Activation-controlled outer-sphere electron transfer reactions: reduction of heteropoly 11-tungstovanadophosphate and heteropoly 10-tungstodivanado phosphate by thiourea in aqueous acid medium

The kinetics of reduction of heteropoly 11-tungstovanadophosphate, [PV^VW₁₁O₄₀]⁴⁻, (HPA1) and heteropoly 10-tungstodivanadophosphate, [PV^VV^VW₁₀O₄₀]⁵⁻, (HPA2) by thiourea has been investigated in HClO₄/phthalate/acetate buffer solutions spectrophotometrically at 25 °C in aqueous medium. The stoichiometry of the reaction is 1:1 in both cases. The HPAs are converted into the corresponding one-electron reduced heteropoly blues, namely, [PV^IVW₁₁O₄₀]⁵⁻ and [PV^IVV^VW₁₀O₄₀]⁶⁻, and thiourea is oxidised to formamidine disulphide. The reaction shows first-order dependence in both [HPA] and [thiourea] at constant pH. The rate–pH profile shows the participation of both the neutral and deprotonated forms of thiourea in the reaction. The reaction proceeds through an outer sphere electron transfer mechanism in which activation-controlled electron transfer is the rate-determining step. Self-exchange rate constants for the couples [PV^VW₁₁O₄₀]⁴⁻/[PV^IVW₁₁O₄₀]⁵⁻, [PV^VV^VW₁₀O₄₀]⁵⁻/[PV^IVV^VW₁₀O₄₀]⁶⁻ and H₂NCSNH₂/H₂NCS^·+NH₂ have been evaluated by Marcus theory.     

Complex formation in Nb6O 198? -WO 42? -H+-H2O system where cNb: cW = 4 : 2

Processes occurring in Nb₆O₁₉⁸⁻-WO₄²⁻-H⁺-H₂O system where c _Nb: c _W = 4: 2, c _Nb+W⁰ = 5 × 10⁻³, 2.5 × 10⁻³, or 10⁻³ mol/L, and ionic strengths I = 0.01–0.14 are created by NaCl background electrolyte were studied by pH titration and mathematical modeling. Solute ion species distribution diagrams were obtained for $ Z = \frac{{c_{H^ + }^0 }} {{c_{Nb + W}^0 }} = 0 - 1.5 $ Z = \frac{{c_{H^ + }^0 }} {{c_{Nb + W}^0 }} = 0 - 1.5 . The concentration constants and thermodynamic constants of formation were calculated for isopolyniobotungstate anions (IPNTAs). H _x Nb₄W₂O₁₉^(6−x)−, (x = 1–5), ions were shown to appear in solution only after Nb₆O₁₉⁸⁻ was protonated and aquapolytungstate anions were formed. The results of modeling were supported by the synthesis of Tl₃H₃Nb₄W₂O₁₉ · 16.5H₂O, Tl₂H₄Nb₄W₂O₁₉ · 11H₂O, and NaTl₃(H₄Nb₄W₂O₁₉)₂ · 22H₂O salts, which were identified by chemical analysis and IR spectroscopy.     

Thermochemistry of copper complex of 6-benzylaminopurine

The copper(II) complex of 6-benzylaminopurine (6-BAP) has been prepared with dihydrated cupric chloride and 6-benzylaminopurine. Infrared spectrum and thermal stabilities of the solid complex have been discussed. The constant-volume combustion energy, Δ_c U, has been determined as −12566.92±6.44 kJ mol⁻¹ by a precise rotating-bomb calorimeter at 298.15 K. From the results and other auxiliary quantities, the standard molar enthalpy of combustion, Δ_c H _m ^θ, and the standard molar of formation of the complex, Δ_f H _m ^θ, were calculated as −12558.24±6.44 and −842.50±6.47 kJ mol⁻¹, respectively.     

