Metathesis of nitrogen atoms within triple bonds involving carbon, tungsten, and molybdenum

(ButO)3Mo triple bond N and W2(OBut)6(M triple bond M) react in hydrocarbons to form Mo2(OBut)6(M triple bond M) and (ButO)3W triple bond N via the reactive intermediate MoW(OBut)6(M triple bond M). (ButO)3W triple bond N and CH3C triple bond N15 react in tetrahydrofuran (THF) at room temperature to give an equilibrium mixture involving (ButO)3W triple bond N15 and CH3C triple bond N. The (ButO)3W triple bond N compound is similarly shown to act as a catalyst for N15-atom scrambling between MeC13 triple bond N15 and PhC triple bond N to give a mixture of MeC13 triple bond N and PhC triple bond N15. From studies of degenerate scrambling of N atoms involving (ButO)3W triple bond N and MeC13 triple bond N in THF-d8 by 13C(1H) NMR spectroscopy, the reaction was found to be first order in acetonitrile and the activation parameters were estimated to be DeltaH = 13.4(7) kcal/mol and DeltaS = -32(2) eu. A similar reaction is observed for (ButO)3Mo triple bond N and CH3C triple bond N15 upon heating in THF-d8. The reaction is suppressed in pyridine solutions and not observed for the dimeric [(ButMe2SiO)3W triple bond N]2. The reaction pathway has been investigated by calculations employing density functional theory on the model compounds (MeO)3M triple bond N and CH3C triple bond N where M = Mo and W. The transition state was found to involve a product of the 2 + 2 cycloaddition of M triple bond N and C triple bond N, a planar metalladiazacyclobutadiene. This resembles the pathway calculated for alkyne metathesis involving (MeO)3W triple bond CMe, which modeled the metathesis of (ButO)3W triple bond CBut. The calculations also predict that the energy of the transition state is notably higher for M = Mo relative to M = W.     

Determination of traces of molybdenum and tungsten by extraction and polarography of their salicoylhydroxamates

Molybdenum and tungsten salicoylhydroxamates have been extracted into methyl isobutyl ketone or a mixture of chloroform and isobutyl alcohol from 1.5M hydrochloric acid and subjected to DC and derivative pulse polarography (DPP) after addition of methanolic lithium chloride solution, phosphoric acid and water in defined proportions. Molybdenum gives two DC waves with E(1 2 ) at -125 and -525 mV and a sharp DPP peak at -75 mV, whereas tungsten shows a single DC wave at -840 mV and a DPP peak at -850 mV. The two metals can be determined down to the sub-ppm level.     

Determination of traces of uranium in tungsten and molybdenum by radiochemical neutron activation analysis via the fission product140Ba

A radiochemical separation procedure has been developed to determine traces of uranium in tungsten and molybdenum. In this procedure the fission product¹⁴⁰Ba, as indicator nuclide for uranium, is selectively separated from the matrix activities and from all other long-lived activation and fission products and obtained at high purity. The radionuclide in the final fraction is sufficiently pure so that it can be measured with high counting efficiency by -counting. The separation procedure consists of two steps: a cation-exchange separation to separate barium from the anionic matrix tungste or molybdate, and many other elements. In the second step the Ba-fraction is further purified by precipitation of barium as barium chloride in 8M hydrochloric acid. The precipitate is then dissolved in water for -counting via the Cerenkov effect. The chemical yield for barium is 94.6±2.6%. When samples of 0.1 g, a thermal neutron flux of 2·10¹³ n·cm^–2·s^–1, an irradiation time of 10 hours and a measuring time of 2 hours were applied, then the detection limit of uranium was 4 ng/g.Presented at the 3rd Intern. Conf. on Nuclear and Radiochemistry, Vienna, September 7–11, 1992.     

Determination of molybdenum traces in grass using a catalytic method

Summary A catalytic method, based on the hydrogen peroxide-iodide-ascorbic acid Landolt reaction is described for the determination of 0.00005–0.0003% molybdenum in grass samples. The sample (5 g) is dry-ashed, then treated with hydrochloric acid and hydrogen peroxide, extracted with benzoin--monoxime in chloroform, the organic phase mineralized and finally the molybdenum content determined by the catalytic method (measurement of reaction time) using potentiometric monitoring. Results are evaluated from a calibration graph; these agree well with those obtained with atomic absorption and plasma emission spectrometry. Relative standard deviations of reaction time measurements are below 2%.

