Binding N2, N2H2, N2H4, and NH3 to transition-metal sulfur sites: modeling potential intermediates of biological N2 fixation

In the quest for low-molecular-weight metal sulfur complexes that bind nitrogenase-relevant small molecules and can serve as model complexes for nitrogenase, compounds with the [Ru(PiPr(3))('N(2)Me(2)S(2)')] fragment were found ('N(2)Me(2)S(2)'(2-)=1,2-ethanediamine-N,N'-dimethyl-N,N'-bis(2-benzenethiolate)(2-)). This fragment enabled the synthesis of a first series of chiral metal sulfur complexes, [Ru(L)(PiPr(3))('N(2)Me(2)S(2)')] with L=N(2), N(2)H(2), N(2)H(4), and NH(3), that meet the biological constraint of forming under mild conditions. The reaction of [Ru(NCCH(3))(PiPr(3))('N(2)Me(2)S(2)')] (1) with NH(3) gave the ammonia complex [Ru(NH(3))(PiPr(3))('N(2)Me(2)S(2)')] (4), which readily exchanged NH(3) for N(2) to yield the mononuclear dinitrogen complex [Ru(N(2))(PiPr(3))('N(2)Me(2)S(2)')] (2) in almost quantitative yield. Complex 2, obtained by this new efficient synthesis, was the starting material for the synthesis of dinuclear (R,R)- and (S,S)-[micro-N(2)[Ru(PiPr(3))('N(2)Me(2)S(2)')](2)] ((R,R)-/(S,S)-3). (Both 2 and 3 have been reported previously.) The as-yet inexplicable behavior of complex 3 to form also the R,S isomer in solution has been revealed by DFT calculations and (2)D NMR spectroscopy studies. The reaction of 1 or 2 with anhydrous hydrazine yielded the hydrazine complex [Ru(N(2)H(4))(PiPr(3))('N(2)Me(2)S(2)')] (6), which is a highly reactive intermediate. Disproportionation of 6 resulted in the formation of mononuclear diazene complexes, the ammonia complex 4, and finally the dinuclear diazene complex [micro-N(2)H(2)[Ru(PiPr(3))('N(2)Me(2)S(2)')](2)] (5). Dinuclear complex 5 could also be obtained directly in an independent synthesis from 1 and N(2)H(2), which was generated in situ by acidolysis of K(2)N(2)(CO(2))(2). Treatment of 6 with CH(2)Cl(2), however, formed a chloromethylated diazene species [[Ru(PiPr(3))('N(2)Me(2)S(2)')]-micro-N(2)H(2)[Ru(Cl)('N(2)Me(2)S(2)CH(2)Cl')]] (9) ('N(2)Me(2)S(2)CH(2)Cl'(2-) =1,2-ethanediamine-N,N'-dimethyl-N-(2-benzenethiolate)(1-)-N'-(2-benzenechloromethylthioether)(1-)]. The molecular structures of 4, 5, and 9 were determined by X-ray crystal structure analysis, and the labile N(2)H(4) complex 6 was characterized by NMR spectroscopy.     

Chromium(III) Hydroxide Solubility in the Aqueous K+-H+-OH?-CO2-HCO 3? -CO 32? -H2O System: A?Thermodynamic Model

Chromium(III)-carbonate reactions are expected to be important in managing high-level radioactive wastes. Extensive studies on the solubility of amorphous Cr(III) hydroxide solid in a wide range of pH (3–13) at two different fixed partial pressures of CO₂(g) (0.003 or 0.03 atm.), and as functions of K₂CO₃ concentrations (0.01 to 5.8 mol⋅kg⁻¹) in the presence of 0.01 mol⋅dm⁻³ KOH and KHCO₃ concentrations (0.001 to 0.826 mol⋅kg⁻¹) at room temperature (22±2 °C) were carried out to obtain reliable thermodynamic data for important Cr(III)-carbonate reactions. A combination of techniques (XRD, XANES, EXAFS, UV-Vis-NIR spectroscopy, thermodynamic analyses of solubility data, and quantum mechanical calculations) was used to characterize the solid and aqueous species. The Pitzer ion-interaction approach was used to interpret the solubility data. Only two aqueous species [Cr(OH)(CO₃)₂²⁻ and Cr(OH)₄CO₃³⁻] are required to explain Cr(III)-carbonate reactions in a wide range of pH, CO₂(g) partial pressures, and bicarbonate and carbonate concentrations. Calculations based on density functional theory support the existence of these species. The log ₁₀ K° values of reactions involving these species [{Cr(OH)₃(am) + 2CO₂(g)⇌Cr(OH)(CO₃)₂²⁻+2H⁺} and {Cr(OH)₃(am) + OH⁻+CO₃²⁻ ⇌Cr(OH)₄CO₃³⁻}] were found to be −(19.07±0.41) and −(4.19±0.19), respectively. No other data on any Cr(III)-carbonato complexes are available for comparisons.     

