Crystal and molecular structure, and P N.M.R. characteristics of di-μ-chlorodi(propionyl)bis(dimethylphenylphosphine)diplatinum(III). Trans-influence of ligands in binuclear complexes

The known complex, $\text{trans}$-(η-C₅H₅)₂Rh₂(CO)₂(CF₃C₂CF₃) is formed in high yield from (η-C₅H₅)Rh(CO)₂ and CF₃FCCF₃ at 100°. The less stable $\text{cis}$-isomer of the complex is obtained in low yield from the same reaction. The infrared, ¹H, ¹⁹F and ¹³C NMR spectra of the two isomers are compared. The $\text{trans}$-isomer undergoes CO scrambling in solution at room temperature, and the variable temperature ¹³C NMR spectra are consistent with a pairwise bridge opening and closing mechanism. The mechanism is extended to account for the isomerization of $\text{cis}$ to $\text{trans}$ isomer, whihc has a half-life of 12 h at room temperature. The ¹³C spectrum indicates that the $\text{cis}$-isomer is static in solution at room temperature. The $\text{trans}$-isomer is reversibly protonated by protonic acids, and BF₄^? and PF₆^? salts of the protonated species can be isolated. The spectroscopic properties of these salts are consistent with protonation at one of the alkynyl-carbons, but it is not possible to distinguish between two alternative structures for the complex cation.Treatment of (η-C₅H₅)₂Rh₂(CO)₂(CF₃C₂CF₃) with (η-C₅H₅)Rh(CO)₂ gives the trinuclear complex (η-C₅H₅)₃Rh₃(CO)(CF₃C₂CF₃) in 80% yield. The analogoug but-2-yne complex is formed from (η-C₅H₅)₃Rh₃(CO)₃ and MeCCMe. The infrared, ¹H, ¹⁹F and ¹³C NMR spectra indicate that the hexafluorobut-2-yne complex exists in two different structural arrangements in solution. One has an edge bridging, and the other a face bridging carbonyl. The proportion of the isomers is affected by the solvent polarity. The spectra of the but-2-yne complex indicate it is fluxional at room temperature, and has a face bridging structure in solution regardless of the polarity of the solvent. Reversible protonation of the hexafluorobut-2-yne complex occurs in protonic acids, and the salt [(η-C₅H₅)₃Rh₃(CO)(CF₃C₂CF₃)H]⁺[BF₄]^?,H₂O can be isolated. The spectroscopic properties of this complex are consistent with a structure incorporating an edge-bridging carbonyl, and probably, an edge-bridging hydride ligand.     

The robinson annulation of some medium ring size 2-hydroxymethylenecyclanones

The adducts formed from methyl vinyl ketone and the 2-hydromethylene derivatives of cyclooctane and cyclodecanone can be cyclised to spiroketones containing the same number of C atoms. The spiroketone from cyclooctanone possesses a single UV maximum at 227 nm; the cyclodecanone compound has a further absorption at ca 246 nm. The constitution and stereochemistry of the ketols produced from the adducts under mild cyclising conditions have been investigated.     

The preparation and molecular structure oftrans-[MoCl2(PMe2Ph)4]

Summary The compoundtrans-[MoCl₂(PMe₂Ph)₄] has been prepared by the reduction of MoCl₅ (by Mg) or of [MoCl₃(PMe₂Ph)₃] (by LiBuⁿ) in the presence of PMe₂Ph in tetrahydrofuran (THF). It has _eff=2.84 B.M. and crystallises in space group P1 witha=11.591(3),b=12.931(3),c=12.703(3) Å, = 95.28(2), =105.97(2), =103.54(2)°. Refinement of the structure gave R=0.036. The Mo-Cl and Mo-P distances average 2.443(6) and 2.534(8) Å, respectively.Low-valent phosphine complexes of the Group VI metals continue to attract much attention because of their involvement in studies of the catalytic activation of dinitrogen⁽¹⁾, dihydrogen(^{2, 3}), alkenes and alkynes(⁴). As a by-product during our studies of dinitrogen(¹) and hydride(²) complexes of molybdenum and tungsten, we obtainedtrans-[MoCl_2- (PMe₂Ph)₄] as yellow, paramagnetic crystals (_eff= 2.84 B.M.). We first obtained the compound during the attempted synthesis ofcis-[Mo(N₂)₂(PMe₂Ph)₄] by reduction of MoCl₅ with Mg in the presence of PMe₂Ph (see Experimental). Upon identification of the compound we found that it could be readily synthesised by treatment of [MoCl₃(PMe₂Ph)₃]⁽⁵⁾ with LiBuⁿ in THF in the presence of PMe₂Ph (experimental).The complex was shown to have thetrans structure by x-ray analysis (Figure). Analogues oftrans-[MoCl₂(PMe₂Ph)₄] have been prepared, namely [CrCl₂(Me₂PCH₂CH₂PMe₂)₂]⁽⁶⁾,trans- [MoCl₂(PMe₃)₄]⁽⁷⁾, [WCl₂(PMe₂Ph)₄]⁽⁸⁾ and [WCl₂(PMe₃)₄]⁽⁴⁾, of which onlytrans-[MoCl₂(PMe₃)₄] has been examined by X-rays⁽⁷⁾. Its principal structural parametersi.e. d(Mo-Cl)= 2.420(6), d(Mo-P)_av=2.496(3) Å⁽⁶⁾ are close to those found here fortrans-[MoCl₂(PMe₂Ph)₄].     