Nucleophilic Substitution Reactions at Planar Tetra-coordinate Bis-cationic-platinum(II) Complexes. Kinetics of Displacement of Pyridine from {Pt[2,6-bis(methylthiomethyl)pyridine](py)}2+, {Pt[bis(2-pyridylmethyl)amine] (py)}2+ and {Pt[bis(2-(pyridylmethyl)sulphide](py)}2+ Cations

The kinetics of nucleophilic substitution of pyridine in bis-cationic [Pt(L)(py)]²⁺ complexes (L=SNS, NNN, NSN) [SNS=bis(methylthiomethyl)pyridine, NNN=bis(2-pyridylmethyl)amine, NSN=bis(2-pyridylmethyl)sulphide] by a series of nucleophiles (Cl⁻, Br⁻, I⁻, N₃⁻, (C₂H₅)₂S, NH₃, thiourea (tu), NO₂⁻, C₅H¹0NH, SeCN⁻, SCN⁻, CN⁻ when L=SNS; Cl⁻, Br⁻, I⁻, N₃⁻, (C₂H₅)₂S, SCN⁻, NH₃, NO₂⁻ when L=NNN; Br⁻, N₃⁻, NO₂⁻, NH₃, C₅H₁₀NH when L=NSN) have been measured in MeOH at 25 °C, μ =0.1 mol dm⁻³ (LiClO₄ or LiCF₃SO₃). The logarithms of the second-order rate constants calculated at μ=0, log k° ₂, do not follow the dependence upon the n° _Pt scale. In particular, the reactivity of the biphilic reagents tu, SeCN⁻, SCN⁻ and, to a lesser extent, NO⁻ ₂, towards these doubly charged substrates is largely lower than expected on the basis of the n° _Ptscale. There are good linear relationships between logk° ₂ for the bis-cationic substrate [Pt(SNS)(py)]²⁺, chosen as the standard, and log k° ₂ for the same reactions with [Pt(NNN)(py)]²⁺, [Pt(NSN)(py)]²⁺ and other double charged complexes previously studied. A new wide nucleophilicity scale based on [Pt(SNS)(py)]²⁺, that is appropriate to all the bis-cationic substrates, is here proposed     

Effect of Substrate Concentration on Dark Fermentation Hydrogen Production Using an Anaerobic Fluidized Bed Reactor

The effect of substrate (glucose) concentration on the stability and yield of a continuous fermentative process that produces hydrogen was studied. Four anaerobic fluidized bed reactors (AFBRs) were operated with a hydraulic retention time (HRT) from 1 to 8 h and an influent glucose concentration from 2 to 25 g L⁻¹. The reactors were inoculated with thermally pre-treated anaerobic sludge and operated at a temperature of 30 °C with an influent pH around 5.5 and an effluent pH of about 3.5. The AFBRs with a HRT of 2 h and a feed strength of 2, 4, and 10 g L⁻¹ showed satisfactory H₂ production performance, but the reactor fed with 25 g L⁻¹ of glucose did not. The highest hydrogen yield value was obtained in the reactor with a glucose concentration of 2 g L⁻¹ when it was operated at a HRT of 2 h. The maximum hydrogen production rate value was achieved in the reactor with a HRT of 1 h and a feed strength of 10 g L⁻¹. The AFBRs operated with glucose concentrations of 2 and 4 g L⁻¹ produced greater amounts of acetic and butyric acids, while AFBRs with higher glucose concentrations produced a greater amount of solvents.     

Three organic–inorganic hybrid complexes based on the Wells–Dawson polyoxoanion

Three new organic–inorganic hybrid complexes based on the Wells–Dawson polyoxoanion, namely (H₂bpp)[Ni₂(bpp)₂(H₂O)₄(P₂W₁₈O₆₂)]·H₂O 1, [Cu₆(Hbpy)₆(bpy)₃(P₂W₁₈O₆₂)₂]·2H₂O 2 and (Him)₅[Cu(im)₂(P₂W₁₈O₆₂)]·4H₂O 3 [bpp = 1,3-bis (4-pyridyl) propane, bpy = 4,4′-bipyridine, im = imidazole] have been synthesized and characterized. Complex 1 exhibits a three-dimensional (6, 3)-connected framework with anatase topology constructed from [α-P₂W₁₈O₆₂]⁶⁻ clusters and [Ni(bpp)]²⁺ fragments. Each [α-P₂W₁₈O₆₂]⁶⁻ anion links to six nickel atoms through six terminal oxygen atoms from four polar and two equatorial WO₆ octahedra, which shows a novel coordination mode of a Wells–Dawson cluster with a transition-metal atom. Complex 2 displays an interesting one-dimensional double-chain structure built from [α-P₂W₁₈O₆₂]⁶⁻ clusters and [Cu₂(bpy)(Hbpy)₂]²⁺ fragments. To our knowledge, complex 2 represents the first double-chain organic–inorganic hybrid complex based on a Wells–Dawson-type cluster. Complex 3 possesses a one-dimensional zigzag chain structure constructed from [α-P₂W₁₈O₆₂]⁶⁻ anions and [Cu(im)₂]⁺ units through weak Cu···O interactions.     