Ethylene tetramerization: a new route to produce 1-octene in exceptionally high selectivities

Linear alpha-olefins, such as 1-hexene and 1-octene, are important comonomers in the production of linear low-density polyethylene (LLDPE). The conventional method of producing 1-hexene and 1-octene is by oligomerization of ethylene, which yields a wide spectrum of linear alpha-olefins (LAOs). While there exists several processes for producing 1-hexene via ethylene trimerization, a similar route for the selective production of 1-octene has so far been elusive. We now, for the first time, report an unprecedented ethylene tetramerization reaction that produces 1-octene in selectivities exceeding 70%, using an aluminoxane-activated chromium/((R2)2P)2NR1 catalyst system.     

Experimental and theoretical characterization of the aromatization, epimerization, and fragmentation reactions of bi-2H-azirin-2-yls prepared from 1,4-diazidobuta-1,3-dienes

1,4‐Diazidobuta‐1,3‐dienes (Z,Z)‐ 10 , 17 , and 21 were photolyzed and thermolyzed to yield the pyridazines 13 , 20 , and 23 , respectively. To explain these aromatic final products, the generation of highly strained bi‐2H‐azirin‐2‐yls 12 , 19 , and 22 and their valence isomerization were postulated. In the case of meso‐ and rac‐ 22 , nearly quantitative formation from diazide 21 , isolation as stable solids, and complete characterization were possible. On the thermolysis of 22 , aromatization to 23 was only a side reaction, whereas equilibration of meso‐ and rac‐ 22 and fragmentation, which led to alkyne 24 and acetonitrile, dominated. Prolonged irradiation of 22 gave mainly the pyrimidine 25 . The change of the configuration at C‐2 of the 2H‐azirine unit was observed not only in the case of bi‐2H‐azirin‐2‐yls 22 but also for simple spirocyclic 2H‐azirines 29 at a relatively low temperature (75 °C). The fragmentation of rac‐ 22 to give alkyne 24 and two molecules of acetonitrile was also studied by high‐level quantum chemical calculations. For a related model system 30 (methyl instead of phenyl groups), two transition states TS‐ 30 – 31 of comparable energy with multiconfigurational electronic states could be localized on the energy hypersurface for this one‐step conversion. The symmetrical transition state complies with the definition of a coarctate mechanism.     

Solubility of (UO2)3(PO4)2?4H2O in H+-Na+-OH?-H2PO?4-HPO2?4-PO3?4-H2O and Its Comparison to the Analogous PuO2 +2 System

The objectives of this study were to address uncertainties in the solubility product of (UO₂)₃(PO₄)₂⋅4H₂O(c) and in the phosphate complexes of U(VI), and more importantly to develop needed thermodynamic data for the Pu(VI)-phosphate system in order to ascertain the extent to which U(VI) and Pu(VI) behave in an analogous fashion. Thus studies were conducted on (UO₂)₃(PO₄)₂⋅4H₂O(c) and (PuO₂)₃(PO₄)₂⋅4H₂O(am) solubilities for long-equilibration periods (up to 870 days) in a wide range of pH values (2.5 to 10.5) at fixed phosphate concentrations of 0.001 and 0.01 M, and in a range of phosphate concentrations (0.0001–1.0 M) at fixed pH values of about 3.5. A combination of techniques (XRD, DTA/TG, XAS, and thermodynamic analyses) was used to characterize the reaction products. The U(VI)-phosphate data for the most part agree closely with thermodynamic data presented in Guillaumont et al.,⁽¹⁾ although we cannot verify the existence of several U(VI) hydrolyses and phosphate species and we find the reported value for formation constant of UO₂PO⁻₄ is in error by more than two orders of magnitude. A comprehensive thermodynamic model for (PuO₂)₃(PO₄)₂⋅4H₂O(am) solubility in the H⁺-Na⁺-OH⁻-Cl⁻-H₂PO⁻₄-HPO²⁻₄-PO³⁻₄-H₂O system, previously unavailable, is presented and the data shows that the U(VI)-phosphate system is an excellent analog for the Pu(VI)-phosphate system.     