In situ soft X-ray studies of ethylene oxidation mechanisms and intermediates on the Pt(111) surface

In situ studies of ethylene oxidation on Pt(111) have been performed using a powerful combination of fluorescence yield soft X-ray methods for temperatures up to 600 K and oxygen pressures up to 0.01 Torr. Absolute carbon coverages have been determined both in steady-state and dynamic catalytic conditions on the Pt(111) surface. Fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) experiments above the carbon K edge were used to identify the structure and bonding of the dominant surface species during oxidation. TP-FYNES experiments of preadsorbed ethylene coverages in oxygen pressures up to 0.01 Torr indicate a stable intermediate is formed over the 215-300 K temperature range. By comparing the intensity of the C-H sigma resonance at the magic angle with the intensity in the carbon continuum, the stoichiometry of this intermediate has been determined explicitly. Based on calibration with known C-H stoichiometries, the intermediate has a C(2)H(3) stoichiometry for oxygen pressures up to 0.01 Torr, indicating oxydehydrogenation occurs before skeletal oxidation. FYNES spectra at normal and glancing incidences were performed to characterize the structure and bonding of this intermediate. Using FYNES spectra of ethylene, ethylidyne, and acetylene as reference standard, this procedure indicates the oxidation intermediate is tri-sigma vinyl. Thus, oxidation of ethylene proceeds through a vinyl intermediate, with oxydehydrogenation preceding skeletal oxidation.     

Time-resolved spectroscopic studies of B(12) coenzymes: a comparison of the primary photolysis mechanism in methyl-, ethyl-, n-propyl-, and 5'-deoxyadenosylcobalamin.

An ultrafast transient absorption study of the primary photolysis of ethyl- and n-propylcobalamin in water is presented. Data have been obtained for two distinct excitation wavelengths, 400 nm at the edge of the UV gamma-band absorption, and 520 nm in the strong visible alphabeta-band absorption. These data are compared with results reported earlier for the B(12) coenzymes, methyl- and adenosylcobalamin. The data obtained for ethylcobalamin and n-propylcobalamin following excitation at 400 nm demonstrate the formation of one major photoproduct on a picosecond time scale. This photoproduct is spectroscopically identifiable as a cob(II)alamin species. Excitation of methyl-, ethyl-, and n-propylcobalamin at 520 nm in the low-lying alphabeta absorption band results in bond homolysis proceeding via a bound cob(III)alamin MLCT state. For all of the cobalamins studied here competition between geminate recombination of caged radical pairs and cage escape occurs on a time scale of 500 to 700 ps. The rate constants for geminate recombination in aqueous solution fall within a factor of 2 between 0.76 and 1.4 ns(-1). Intrinsic cage escape occurs on time scales ranging from 相似文献   

Sign changes in the prime number theorem

The Ramanujan Journal - Let V(T) denote the number of sign changes in $$\psi (x) - x$$ for $$x\in [1, T]$$ . We show that $$\liminf _{T\rightarrow \infty } V(T)/\log T\ge \gamma _{1}/\pi +...     

The cohomology of invariant variational bicomplexes

Let: E M be a fiber bundle and let be an infinitesimal Lie transformation group acting onE. We announce various new results concerning the cohomology of the invariant variational bicomplex ( ^*,* (J(E)), d_H, d_V) and the associated invariant Euler-Lagrange complex. As one application of our general theory, we completely solve the local invariant inverse problem of the calculus of variations for finite-dimensional infinitesimal Lie transformation groups.