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A new two-electron photosensitizer, H2BBQ2+[ZnCl4]2−/[(ZnCl)2(μ-BBH)] (BBQ stands for 2,5-bis[bis(pyridin-2-ylmethyl)amino]-1,4-quinone and BBH stands for 2,5-bis[bis(pyridin-2-ylmethyl)amino]-1,4-hydroquinone), has been synthesized and the oxidation state of the ligand was determined by X-ray crystallography and NMR spectroscopy. Under light illumination the H2BBQ2+[ZnCl4]2− + ZnCl2 is reduced quantitatively to [(ZnCl)2(μ-BBH)] (pH ∼ 5) oxidizing H2O to H2O2 as is evident by trap experiments. Electrochemistry gave a reversible two-electron ligand-centered redox wave for [(ZnCl)2(μ-BBH)]. UV-Vis, luminescence and EPR spectroscopies reveal the slow formation of a stable quinone diradical, intermediate of the reaction. DFT calculations are in agreement with the proposed mechanism. Based on this property an aqueous {[(ZnCl)2(μ-BBH)] H2O2} solar rechargeable galvanic cell has been constructed. © The Royal Society of Chemistry. 46 11 3688 3699

New Eu3+, Sm3+ and Gd3+ complexes of triazine ligands 2,4,6-tri(2-pyridyl)-1,3,5-triazine, 3,5,6-tri(2-pyridyl)-1,2, 4-triazine and 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine were synthesized. The structure of five of them was established by single crystal X-ray crystallography. Various aspects of the triazine ligands such as steric interactions, deviation from the planarity and coordination mode define the structure of the complexes. The 1H NMR spectra of the solutions of Eu3+ and Sm3+ molecules showed broad paramagnetic shifted peaks assigned to the protons of the ligated triazines. The powder EPR spectra of the Gd3+ complexes exhibited a broad peak centered at g = 2. All Eu3+ and Sm3+ complexes display characteristic lanthanide emission upon UV excitation. The enhancement of the luminescence is attributed to the effective energy transfer from the triazine ligand to the lanthanide ion. 1,2,4-Triazines show to be better sensitizers for Eu3+ and Sm 3+ than the 1,3,5-triazine ligand. Co-coordination of the triazines with β-diketonate ligands further enhances the luminescence of the lanthanides. This enhancement was attributed to the synergistic effect of the two ligands and/or the decrease of the vibrational quenching processes. 52 856 865 Cited By :12

Reaction of NH4VO3 with (phenolate/hydroquinonate) iminodiacetate tripod ligands in aqueous solution in the pH range 7–8 results in the isolation of three new dioxido vanadium(V) complexes, one dinuclear (NH4)4{(VVO)2[μ-bicah(6-)-N,O,O,O]}·8H2O, (H6bicah, 2,5-bis[N,N'-bis(carboxymethyl)aminomethyl]-hydroquinone), and two mononuclear (NH4)2{(VVO)2[Hcah(3-)-N,O,O,O]·2H2O, (H4cah, 2-[N,N'-bis(carboxymethyl)aminomethyl]-hydroquinone) and (NH4)2{(VVO)[Hcacp(3-)-N,O,O,O]}·4H2O, (H4cacp, 2-[N,N'-bis(carboxymethyl)aminomethyl]-4-carboxyphenol). The 51V, 1H, and 13C NMR spectra in D2O show the coordination environment of vanadium in the above complexes to be octahedral, with four out of six positions to be occupied by the two carboxylate oxygen, one hydroquinonate oxygen, and one amine nitrogen atoms of the tripod ligand. The two acetate arms are in cis position to each other. Variable-temperature 1H and 13C NMR measurements show that the complexes are kinetically labile. An intramolecular exchange that proceeds through the opening of the phenolate/hydroquinonate chelate ring has been revealed by 2D{1H} NMR EXSY (exchange spectroscopy).

The reaction of 2,5-bis[N,N′-bis(2-pyridyl-aminomethyl)aminomethyl]-p-hydroquinone (H2bpymah) with VO2+ salts in acetonitrile or water at a low pH (2.2-3.5) results in the isolation of [{VIV(O)(Cl)}2(μ-bpymah)], the p-semiquinonate complex [{VIV(O)(Cl)}2(μ-bpymas)](OH), the cyclic mixed-valent hexanuclear compound [{VV(O)(μ-O)VIV(O)}(μ-bpymah)]3, and [(VVO2)2(μ-bpymah)]. [{VIV(O)(Cl)}2(μ-bpymas)](OH) is an intermediate of the radical-mediated oxidation of [{VIV(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 [{VIV(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 [{VV(O)(μ-O)VIV(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 [{VIV(O)(Cl)}2(μ-bpymah)]-[{VIV(O)(Cl)}2(μ-bpymas)](OH) redox process is electrochemically reversible. The VIV 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. © 2015 American Chemical Society. 54 15 7218 7229 Cited By :7

