High tetraalkylaluminate fluxionality in half-sandwich complexes of the trivalent rare-earth metals

Steric factors govern the formation of half-sandwich complexes (C5Me4R)Ln[N(SiHMe2)2]2 according to acid-base reactions utilising Ln[N(SiHMe2)2)3(thf)2 and substituted cyclopentadienes. Subsequent trimethylaluminium-promoted silylamide elimination produces the first half-sandwich bis(tetramethylaluminate) complexes (C5Me4R)Ln(AlMe4)2.     

Collision induced dissociation of stored gold cluster ions

The stability of gold cluster ions Au _n ⁺ (2n23) has been investigated via collision induced dissociation in a Penning trap. Threshold energies and dissociation channels have been determined. The cluster stability exhibits a pronounced odd — even alternation: Clusters with an odd number of atoms,n, are more stable than the even-numbered ones. Enhanced stabilities are found for Au ₃ ⁺ , Au ₉ ⁺ , and Au ₁₉ ⁺ in accordance with the Clemenger-Nilsson and the deformed jellium model of delocalized valence electrons. Excited odd cluster ions withn15 predominantly decay by evaporation of dimers; all others decay by monomer evaporation. From the dissociation channels estimates of the binding energies are deduced.This publication comprises part of the thesis of St. Becker     

Trace analysis of bromate and bromide with ion chromatography on coated reversed phase materials

 The ion-chromatographic method for trace analysis of bromate and bromide presented in this paper is based on coating of reversed phase (RP) material with an ionogenic agent, tetrakisdecylammonium bromide, to obtain a pseudo ion-exchange column. The analysis is carried out with usual HPLC pump and UV-detection near 200 nm. Some commercially available RP materials were tested for the coating procedure. The differences between the reversed phases are not significant. All HETP values are calculated between 0.02 and 0.14 mm. The calibration, the sensitivity of the method and the long-time stability of the coated column were tested with one selected RP material. It is shown that the simultaneous trace analysis of bromate and bromide in surface and drinking waters with chloride concentrations up to 50 mg/L is possible without any clean-up on Ag precolumns. A comparison of performance data with a determination method for bromate and bromide employing a commercially available equipment demonstrates the efficiency of the new technique. Received: 23 February 1996/Received: 10 May 1996/Accepted: 14 May 1996     

Synthesis of (-)-centrolobine by Prins cyclizations that avoid racemization

[formula: see text] The segment-coupling Prins cyclization avoids two of the problems common to other Prins cyclization protocols: side-chain exchange and partial racemization by reversible 2-oxonia Cope rearrangement. Model studies demonstrate the stereochemical fidelity of Prins cyclizations using alpha-acetoxy ethers compared with direct aldehyde-alcohol Prins reactions. Furthermore, we propose a mechanism for the racemization observed in some intermolecular Prins cyclizations. Two straightforward syntheses of optically pure (-)-centrolobine highlight the utility of Prins cyclizations.     

Unusual pathways for metal-assisted C[bond]C and C[bond]P coupling reactions using allenylidenerhodium complexes as precursors