Two New {P<Subscript>8</Subscript>W<Subscript>49</Subscript>} Wheel-shaped Tungstophosphates Decorated by Co(II), Ni(II) Ions

Two new wheel-shaped tungstophosphates based on 3d-transition metals (Co(II), Ni(II)) ions decorated [P₈W₄₉O₁₈₇]⁴⁰⁻ anion (TM-{P₈W₄₉}), K₄Na₂₂{[Co(H₂O)₂Cl][Co(H₂O)₃]₂[Co(H₂O)₅]_1.5 [Co(H₂O)₃H₄P₈W₄₉O₁₈₇(H₂O)]}·2NaCl·41·5H₂O 1 and Na₃₀{[Ni(H₂O)₃]₂[{Ni(H₂O)₃}_1.5H₃P₈W₄₉O₁₈₇ (H₂O)]}·41.5H₂O 2 have been synthesized by routine synthetic reaction of hexavacant Dawson polyoxonanion [P₂W₁₂O₄₈]¹⁴⁻ with divalent 3d transition-metal ions in aqueous solution. The two compounds are characterized by elemental analysis, IR spectroscopy, TG analysis, electrochemical analysis, and single-crystal X-ray diffraction. Both compounds contain a 2D layer-like structure constructed from 1D chains of wheel-type [P₈W₄₉O₁₈₇]⁴⁰⁻ anions bridged via CoO₆ or NiO₆ units. Cyclic voltammograms and ³¹P NMR analysis suggest that the polyanion [P₈W₄₉O₁₈₇]⁴⁰⁻ of both compounds are stable in aqueous solution (pH = 4).     

Platinum–tin complexes as catalysts for the anodic oxidation of borohydride

Platinum–tin complexes were prepared by the reduction of Pt(IV) with Sn(II) in HCl media and studied by light absorption spectrometry, X-ray photoelectron spectroscopy (XPS), and electron microscopy. The formation of three complexes, H₃[Pt(SnCl₃)₅], H₂[Pt(SnCl₃)₂Cl₂], and H₂[Pt₃(SnCl₃)₈], depending on HCl and SnCl₂ concentrations, has been shown. The glassy carbon (GC) electrode modified in the complexes solutions was found to be an electrocatalyst for borohydride oxidation in a 1.0-M NaOH solution. Comparison of BH₄ ⁻ electrooxidation on Pt and on GC modified with platinum–tin complexes has shown that catalytic hydrolysis of BH₄ ⁻ did not proceed in the latter case in contrast to its oxidation on the Pt electrode, and only direct BH₄ ⁻ oxidation has been observed in the positive potentials scan. The activity of Pt–Sn complexes for BH₄ ⁻ oxidation changes with time and eventually decreases due to Sn(II), bound in the complex with Pt(II), oxidation by atmospheric oxygen. The complexes may be renewed by addition of missing amounts of SnCl₂ and HCl.     