Development and validation of a rapid LC-ESI-MS/MS method for quantification of fluoxetine and its application to MS binding assays

In the present study, a rapid and sensitive LC-ESI-MS/MS method for quantification of (S)-fluoxetine as a native marker in mass spectrometry (MS) binding assays addressing the human serotonin transporter (hSERT) was developed and validated. The concept of MS binding assays based on mass spectrometric quantification of a nonlabeled marker recently introduced by us represents a promising alternative to conventional radioligand binding without the drawbacks inherently connected with radioisotope labeling. For high-performance liquid chromatography (HPLC), a 20 × 2-mm RP-18 column with a mobile phase composed of acetonitrile and ammonium bicarbonate buffer (5 mmol L⁻¹, pH 9.5) at a ratio of 80:20 (v/v) and a flow rate of 800 μL min⁻¹ in an isocratic mode were used, resulting in a chromatographic cycle time of 60 s. Employing [²H₅]fluoxetine as internal standard enabled ESI-MS/MS quantification of (S)-fluoxetine between 3 nmol L⁻¹ and 50 pmol L⁻¹ (LLOQ) in matrix obtained from binding experiments without the need of any sample preparation. Validation of the method showed that linearity, intra-, and inter-batch accuracy as well as precision meet the requirements of the FDA guidance for bioanalytical method validation. Considering sensitivity and speed, the established method is clearly superior to those published for biological matrices so far. Furthermore, the method was transferred to other RP-18 columns of different lengths and respective validation experiments demonstrated its versatility and chromatographic robustness. Finally, the newly developed method was successfully applied to MS binding assays for hSERT. The affinity determined for (S)-fluoxetine in saturation experiments was in good agreement with literature data obtained in respective radioligand binding assays.     

Zur Bestimmung der gesamten und freien schwefligen S?ure im Wein

Zusammenfassung Ein Vergleich der Methoden von Weinmann u. Deibner zur Bestimmung der gesamten schwefligen Säure zeigte, daß die letzte Methode zu exakteren Werten führt,vor allem dadurch bedingt, daß das überdestillierte Schwefeldioxyd in einer sauren gekühlten Vorlage aufgefangen wird. Ein Nachteil der Methode ist, daß bei der Titration der Vorlage mit Jodlösung der p_H-Wert mehrfach geändert werden muß. Es gelang uns, diesen Mangel ohne Einbuße an Analysengenauigkeit dadurch zu beseitigen, daß wir als Auffanglösung ein Puffergemisch (p_H 3,5) benutzen und nicht in saurem, sondern in schwach alkalischem Milieu mit Jodlösung titrieren. In dieser Form kann das Verfahren zur Bestimmung der gesamten schwefligen Säure im Wein empfohlen werden.Zur genauen Bestimmung der freien schwefligen Säure arbeiteten wir ein polarographisches Verfahren aus, bei dem im Gegensatz zur jodometrischen Methode die koordinativ gebundene schweflige Säure nicht miterfaßt wird. Der verhältnismäßig große apparative Aufwand bei der Polarographie spricht jedoch gegen die Anwendung dieses Verfahrens in der routinemäßigen Weinanalyse. Es wird deshalb eine vereinfachte, aber recht genaue jodometrische Methode zur Bestimmung der freien schwefligen Säure vorgestellt, die geeignet ist, Aufnahme in die neue Allgemeine Verwaltungsvorschrift für die Untersuchung von Wein und ähnlichen alkoholischen Erzeugnissen sowie von Fruchtsäften zu finden, die an die Stelle der alten Amtlichen Anweisung getreten ist.Für die Untersuchungen wurden Mittel des Bundesernährungsministeriums zur Verfügung gestellt, wofür auch an dieser Stelle bestens gedankt sei.Die Arbeit stellt einen Ausschnitt dar aus der Diplomarbeit von D. Hess: Beitrag zur Bestimmung der schwefligen Säure im Wein (Universität Frankfurt/M. 1957) und der Dissertation von D. Hess: Über den Einfluß von schwefliger Säure und l-Ascorbinsäure bei der Weinbereitung (Frankfurt/M. 1960).     