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Bestimmung von Molybdänspuren in Gras mit Hilfe einer katalytischen Methode

Zusammenfassung Das beschriebene Verfahren beruht auf der Landolt-Reaktion zwischen Wasserstoffperoxid, Iodid und Ascorbinsäure und eignet sich zur Bestimmung von 0,00005–0,0003% Mo in Gras. Eine Probe von 5 g wird trocken verascht, mit Salzsäure und Wasserstoffperoxid behandelt, und mit Benzoin--monoxim in Chloroform extrahiert. Die organische Phase wird mineralisiert und schließlich der Molybdängehalt mit Hilfe der katalytischen Reaktion (Reaktionszeitmessung mit potentiometrischer Anzeige) bestimmt. Die Ergebnisse zeigen gute Übereinstimmung mit den Resultaten aus AAS und Plasma-Emissionsspektrometrie. Die relativen Standardabweichungen der Reaktionszeit-Messungen liegen unter 2%.

     

Determination of traces molybdenum by catalytic adsorptive stripping voltammetry

A reliable and very sensitive procedure for the determination of ultra trace of molybdenum is proposed. Molybdenum was determined by cathodic stripping differential pulse voltammetry based on the adsorption collection of the Mo(VI)-Tiron complex on a hanging mercury drop electrode (HMDE). The variation of peak current with pH, concentration of Tiron and chlorate, plus several instrumental parameters such as accumulation time, accumulation potential and scan rate, were optimized. Under optimized condition, the relationship between the peak current and molybdenum concentration is linear in the range of 0.010-21.0 ng ml⁻¹. The limit of detection was found to be 0.006 ng ml⁻¹. The relative standard deviation for 10 replicates determination of 0.6 and 10 ng ml⁻¹ Mo(VI) is equal to 1.3 and 0.9%, respectively. The method was applied to the determination of molybdenum in river water, tap water, well water, plant foodstuff samples such as cucumber, tomato, carrot, and certified steel reference materials.     

Determination of boron, carbon, nitrogen and oxygen in nickel by charged particle activation analysis

Summary The determination of boron, carbon, nitrogen and oxygen in nickel by charged particle activation using the reactions ¹⁰B(d, n)¹¹C, ¹²C(d, n)¹³N, ¹⁴N(p, )¹¹C and ¹⁶O(³He, p)¹⁸F is studied. The interference of ¹¹B(p, n)¹¹C with the ¹⁴N(p, )¹¹C reaction is taken into account. ¹¹C, ¹³N and ¹⁸F are separated by oxygen combustion followed by trapping of ¹¹CO₂ in NaOH and by steam distillation of ¹³NH₃ and of H₂Si¹⁸F₆, respectively. The results obtained were 0.129±0.035 g g^–1 for boron, 86.4±8.3 g g^–1 for carbon, 1.077±0.037 g g^–1 for nitrogen and 8.91±1.00 g g^–1 for oxygen. The results for nitrogen and oxygen agreed satisfactorily with those of other analytical methods.Grateful acknowledgement is made to Prof. J. Hoste for his interest shown in this work, to J. Pauwels (B.C.M.N., Geel) for providing the samples, to B. F. Schmitt (Bundesanstalt für Materialprüfung, Berlin) for helpful information concerning the radiochemical separation of ¹¹C and to the NFWO and the IIKW for financial support.     

Direct substitution of propargylic alcohol with oxygen, nitrogen, and carbon nucleophiles catalyzed by molybdenum(VI)

Efficient and direct substitution of propargylic alcohol with a variety of oxygen, nitrogen, and carbon nucleophiles catalyzed by MoO₂(acac)₂/NH₄PF₆ system was developed. The functional alkynes were obtained in modest to good yields with this versatile and practical protocol.     

Use of long-chain alkylamines for preconcentration and determination of traces of molybdenum, tungsten and rhenium by atomic-absorption spectroscopy-II: molybdenum in soils, sediments and natural waters

Molybdenum is extracted as the thiocyanate complex with the quaternary long-chain aliphatic amine Aliquat 336 in chloroform, followed by evaporation of the solvent, dissolution in MIBK, and atomic-absorption spectroscopy. The method is simple, rapid and sensitive, with few interference problems for the determination of the Mo content of soils and sediments in the range 0.1-1.0 ppm with a relative standard deviation better than 5% when 1-g samples are used. Quantitative extraction from large volumes of aqueous solution has also been confirmed, allowing the determination of Mo in natural waters in the ppM range.     