Chitosan-encapsulated Keggin Anion [Ti2W10PO40]7−. Synthesis, Characterization and Cellular Uptake Studies

The Keggin type polyoxotungstate [Ti₂W₁₀PO₄₀]⁷⁻ forms stable associates with the biopolymer chitosan in the nanometer size range. The cluster compound crystallizes from aqueous solution as K₄H₃[Ti₂W₁₀PO₄₀] · 15H₂O having a tetragonal structure. Both, the cluster compound and the chitosan/[Ti₂W₁₀PO₄₀] associates show a high hydrolytic stability at pH 7.4. The associates formed between the cluster anion [Ti₂W₁₀PO₄₀]⁷⁻ with the polyaminosaccharide chitosan have been characterized by photon correlation spectroscopy, scanning electron microscopy, filtration, centrifugation and zeta potential measurements. The size of the associates formed is in the range of ca. 5×10¹ to 5×10² nm. These particles have a defined stoichiometry with 5–6 cluster anions bound per molecule chitosan. The isoelectric point determined by zeta potential measurements was found for a cluster anion to chitosan molar ratio of 5.5, indicating the charge neutralization between protonated chitosan and [Ti₂W₁₀PO₄₀]⁷⁻ anions. Cellular uptake studies with [Ti₂W₁₀PO₄₀]⁷⁻ using tumor cell lines FaDu (human squamous carcinoma) and HT-29 (human adenocarcinoma) showed that the tungsten amount inside the cells is remarkably enhanced in the presence of chitosan.     

Synthesis and Structural Chemistry of Tungstoarsenates(V)

An overview of the synthesis and of the structural chemistry of tungstoarsenates(V) of the 1:12 and 2:21 families is presented with special emphasis on the 2:20 species. The molecular structure of [As₂W₂₀O₇₁(H₂O)₃]¹²⁻ has been determined by single-crystal X-ray diffraction of K₁₂[As₂W₂₀O₇₁(H₂O)₃]·23H₂O and that of [As₂W₂₀O₇₀(H₂O)₂]¹⁰⁻ has been inferred from a ¹⁸³W NMR study in aqueous solution.This paper is dedicated to Professor Michael Pope in recognition of his invaluable contribution to polyoxometalate chemistry.     

Methane conversion to C2 hydrocarbons in solid electrolyte membrane reactors

Solid electrolyte membrane reactors (SEMRs) have been used to both study and influence catalytic reaction rates. Methane coupling is the reaction most thoroughly and intensively studied in these membrane reactors. In the last 20 years, oxygen ion (O²⁻), proton (H⁺) and mixed (O²⁻-e⁻, H⁺-e⁻) conducting membranes have been tested in order to maximize the conversion of methane to C₂ compounds. The present review contains the fundamental operating principles of the various SEMR types and their applications in this reaction. The difficulties that should be overcome in order to promote this SEMR process to an industrial scale are discussed.     

Effect of polyether diamine on gas permeation properties of organic-inorganic hybrid membranes

The quantitative incorporation and high dispersion of platinum nanoparticles into MCM-41 has been carried out by the coordination between Pt(IV) ion and APTMS-anchored MCM-41. Before and after calcination of Pt/APTMS/MCM41 samples, the Pt content in samples was evaluated from home-made photoacoustic spectrometer (PAS). The PAS bands at 350 nm and 450 nm can be assigned to d–d transition bands of Pt complexes. By increasing the concentration of Pt solution, the PAS intensity of Pt/APTMS/MCM41 was increased proportionally up to 1.0×10⁻² M, and remained constant above 1.0×10⁻² M. It can be attributed to the saturation of Pt content within Pt/APTMS/MCM41. The Pt content in the saturated Pt/APTMS/MCM41 was 8.5 wt% (the theoretical value), 9.7 wt% (measured by EDX) and 9.2 wt% (measured by ESCA), respectively. This indicates that the content of Pt precursor within MCM-41 could be controlled by the concentration of Pt precursor solution. The PAS intensity of calcined Pt/APTMS/MCM41's in H₂ flow was increased up to 1.0×10⁻² M and remained nearly constant above 1.0×10⁻² M. Therefore, we suggest that the formation of Pt complexes with APTMS-anchored MCM-41 made it possible to incorporate quantitatively Pt nanoparticles in the range of 0.5–9.2 wt% within MCM-41 channels.     

