Synthese,Struktur und Thermolyse von NH4[Re3Br10]

Synthesis, Structure and Thermolysis of NH₄[Re₃Br₁₀] NH₄[Re₃Br₁₀] crystallizes as dark brown single crystals upon slow cooling of a hot, saturated hydrobromic-acid solution of [Re₃Br₉(H₂O)₂] after the addition of NH₄Br. The crystal structure (monoclinic, C2/m (Nr. 12); Z = 4; a = 1461.6(7), b = 1 085.6(4), c = 1030.3(7) pm, β = 92.63(4)°, V_m = 245.9(4)cm³/mol; R = 0.097, R_w = 0.043) contains [Re₃Br₁₂]^? units that share two common edges. These chains run along [010] and are held together by NH₄⁺ ions. Each NH₄⁺ is surrounded by eight Br^? from four different chains. The first step of the thermal decomposition at 290°C is the disproportionation to ReBr₃ (ReCl₃ type), rhenium metal and (NH₄)₂[ReBr₆]. Secondly, the internal reduction of (NH₄)₂[ReBr₆] at 390°C to rhenium metal takes place.     

New Benzo[<Emphasis Type="Italic">b</Emphasis>]xanthones from Diels-Alder Reactions of Chromone-3-carboxaldehydes with <Emphasis Type="Italic">ortho</Emphasis>-Benzoquinodimethanes

Summary.  New benzo[b]xanthone derivatives, having substituents in the A and D rings, were prepared from cycloaddition reactions of chromone-3-carboxaldehydes with ortho-benzoquinodimethanes, generated in situ from 1,3-dihydrobenzo[c]thiophene 2,2-dioxide, followed by oxidation of the obtained diastereomeric adducts. The structure of all compounds was fully established by 1D and 2D NMR, MS, and elemental analysis. The stereochemistry of the obtained diastereomeric cycloadducts was established by NOESY experiments. Corresponding author. E-mail: arturs@dq.ua.pt Received September 16, 2002; accepted September 20, 2002     

The ferrous verdoheme-heme oxygenase complex is six-coordinate and low-spin

A biosynthetic and enzymatic method was developed for the preparation of 13C-labeled verdoheme, which permits the 13C NMR spectroscopic characterization of this elusive intermediate in the heme oxidation path catalyzed by the enzyme heme oxygenase. The 13C NMR data indicate that the ferrous verdoheme complex of Neisseria meningitides heme oxygenase is hexacoordinate and diamagnetic, with a proximal histidine and likely a distal hydroxide as axial ligands. The coordination number and spin state of the ferrous verdoheme-heme oxygenase complex is in stark contrast to the pentacoordinate and paramagnetic nature of the heme-heme oxygenase complex and heme centers in general.     

Temperature Dependence of the Crystal Structure and EPR Spectrum of Bis(1,3,5-Trihydroxycyclohexane)copper(II) Tosylate. A Unified Interpretation Using a Model of Dynamic Vibronic Coupling

The crystal structure of bis(1,3,5-trihydroxycyclohexane)copper(II) tosylate is reported at temperatures of 293, 233, 188, 163, and 93 K, as are the structures of the Zn(II) and Ni(II) analogues at room temperature for comparison. The isomorphous compounds are triclinic, space group P&onemacr;, with one formula unit in the unit cell. The unit cell parameters of the Cu compound at 293 K are a = 6.456(5) ?, b = 9.505(3) ?, c = 12.544(3) ?, alpha = 76.57(2) degrees, beta = 87.48(4) degrees, gamma = 76.65(4) degrees. The centrosymmetric ZnO(6) and NiO(6) octahedra are tetragonally compressed with a slight orthorhombic distortion. The Cu(2+) polyhedra exhibit similar geometries, but with considerably larger deviations from a regular octahedron. Two of the three independent Cu-O bond lengths and two of the g-values change significantly as a function of temperature. A model of dynamic vibronic coupling is presented which explains both the EPR and structural data. Vibronic wave functions associated with a Jahn-Teller potential energy surface modified by an orthorhombic lattice "strain" are given. The temperature dependence of the structures is calculated from the nuclear parts and that of the g-values from the electronic parts of the wave functions. The temperature dependence of the structures and g-values is also interpreted using a simpler model involving an equilibrium between two forms of the complex which differ solely in their orientation in the crystal lattice, and the results of the two approaches are compared.     