Determination of manganese in high-purity niobium, tantalum, molybdenum and tungsten metals with pan

A spectrophotometric method for the determination of 0.0005-0.10% of manganese in high-purity niobium, tantalum, molybdenum and tungsten metals is described. The matrix materials are separated from the manganese by extraction as cupferrates, after sample dissolution, then the red complex formed between manganese(II) and 1-(2-pyridylazo)-2-naphthol, PAN, is extracted into chloroform from an ammoniacal tartrate-cyanide medium. The absorbance of the extract is determined at 562 mmicro. With the exception of zinc and lead, other impurities present in the four high-purity metals described do not interfere with the proposed method.     

Determination of traces of molybdenum in foods by electrothermal atomic absorption spectrometry

Low levels of molybdenum in different food samples were determined by atomic absorption spectrometry with electrothermal atomization after destruction of organic matter using four different methods, involving dry ashing at 600°C, wet digestion with heating with nitric/sulphuric and nitric/sulphuric/perchloric acid mixtures or direct extraction with dilute hydrochloric acid. In all cases the working conditions, reproducibility, precision and accuracy were investigated. The dry ashing method was generally the optimum destruction method.     

Determination of vanadium,molybdenum and tungsten in chloralkali electrolysis brines by x-ray fluorescence spectrometry

The levels of vanadium, molybdenum and tungsten are critical in chloralkali electrolysis brines because these elements catalyze the hydrogen-producing side-reaction. Preconcentration by chelation with 8-quinolinol and subsequent adsorption on activated carbon, can be combined with energy- or wavelength-dispersive x-ray fluorescence. Under optimized conditions, the detection limits are in the μg 1^-1 range, and below the critical levels. Up to 100 /smg l^-1, reliable results are obtained for V, Mo and W. The coefficient of variation is typically 15% in the 25–100 μg l^-1 range.     

Photochemical interaction of molybdenum and tungsten phosphine hydride complexes with carbon dioxide

UV, IR, and¹H NMR spectra of photoproducts obtained by irradiation of phosphine hydride complexes MH₄L₄ (M = Mo and W; L = PHPh₂, PMePh₂, PEtPh₂, PEt₂Ph, and PBuPh₂) in an atmosphere of CO₂,¹³CO₂, and C¹⁸O₂ have been studied. The photoproducts can be formed due to the insertion of carbon dioxide both into the M-H bond and M-C and M-Ph bonds.Deceased.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2324–2326, September, 1996.     

Potentialities of infrared spectroscopy in determining traces of carbon or oxygen as carbon dioxide

The determination of traces of carbon dioxide by infrared spectroscopy has been investigated. For maximum sensitivity the absorption band at 2350 cm(-1) is measured, the total pressure being raised to one atmosphere by addition of an inert gas. With a micro gas-cell and a beam-condenser the limit of detection is 0.02 mug of carbon, and for 4, mug of carbon the coefficient of variation is 1.5%.     

Decomposition of electron-excited molecules of molybdenum and tungsten phosphine hydride and phosphine nitrogen complexes

The kinetics and mechanism of photodehydrogenation of the phosphine hydride complexes MH₄L₄ (M = Mo, W; L are phosphine ligands) and the formation of coordinatively unsaturated species M_L4 were studied by the absorbance of long-wavelength bands with λ_max at 450–460 nm appeared in the absorption spectra of the photoproducts. The rate constants of the reactions of the coordinatively unsaturated M(DPPE)₂ species (M = Mo, W; DPPE = Ph₂PCH₂CH₂PPh₂) with molecular nitrogen in benzene were determined (k _W = 200 s⁻¹, k _Mo = 8700 s⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 282–284, February, 2008.     

Trace determination of carbon and oxygen in tungsten by spark-source mass-spectrography

A high pumping-speed vacuum system, incorporating a mechanical cryo-pump, was designed and built to attach to the source of a spark-source mass-spectrometer to reduce the partial pressure of carbon- and oxygen-containing gases. Pressures in the 10(-10) mmHg range were obtained, which provide a suitable environment for the determination of carbon and oxygen in tungsten. The effect of the partial pressure of CO on carbon and oxygen determination has been studied and characterized. A determination limit of 1 ppm has been achieved for these elements in tungsten.