Reaction of the N,N-disubstituted bis(hydroxylamino) ligand 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H2bihyat) with cis-[MoVIO2(acac)2] in tetrahydrofuran resulted in isolation of the mononuclear compound cis-[MoVIO 2(bihyat)] (1). The treatment of Na2MoVIO 4·2H2O with the ligand H2bihyat in aqueous solution gave the dinuclear compounds cis-[MoVI 2O4(bihyat)2(H2O)2] (2) and trans-[MoVI2O4(bihyat)2(H 2O)2] (3) 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 1 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) bihyat2- 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 {Mo 2VIO2(μ2-O2)} 4+ moiety. The potentiometry revealed that the Mo VIbihyat2- 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 1 with ligation of a weakly bound water molecule trans to the oxido ligand (1w) renders the equatorial oxido group in 1w more nucleophilic than that in 1, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds 2 and 3 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 (1 → 2/3). 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 1-3 have been studied by means of density functional theory calculations. © 2012 American Chemical Society. 51 24 13138 13147 Cited By :3

Bisphosphonates are biologically relevant therapeutics for bone disorders and cancer. Reaction ofγ-chlorobutyric acid, phosphorus acid, and phosphorus trichloride without the use of solvent gave the tetrahydrofuranyl-2,2-bisphosphonate sodium salt (Na2H2L). TheNa2H2Lwas isolated, characterized in solution byH,C, andPNMR spectroscopy and in solid state by single X-Ray crystallography. The crystal structure showed that theNa2H2Lforms in the crystal infinite two-dimensional sheets stacked one parallel to the other. A comparison of the chelating properties ofH2L2−with similar hydroxyl bisphosphonate ligands shows that the strength of the Na–O(furanyl/hydroxyl) bond is directly related to the total charge of the ligand anion.

Copper ions in the active sites of several proteins/enzymes interact with phenols and quinones, and this interaction is associated to the reactivity of the enzymes. In this study the speciation of theCu2+with iminodiacetic phenolate/hydroquinonate ligands has been examined by pH-potentiometry. The results reveal that the iminodiacetic phenol ligand forms mononuclear complexes withCu2+at acidic and alkaline pHs, and a binuclearOphenolate-bridged complex at pH range from 7 to 8.5. The binucleating hydroquinone ligand forms only 2 : 1 metal to ligand complexes in solution. The pK values of the protonation of the phenolate oxygen of the two ligands are reduced about 2 units after complexation with the metal ion and are close to the pK values for the copper-interacting tyrosine phenol oxygen in copper enzymes.

Reaction of the ligand 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5- triazine (H2bihyat) with NaVVO3 in aqueous solution followed by addition of either Ph4PCl or C(NH 2)3Cl, respectively, gave the mononuclear vanadium(V) compounds Ph4P[VVO2(bihyat)]·1.5H 2O (1) and C(NH2)3[VVO 2(bihyat)] (2). Treatment of VIVOSO4· 5H2O with the ligand H2bihyat in methyl alcohol under specific conditions gave the oxo-bridged dimer [VV2O 2(μ2-O)(bihyat)2] (3). The structures for 1 and 3 were determined by X-ray crystallography and indicate that these compounds have distorted square-pyramidal arrangement around vanadium. The ligand bihyat2- is bonded to vanadium atom in a tridentate fashion at the pyridine-like nitrogen atom and the two deprotonated hydroxylamino oxygen atoms. The high electron density of the triazine ring nitrogen atoms, which results from the resonative contribution of electrons of exocyclic nitrogen atoms (Scheme 4), leads to very strong V-N bonds. The cis-[VVO 2(bihyat)]- species exhibits high hydrolytic stability in aqueous solution over a wide pH range, 3.3-11.0, as it was evidenced by 1H and 51V NMR spectroscopy and potentiometry. The high affinity of the H2bihyat ligand for the VVO 2+ unit, its tridentate character, as well as its small size, paves the way for potential applications in medicine, analysis, and catalysis for the C(NH2)3[VVO 2(bihyat)] compound. The molecular structures, vibrational and electronic spectra, and the energetics of the metal-ligand interaction for compounds 1 and 3 have been studied by means of density functional calculations. © 2008 American Chemical Society. 47 24 11698 11710 Cited By :17