Synthesis,structure, spectroscopy and redox energetics of a series of uranium(IV) mixed-ligand metallocene complexes

A series of uranium(IV) mixed-ligand amide–halide/pseudohalide complexes (C₅Me₅)₂U[N(SiMe₃)₂](X) (X = F (1), Cl (2), Br (3), I (4), N₃ (5), NCO (6)), (C₅Me₅)₂U(NPh₂)(X) (X = Cl (7), N₃ (8)), and (C₅Me₅)₂U[N(Ph)(SiMe₃)](X) (X = Cl (9), N₃ (10)) have been prepared by one electron oxidation of the corresponding uranium(III) amide precursors using either copper halides, silver isocyanate, or triphenylphosphine gold(I)azide. Agostic U?H–C interactions and η³-(N,C,C′) coordination are observed for these complexes in both the solid-state and solution. There is a linear correlation between the chemical shift values of the C₅Me₅ ligand protons in the ¹H NMR spectra and the U^IV/U^III reduction potentials of the (C₅Me₅)₂U[N(SiMe₃)₂](X) complexes, suggesting that there is a common origin, that is overall σ-/π-donation from the ancillary (X) ligand to the metal, contributing to both observables. Optical spectroscopy of the series of complexes 1–6 is dominated by the (C₅Me₅)₂U[N(SiMe₃)₂] core, with small variations derived from the identity of the halide/pseudohalide. The considerable π-donating ability of the fluoride ligand is reflected in both the electrochemistry and UV-visible-NIR spectroscopic behavior of the fluoride complex (C₅Me₅)₂U[N(SiMe₃)₂](F) (1). The syntheses of the new trivalent uranium amide complex, (C₅Me₅)₂U[N(Ph)(SiMe₃)](THF), and the two new weakly-coordinating electrolytes, [Pr₄N][B{3,5-(CF₃)₂C₆H₃}₄] and [Pr₄N][B(C₆F₅)₄], are also reported.     

Synthesis and Diels-Alder Reactions of a New Kind of Chiral Dienophiles: Cyclic Vinyl-p-tolylsulfilimines

A new kind of chiral dienophiles, cyclic vinyl-p-tolylsulfilimines (2a and 2b), were obtained from the corresponding (Z)-sulfinylacrylonitriles with HBF(4) and methanol. The asymmetric Diels-Alder reaction of optically pure 2a with cyclopentadiene under mild thermal or catalyzed conditions afforded only the endo-4a adduct with complete endo and pi-facial selectivities. The ability of the sulfilimine moiety to enhance the dienophilic reactivity of the double bond is similar to that of the sulfinyl group.     

A straightforward means of coupling preparative high-performance liquid chromatography and mass spectrometry

Flow splitting to a mass spectrometer is a common way of coupling a highly specific detector to preparative or semi-preparative high-performance liquid chromatography (HPLC) purification of combinatorial libraries, drug metabolites, and characterizable impurities. The sensitive mass spectrometer consumes only a small fraction of the analyte while providing online structure-specific detection, and its output can thus be used to trigger collection of the desired fraction. Coupling mass spectrometry to preparative HPLC is difficult due to the susceptibility of the detector to fouling under conditions of high analyte concentration or solute amount, or to changes in solvent composition. We report here on a device, the mass rate attenuator (MRA), which automatically produces split ratios over a range of 100:1 to 100 000:1 under programmable user control. The MRA is a flow-control device that periodically gates a small aliquot from one liquid stream into another. The design allows the user to set the frequency of the gating without interruption of the HPLC flow stream. The MRA also allows control of the volume of the aliquot that is transferred between the flow streams. This additional control, compared to passive splitting devices, facilitates optimization of the tubing connecting the separation, detection and collection events. We demonstrate that such optimization can reduce the volume of the collected fraction without compromising recovery, thus reducing the time spent in evaporating solvents to reclaim purified products.     

Toward actinide molecular magnetic materials: coordination polymers of U(IV) and the organic acceptors TCNQ and TCNE

Molecular materials of transition metal ions and organic acceptors of the general formula M(TCNX)2 M=V, Mn, Fe, Co, Ni; X=Q=7,7,8,8-tetracyano-p-quinodimethane, E=tetracyanoethylene, are known to exhibit magnetic ordering resulting from magnetic interactions between the 3d metal orbitals of the paramagnetic metal ion and pi* orbitals of the organic radical spin centers. The reaction of THF solutions of UI3(THF)4 with neutral TCNQ and TCNE instantaneously produce insoluble coordination polymers that can be assigned a formula of [U(TCNX)2I2(THF)2]n on the basis of elemental analysis. Similar reactions between (K-18-crown-6)(TCNX) reagents and UI3(THF)4 produce materials with slightly different phase compositions that are structurally distinct from the neutral TCNQ and TCNE reaction products, as judged by infrared spectroscopy. In all cases the magnetic response of these materials is consistent with the presence of the U(IV) rather than U(III) ion. Voltammetric data obtained for all the synthetic precursors are consistent with the U(IV)/TCNX radical anion formulation. None of the materials undergo magnetic ordering, presumably due to the singlet magnetic ground state that results from the U(IV) ion in a low-symmetry ligand field.     

Zeros of the Whittaker function associated to Coulomb waves

The zeros in the complex z plane of the Whittaker function W_c/z,µ(z),closely related to spherical waves in the quantum-mechanicalCoulomb problem, are investigated for varying real values ofthe parameters c and µ     

Charge reversal of the conjugate base of formamide

Charge reversal collisional activation mass spectremetry of negative ions has been used in conjunction with positive ion collisional activation to investigate several isomeric [H₂, C, N, O]⁺ ions. Generation of m/z 44 ions from formamide, acetamide, JV-methylformamide, acetaldoxime and by charge reversal of the [M–1]^? ion formed from formamide yields several different isomeric structures. Charge reversal of the conjugate base of formamide appears to yield a mixture of singlet and triplet H₂NC?O⁺ ions; experiments with deuterium-labeled compounds have been used to support this. Ab initio molecular orbital calculations indicate that the triplet ion is a stable structure, existing in a potential minimum 390.6 kJ mol^?1 above the ground state singlet.