Kinetics and mechanism of aqua ligand substitution from cis-diaqua(cis-1,2-diaminocyclohexane)platinum(II)perchlorate by diethyldithiocarbamate anion in aqueous medium

The kinetics of the interaction of diethyldithiocarbamate (Et₂DTC) with [Pt(dach)(H₂O)₂]²⁺ (dach = cis-1,2-diaminocyclohexane) have been studied spectrophotometrically as a function of [Pt(dach)(H₂O)₂ ²⁺], [Et₂DTC] and temperature at a particular pH (4.0). The reaction proceeds via rapid outer sphere association complex formation followed by two slow consecutive steps. The first step involves the transformation of the outer sphere complex into an inner sphere complex containing a Pt–S bond and one aqua ligand, while the second step involves chelation when the second aqua ligand is replaced. The association equilibrium constant K _E and two rate constants k ₁ and k ₂ have been evaluated. Activation parameters for both the steps have been calculated (∆H ₁ ^# = 66.8 ± 3.7 kJ mol⁻¹, ∆S ₁^# = −81 ± 12 JK⁻¹ mol⁻¹ and ∆H ₂^# = 95.1 ± 2.8 kJ mol⁻¹, ∆S ₂^# = −34.4 ± 9.1 JK⁻¹ mol⁻¹). The low enthalpy of activation and negative entropy of activation indicate an associative mode of activation for both the steps.     

Reduction of the photoexcited uranyl ion by water

Photooxidation of water by the uranyl ion was studied. Solutions of uranyl in 0.01–4.0M H₂SO₄, HClO₄, or 0.1–1.0M Na₂SO₄ and NaClO₄ containing “lacunary” heteropolytungstate (HPT) K₁₀P₂W₁₇O₆₁ or K₈SiW₁₁O₃₉ were irradiated with a nitrogen laser, a mercury or xenon lamp, or visible light. Spectrophotometric analysis showed that the irradiation results in the accumulation of U^IV. Simultaneously the formation of H₂O₂ proceeds. The quantum yield Φ of the reaction increases as the concentration of the acid or salt increases. For aerated solutions of 1M H₂SO₄ or 1M HClO₄, irradiation by light with λ=337.1 Φ is close to (1.5–2)·10⁻³. The irradiation of solutions with pH −4 for many days leads to an almost quantitative transformation of UO₂ ²⁺ into U^IV. When the irradiation was carried out in the absence of HPA, U^IV was not detected, although hydrogen peroxide was observed in the solution. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 2, pp. 282–287, February, 2000.     

Synthesis,spectroscopic analysis and structure deduction of gold(III), palladium(II) and platinum(II) complexes with the tripeptide glycyl-<Emphasis Type="SmallCaps">l</Emphasis>-phenylalanyl-glycine

The coordination behaviour of the tripeptide glycyl-l-phenylalanyl-glycine (H-Gly-Phe-Gly-OH) with Au(III), Pd(II), and Pt(II) in both solution and in the solid state has been investigated experimentally. In addition, quantum chemical calculations have been carried out with a view to obtain the structures and spectroscopic properties of the ligand and its complexes. Both in solution and in the solid state the tripeptide interacts in a tetradentate manner with the Au(III) and Pd(II) ions through the NH₂, two deprotonated amide N atoms and the COO⁻group, forming [Au(H-Gly-Phe-Gly-OH)H₋₂)] × H₂O and [Pd(H-Gly-l-Phe-Gly-OH)H₋₂)]Na × H₂O complexes. The MN₃O chromophores are calculated to be near planar. Interaction with cisplatin leads to the formation of a mononuclear complex with tridentate coordination of the ligand by NH₂ and two N- atoms from the deprotonated amide groups ([Pt(H-Gly-l-Phe-Gly-OH)H₋₂)NH₃] × 2H₂O). The fourth coordination position of the Pt(II) is occupied by an NH₃ ligand. The PtN₄ chromophore is flat with a deviation from planarity of 0.3°. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

One-dimensional inorganic-organic hybrid material [Cu(En)2]2H2[{Cu(En)2}2(H2W12O42)] · 6.5H2O: Hydrothermal syntheses and crystal structure

A novel inorganic-organic hybrid material, [Cu(En)₂]₂H₂[{Cu(En)₂}₂(H₂W₁₂O₄₂)] · 6.5H₂O (I) based on polyoxotungstates and Cu²⁺ ions, has been successfully synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction, IR spectra, and TG analysis. Structural analysis indicates that the structure consists of a one-dimensional structure constructed from the polyoxoanion [H₂W₁₂O₄₂]¹⁰⁻ bridged by {Cu(En)₂}²⁺ groups. Each [H₂W₁₂O₄₂]¹⁰⁻ cluster in the polymeric chain is linked to two neighbors through four coordination groups of μ₂-[Cu(En)₂]²⁺. Furthermore, it extends into a three-dimensional supramolecular network by hydrogen bonding interactions.     

