Selective Homogeneous Palladium(0)-Catalyzed Hydrogenation of Alkynes to (Z)-Alkenes

Zero-valent palladium precatalysts containing rigid bidentate bis(arylimino)acenaphthene ligands (shown schematically) facilitate the highly stereoselective homogeneous catalytic hydrogenation of alkynes to (Z)-alkenes. Internal, terminal, aryl-substituted, and cyclic alkynes are suitable substrates, as are some enynes, which are chemoselectively hydrogenated to dienes. E=CO(2)Me; R(1), R(2)=4-OCH(3), 4-CH(3), 2,6-(CH(3))(2).     

A critical investigation of the effects of the radio frequency potential on the trapping of externally injected ions in ion trap mass spectrometry

The sensitivity of all ion trap mass spectrometry (ITMS) methods is dependent on the trapping efficiency of the instrument. For ITMS instruments utilizing external ion sources, such as laser desorption, trapping efficiency is known to depend on the phase and amplitude of the radio frequency (RF) potential applied to the ring electrode at the time of ion introduction. It is remarkable that, in a considerable body of literature, no consensus exists regarding the effects of these parameters on the efficacy of trapping externally generated ions. In this paper, a summary of the literature is presented in order to highlight significant discrepancies. New laser desorption ion trap mass spectrometry (LD-ITMS) data are also presented, from which conclusions are drawn in our effort to clarify some of the confusion. Copyright 1999 John Wiley & Sons, Ltd.     

Photoelastic determination of stresses in multiple-pin connectors

The design, fabrication, and testing of photoelastic models of double-lap, multiple-pin connectors are discussed. Interest is in the stresses in the inner laps. These stresses are determined by constructing models with photoelastic inner laps and transparent-acrylic outer laps. The connectors have two pins, in tandem, parallel to the load direction. A photoelastic-isotropic point is shown to permit the evaluation of load sharing between the two pins. A numerical scheme, utilizing the isochromatic- and isoclinic-photoelastic data and a finite-difference representation of the planestress equilibrium equations, is used to compute the stresses around the two pins. Representative stress distributions and stress-concentration factors are shown.     

Spectroscopic studies of [M(CO)5M′(CO)3(1,4-diazabuta-1,3-diene)] (M,M' = Mn,Re) and their photolysis products

Photolysis of [M(CO)₅M′(CO)₃(dab)] (M, M′ = Mn, Re; dab = 1,4-diazabuta-1,3-diene, RNCHCHNR) in 2-Me-THF leads to both homolytic and heterolytic splitting of the metalmetal bond depending on the solution temperature. In a rigid medium such as a CH₄-matrix no breaking of the metalmetal bond is observed but instead formation of [M(CO)₃M′(CO)₃(dab)] in which compound the dab-ligand is σ,σ,π,π bridging between M and M′.     

Isostructural triazenido complexes of first row transition metals. Part 2. Synthesis and properties of [(η5-C5H5)L(RN3R)CoIII]PF6, L = PEt3, PPh3, P(OMe)3 and P(OPh)3

Summary The synthesis and properties of the [(⁵-C₅H₅)L(RN₃R)Co^III]PF₆ complexes, with L = PEt₃, PPh₃, P(OMe)₃ or P(OPh)₃, are reported. A six coordinate configuration containing a chelating triazenido ligand is proposed which is isostructural with the known complexes of iron and nickel.The spectroscopic properties of the isoelectronic Co and Fe complexes, (⁵-C₅H₅)L(RN₃R)M, are compared in relation to the charge on the central metal atom. The complex with L = CO could not be prepared, but the carbonyl inserted product (⁵-C₅H₅)(L{RNNN(R)C(O)}Co was isolated. In one of the reactions, the novel ring-bound triphenylphosphine complex, [⁵-C₅H₅)(⁵-C₅H₄PPh₃)Co^III](PF₆)₂, was isolated as a side product. The mechanism of this reaction is discussed.     

Isostructural triazenido complexes of first row transition metals. Part 3. Low spin cyclopentadienyl complexes of cobalt(II) (η5-C5H5)(PPh3)(RN3R)Co

Summary Reaction of (PPh₃)₂Cl₂Co^II with TlC₅H₅ gave the new ( ⁵-C₅H₅)(PPh₃)ClCo^II complex. This complex reacted with Ag(RN₃R) or Cu(RN₃R) (R=p-MeC₆H₄,p-ClC₆H₄ or 3,5-Cl₂C₆H₃) to give the novel compounds ( ⁵-C₅H₅)(PPh₃)(RN₃R)Co^II, with a spin state of one half, which are representatives of the very small class of paramagnetic monocyclopentadienyl transition metal complexes. The structural conclusions are based on comparison with the isostructural [ ⁵-C₅H₅)(PPh₃)(RN₃R)Co^III]PF₆ complex, on the i.r. spectra and on the observed redox behaviour. The e.p.r. spectra are reported, but are not well enough resolved for the determination of the hyperfine interactions. The orbital scheme of the new complex is discussed in terms of a distorted octahedral crystal field in relation to the series of isostructural ( ⁵-C₅H₅)(PPh₃)(RN₃R)M^II (M = Fe, Co or Ni) complexes.