Toward the development of new photolabile protecting groups that can rapidly release bioactive compounds upon photolysis with visible light

The synthesis and characterization of a new photolabile protecting group (caging group) for carboxylic acids, the 2-(dimethylamino)-5-nitrophenyl (DANP) group, is described. This compound has a major absorption band in the visible wavelength region with a maximum near 400 nm (epsilon400 = 9077 M(-1) cm(-1) at pH 7.4 and 21 degrees C). The caging group is attached through an ester linkage to the carboxyl functionality of beta-alanine, which activates the inhibitory glycine receptor in the mammalian central nervous system. Such caged compounds play an important role in transient kinetic investigations of fast cellular processes. Upon photolysis of DANP-caged beta-alanine, the caging group is released within 5 micros. Quantum yields of 0.03 and 0.002 were obtained in the UV region (308 and 360 nm) and the visible region (450 nm), respectively. Laser-pulse photolysis experiments, using 337 or 360 nm light, were performed with the caged compound equilibrated with HEK 293 cells transiently transfected with cDNA encoding the alpha1 homomeric, wild-type glycine receptor. The experiments demonstrated that neither DANP-caged beta-alanine nor its byproducts inhibit or activate the glycine receptors on the cell surface. Under physiological conditions, the DANP-caged beta-alanine is water-soluble and stable and can be used for transient kinetic measurements.     

In situ FTIR diagnostics of the radio-frequency plasma decomposition of N2O

The decomposition of N₂O in a 13.56-MHz parallel-plate system was studied usingin situ Fourier transform infrared (FTIR) spectroscopy. Areas of two infrared absorption bands of N₂O recorded at 8 cm^–1 resolution were used to estimate relative gas-phase dissociation as a function of rf power and flow rate at 500 mT. Flow rate was found to strongly affect band areas over the range of powers investigated (10–90 W). The effect of rf power on band areas diminished above 40 W, probably due to poor plasma confinement. Distortion of the band shapes by the plasma permitted rotational temperatures to be estimated. Rotational temperature increased essentially linearly with power at constant flow rate, reaching 450 K at 80 W, but was independent of flow rate at constant power. Rotational temperatures were also found to depend on the temperature of the electrodes, which were heated by plasma exposure. No infrared-active product species were observed even under batch conditions where all N₂O was irreversibly dissociated. This lack of detectable products and a 50% pressure rise observed in a batch study suggest that N₂ and O₂ are the primary stable discharge products.     

Cl (Pj)_ fine structure quantum yields from UV dissociation of PCl3, CCl4, CHCl3 and Cl2 in a supersonic jet

Jet-cooled phosphorus trichloride, carbon tetrachloride, chloroform and molecular chlorine are photodissociated in the UV wavelength range 235–238 nm. Chlorine atom photofragments Cl (²P_3/2) and Cl* (²P_1/2) are detected via resonance-enhanced (2+1) ionization throughout the 232–238 nm wavelength region. The relative Cl* yields, φ*=[Cl*]/([Cl]+[Cl*]), are measured for both ³⁵Cl and ³⁷Cl isotopes using the two-photon atomic transitions at 235.3 nm for Cl and 237.8 nm for Cl*. Preliminary results indicate that the Cl* yields are different for the two isotopes for some of the precursors. In addition, we obtained the two-photon oscillator strength of the Cl transition at 237.7 nm relative to the Cl* transition at 237.8 nm. The advantage of using the two-photon Cl transition at 237.7 nm for quantum yield measurements is discussed.