Electrochemistry of uranium in LiF–BeF2 melt

A study of the electrochemistry of uranium in LiF–BeF₂ system important for molten salt reactor concept was conducted at W and Ni electrodes. Cyclic voltammetry and chronopotentiometry methods were used. Two-step reduction mechanism for U⁴⁺ ions involving one electron exchange in soluble/soluble U⁴⁺/U³⁺ system and three electron exchange in the second step were found on W electrode. Both processes were identified as reversible and diffusion-controlled. Based on voltammetric and chronopotentiometric measurements, the diffusion coefficient of U⁴⁺ ions at 813 K was calculated: D(U⁴⁺) = 1.26 × 10⁻⁶ cm² s⁻¹ and D(U⁴⁺) = 1.28 × 10⁻⁶ cm² s⁻¹, respectively. Formation of U–Ni alloys was observed on Ni electrode.     

Measurement of neutron flux and albedo of water for thermal neutrons with foils of indium in a subcritical nuclear reactor

A subcritical nuclear reactor, Model 9000, Nuclear Chicago, is installed and operating at the Aristotle University of Thessaloniki, in the Atomic and Nuclear Physics Laboratory, at Thessaloniki, Northern Greece. The fuel is about 5500 lbs (2495 kgs) natural uranium metal (U₃U₈), the moderator about 3600 lbs (1633 kgs) light water, H₂O and the reflector is also light, water, H₂O. The lattice core is hexagonal, 42 inches (1.07 m) high and of 35 inches (88.90 cm) maximum diameter. The neutron source at the core is Am-Be 5 curies (185 GBq), 1.1·10⁷n·s⁻¹. The reactor is used for the activation of various materials by neutrons such as indium, the determination of the thermal neutrons flux, the horizontal and the vertical distribution of the neutron flux, material buckling, B, and geometric buckling, B, the parameters of the reactor, and the albedo of water for thermal neutrons with foils of indium. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis, structure, and characterization of a 1D monocopper substituted Dawson-type arsenotungstate (H2En)0.5[Cu(En)2]0.5{[Cu(En)2(H2O)]2[Cu(En)2] (α1-As2W17CuO61)} · 8H2O

A new 1D organic-inorganic hybrid monocopper substituted Dawson-type arsenotungstate (H₂En)_0.5[Cu(En)₂]_0.5{[Cu(En)₂(H₂O)]₂[Cu(En)₂](α₁-As₂W₁₇CuO₆₁)} · 8H₂O (I) has been hydrothermally synthesized by the reaction of Na₈[A-α-HAsW₉O₃₄] · 11H₂O, CuCl₂ · 2H₂O, YCl₃, with enthylenediamine (En) and characterized by elemental analyses, IR spectra, UV spectra, powder X-ray diffraction, thermogravimetric analysis, as well as single-crystal X-ray diffraction. Single-crystal structural analysis shows that I displays an interesting 1D chain constructed from [α₁-As₂W₁₇CuO₆₁]⁸⁻ subunits by means of common oxygen atoms and [Cu(En)₂]²⁺ bridges. It should be noted that the structural transformation of the trivacant Keggin polyoxoanion [A-α-AsW₉O₃₄]⁹⁻ to the monovacant Dawson [α-As₂W₁₇O₆₁]¹⁰⁻ polyoxoanion was observed. The TG curve of I exhibits three stages of weight loss.     

Thermodynamic parameters for the protonation of urea at different temperatures and ionic strengths

Potentiometric measurements of aqueous urea solutions (less than2.5 mol-dm⁻³ of urea) were performed at different temperature and ionic strengths (10≤T≤45°C; 0.02≤I≤0.6 mol-dm⁻³) to determine thermodynamic protonation parameters. The formation of the species U₂H⁺ was hypothesized to explain the concentration dependence of the protonation constant.