Synthesis and characterization of Nb(OR)4[OC(O)OR] (R = Me,Et, Allyl) and their reaction with the parent alcohol to afford organic carbonates

Nb(OR)(4)[OC(O)OR] (R = Me, Et, allyl) compounds have been synthesized by the reaction of dimeric alkoxo complexes [Nb(OR)(5)](2) with CO(2). The kinetics of the reaction has shown that the monomer in equilibrium with the dimer is the species that reacts with CO(2). The resulting compounds are monomeric with a eta(2)-O,O-bonded hemicarbonate group. The CO(2) uptake is reversible. The hemicarbonates react with the parent alcohol to afford the relevant organic carbonate (RO)(2)CO with a turnover number (TON) of >2.     

Qualification of automated low-field NMR relaxometry for quality control of polymers in a production setting

Implementation of a low-field time-domain nuclear magnetic resonance (NMR) scanner as a diagnostic tool in the production of new polymer components is described in the context of qualification of a new QA/QC device. A study to determine the optimal experimental parameters was performed and a robotic autosampler was built to enable scanning of multiple pads. Relationships between T₂ values and physical properties of DC745 slabs were investigated, and the appropriate sampling parameters for the production setting were determined. Two versions of a robotic autosampler were built and, for the component described here, a fourth radial axis was required in addition to traditional X, Y, and Z movement to eliminate the large variability in T₂ due to inconsistent sample coverage caused by the complex rib geometry of the component. Data show that with appropriate choice of experimental conditions of the NMR detector and the detection geometry of the robotic autosampler, sufficient resolution of variations in cross-link density on the millimeter scale could be determined. All data to date demonstrate that low-field NMR devices are a feasible tool for use in production settings for non-destructive quality control of polymer components.     

Comparison of microwave-assisted extraction and waste extraction test (WET) preparation for inductively coupled plasma spectroscopic analyses of waste samples

The Waste Extraction Test (WET) is used in California as a complement to the Toxicity Characteristic Leaching Procedure (TCLP). The WET protocol consists of shaking a sample with citrate buffer and determining the metal content in the solution produced. This procedure requires a 1-to-10 waste-to-liquid ratio and 48 h for extraction. Although the WET protocol proves to be very useful, it is a time-consuming step in the determination of leaching and mobility. Therefore, a microwave extraction procedure was optimized to emulate the relative extraction efficiency obtained by the WET protocol. Lead, arsenic, and copper concentrations were measured by inductively coupled plasma mass or atomic spectrometry (ICP-MS or ICP-AES), following a strict quality assurance protocol. Results obtained with this new methodology were statistically comparable to those obtained by the WET protocol. This microwave extraction approach proved simple and fast, reducing sample treatment by almost 280%. A significant reduction also occurs in waste production, materials, labor, and chemical usage. Therefore, the microwave extraction procedure is recommended as a rapid and cost effective monitoring tool for waste samples when combined with or supplemented by the traditional WET protocol.     

C-C coupling of aryl groups and allyl derivatives on Pt(II)-phenanthroline fragments: crystal and molecular structure of the tbp [(η,η-2-allyl,5-methyl-phenyl)iodo(1,10-phenanthroline)platinum(II)] complex containing the N-N ligand in axial-equatorial coordination mode

The reactions between allyl compounds CH₂CHCH₂Fn bearing electron-withdrawing functional (Fn) groups and cationic {Pt(aryl)(1,10-phenanthroline)}⁺ fragments generated in situ are described. The aryl and platinum addition to the terminal and, respectively, internal unsaturated carbon is generally observed. The subsequent H-C_aryl bond activation, followed by HFn elimination, affords the ortho organic fragment (Pt)-aryl-CH₂CHCH₂ η¹,η²-chelate to the platinum. This process does not occur when the regiochemistry is of Markownikov type and the Pt-CH₂CH(aryl)CH₂Fn fragment is formed. The described results are compared with those concerning the behaviour of unsubstituted α-olefins. The X-ray crystal structure of the title five-coordinate complex shows a distorted tbp arrangement of the ligands with the iodide in the equatorial plane and the unusual axial-equatorial coordination mode of the 1,10-phenanthroline.