- by [VIV(═O)(κ3-L)(Η2Ο)2]+. The galvanic cell {Zn|VIII,VII||Id, [VIVO(κ3-L)(H2O)2]+|O2|C(s)} was manufactured, demonstrating the important applicability of this new chemistry to Zn|H2O2 fuel cells technology generating H2O2 in situ from the atmospheric O2.

Four electron selective O2 reduction by a tetranuclear vanadium(IV/V)/hydroquinonate catalyst: application in the operation of Zn–air batteries

A tetranuclear vanadium(IV/V) hydroquinonate electrocatalyst for oxygen reduction through proton-coupled electron transfer. The complex enhances the current and power of Zn–air batteries.

Synthesis of vitamin E and aliphatic lipid vanadium(IV) and (V) complexes, and their cytotoxic properties

Novel vitamin E chelate derivatives and their V^IV/V complexes have been synthesized and characterized, and their anticancer properties have been evaluated. The new complexes have been designed to exhibit enhanced cytotoxicity by combining high lipophilicity with the properties of vanadium to induce the formation of reactive oxygen species (ROS). In particular, the β-tocopherol derivatives with iminodiethanol (β-tocDEA) and dipicolylamine (β-tocDPA) as well their V^V and V^IV complexes, [V^VO(β-tocDEA] and [V^IVO(β-tocDPA] have been synthesized and characterized by Nuclear Magnetic Resonance (NMR), Ultra Violet-Visible (UV-Vis) and Electron Paramagnetic Resonance (EPR) spectroscopies. Although the β-tocopherol compounds exhibit antioxidant activity their complexes induce formation of radicals. In addition, two vanadium amphiphilic complexes of 2,2'-((2-hydroxyoctadecyl)azanediyl)bis(ethan-1-ol) (C18DEA) and 1-(bis(pyridin-2-ylmethyl)amino)octadecan-2-ol (C18DPA) known to activate O₂ and produce ROS were synthesized and characterized (C. Drouza, A. Dieronitou, I. Hadjiadamou, M. Stylianou, J. Agric. Food. Chem., vol. 65, 2017, pp. 4942-4951). The four amphiphilic vanadium complexes exhibit enhanced hydrolytic stability. All compounds found to be cytotoxic for cancer cells exhibiting activity similar or higher to cis-platin.

Investigation of dioxygen activation by copper(ii)–iminate/aminate complexes

Cu^II amidate/iminate complexes activate dioxygen by a ligated to Cu^II, –HCN– moiety.

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 (t_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.

Coordination of o-benzosemiquinonate, o-iminobenzosemiquinonate and aldimine anion radicals to oxidovanadium(iv)

o-Benzosemiquinonate, o-iminobenzosemiquinonate and hitherto unknown aldimine anion radical complexes of oxidovanadium(iv) are reported.

Synthesis of new photosensitive H2BBQ2+[ZnCl4]2−/[(ZnCl)2(μ-BBH)] complexes, through selective oxidation of H2O to H2O2

A two-electron photosensitive H₂O to H₂O₂ oxidizer, H₂BBQ²⁺[ZnCl₄]²⁻/[(ZnCl)₂(μ-BBH)], has been synthesized. An aqueous {[(ZnCl)₂(μ-BBH)]||H₂O₂} solar rechargeable galvanic cell has been constructed.

Arylamino radical complexes of ruthenium and osmium: dual radical counter in a molecule

Radical and non-radical ruthenium and osmium complexes of 1-amino-9,10-anthraquinone (AqNH₂), which is defined as a molecule of dual radical counter, are disclosed. 1-Amido-9,10-anthraquinone (AqNH^-) complexes of the types trans-[Ru^II(AqNH^-)(PPh₃)₂(CO)Cl] (1), trans-[Os^II(AqNH^-)(PPh₃)₂(CO)Br] (2) and trans-[Ru^III(AqNH^-)(PPh₃)₂Cl₂] (3) were isolated. AqNH^- of 1-3 is redox active and undergoes oxidation reversibly at +(0.05-0.35) V to the 1-amino-9,10-anthraquinone radical (AqNH˙) and reduction at -(0.86-1.60) V to the 1-amido-9,10-anthrasemiquinonate anion radical (Aq^NHSQ˙^2-). The reaction of 2 with I₂ in CH₂Cl₂ afforded a crystalline AqNH˙ complex of the type trans-[Os^II(AqNH˙)(PPh₃)₂(CO)Br]⁺I₅^-·½I₂ (2⁺I₅^-·½I₂). AqNH˙ and Aq^NHSQ˙^2- complexes of the types trans-[Ru^II(AqNH˙)(PPh₃)₂(CO)Cl]⁺ (1⁺), trans-[Ru^III(AqNH˙)(PPh₃)₂Cl₂]⁺ (3⁺), trans-[Ru^II(Aq^NHSQ˙^2-)(PPh₃)₂(CO)Cl]^- (1^-) and trans-[Os^II(Aq^NHSQ˙^2-)(PPh₃)₂(CO)Br]^- (2^-) were generated chemically/electrochemically in solution. The electronic states of the complexes were authenticated by single crystal X-ray structure determinations of 1, 2·5/4 toluene, 3 and 2⁺I₅^-·½I₂, EPR spectroscopy and density functional theory (DFT) calculations. AqNH˙ instigates a 2c-3e p_π-d_π interaction and the length in 2⁺I₅^-·½I₂, 1.978(5) Å, is relatively shorter than the Os^II-NH_Aq^- length, 2.037(2) Å, while the Aq-NH˙ bond, 1.365(8) Å, is longer than the Aq-NH^- bond, 1.328(3) Å. DFT calculations predicted that the atomic spin is delocalized over the ligand backbone (1⁺, 56%) particularly in one of the p-orbitals of the nitrogen and the metal atoms of the 1⁺ and 2⁺ ions, while the spin is dominantly localized on the anthraquinone fragment of the 1^- and 2^- ions. TD DFT calculations were employed to elucidate the origins of the lower energy absorption bands of the neutral complexes. Hypsochromic shifts of the UV-vis-NIR absorption maximum during 1→1⁺, 2→2⁺ and 3→3⁺ conversions were recorded by spectroelectrochemical measurements.

Coordination of o-benzosemiquinonate, o-iminobenzosemiquinonate, 4,4′-di-tert-butyl-2,2′-bipyridine and 1,10-phenanthroline anion radicals to oxidovanadium(iv)

Coordination of semiquinonate and heterocyclic α-diimine anion radicals to oxidovanadium(iv) is authenticated.