The rhodium allenylidenes trans-[RhCl[[double bond]C[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] [R = Ph (1), p-Tol (2)] react with NaC(5)H(5) to give the half-sandwich type complexes [(eta(5)-C(5)H(5))Rh[[double bond]C[double bond]C[double bond]C(Ph)R](PiPr(3))] (3, 4). The reaction of 1 with the Grignard reagent CH(2)[double bond]CHMgBr affords the eta(3)-pentatrienyl compound [Rh(eta(3)-CH(2)CHC[double bond]C[double bond]CPh(2))(PiPr(3))(2)] (6), which in the presence of CO rearranges to the eta(1)-pentatrienyl derivative trans-[Rh[eta(1)-C(CH[double bond]CH(2))[double bond]C[double bond]CPh(2)](CO)(PiPr(3))(2)] (7). Treatment of 7 with acetic acid generates the vinylallene CH(2)[double bond]CH[bond]CH[double bond]=C=CPh(2) (8). Compounds 1 and 2 react with HCl to give the five-coordinate allenylrhodium(III) complexes [RhCl(2)[CH[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] (10, 11). An unusual [C(3) + C(2) + P] coupling process takes place upon treatment of 1 with terminal alkynes HC[triple bond]CR', leading to the formation of the eta(3)-allylic compounds [RhCl[eta(3)-anti-CH(PiPr(3))C(R')C[double bond]C[double bond]CPh(2)](PiPr(3))] [R' = Ph (12), p-Tol (13), SiMe(3) (14)]. From 12 and RMgBr the corresponding phenyl and vinyl rhodium(I) derivatives 15 and 16 have been obtained. The previously unknown unsaturated ylide iPr(3)PCHC(Ph)[double bond]C[double bond]C[double bond]CPh(2) (17) was generated from 12 and CO. A [C(3) + P] coupling process occurs on treatment of the rhodium allenylidenes 1, 2, and trans-[RhCl[[double bond]C[double bond]C[double bond]C(p-Anis)(2)](PiPr(3))(2)] (20) with either Cl(2) or PhICl(2), affording the ylide-rhodium(III) complexes [RhCl(3)[C(PiPr(3))C[double bond]C(R)R'](PiPr(3))] (21-23). The butatrienerhodium(I) compounds trans-[RhCl[eta(2)-H(2)C[double bond]C[double bond]C[double bond]C(R)R'](PiPr(3))(2)] (28-31) were prepared from 1, 20, and trans-[RhCl[[double bond]C[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] [R = CF(3) (26), tBu (27)] and diazomethane; with the exception of 30 (R = CF(3), R' = Ph), they thermally rearrange to the isomers trans-[RhCl[eta(2)-H(2)C[double bond]C[double bond]C[double bond]C(R)R'](PiPr(3))(2)] (32, 33, and syn/anti-34). The new 1,1-disubstituted butatriene H(2)C[double bond]C[double bond]C[double bond]C(tBu)Ph (35) was generated either from 31 or 34 and CO. The iodo derivatives trans-[RhI(eta(2)-H(2)C[double bond]C[double bond]C[double bond]CR(2))(PiPr(3))(2)] [R = Ph (38), p-Anis (39)] were obtained by an unusual route from 1 or 20 and CH(3)I in the presence of KI. While the hydrogenation of 1 and 26 leads to the allenerhodium(I) complexes trans-[RhCl[eta(2)-H(2)C[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] (40, 41), the thermolysis of 1 and 20 produces the rhodium(I) hexapentaenes trans-[RhCl(eta(2)-R(2)C[double bond]C[double bond]C[double bond]C[double bond]C[double bond]CR(2))(PiPr(3))(2)] (44, 45) via C-C coupling. The molecular structures of 3, 7, 12, 21, and 28 have been determined by X-ray crystallography.     

Retardation (Casimir) energy shifts for Rydberg helium-like low-Z ions

The development of storage rings and electromagnetic traps for heavy charged particles is opening up new regimes of atomic physics, including, in particular, spectroscopic studies of Rydberg helium-like ions — with nuclear chargeZ, one electron in the 1s state, and one electron in a near-hydrogenic state of highn andl <n, withn andl the principal and orbital quantum numbers, respectively. We consider the possibility of detecting energy shifts due to retardation, ΔE _ret (n,l), Casimir-like effects. These are quantum electrodynamic (QED) retardation effects associated with the finite speed of light. (As opposed to basically kinematic and dynamic QED effects for small quantum numbersn andl, the appropriate expansion parameter forn andl large for retardation QED corrections is notZ(e ²/?c) but [(Z ? 1)/n ² Z ²](?c/e ²).) We wish to provide some orientation to those planning experiments in the area, with regard to the choices ofn,l, andZ most likely to be able to generate a high-precision confirmation of a retarded interaction. To do so, we provide extensive tables of estimates, for 1s,nl states, of ΔE _ret(n,l), of radiative widths, and ofE, the spin-independent (“electric” fine structure) energy in the absence of retardation shifts, for (nuclear spin zero) ions withZ=2, 6, 8, 16 and 20. These ions might be experimentally accessible in storage rings, and theZ's are low enough that virtual pair production effects may not yet be significant. There is also a brief survey of possible experimental techniques.     

Inelastic Interactions of Slow Electrons with Adsorbed Particles

When slow electrons impinge on metal surfaces which are covered with an adsorbed layer, desorption of neutral particles as well as of positive ions, partly accompanied by fragmentation of the adparticles, is observed. The mechanism of these processes is of interest in connection with existing concepts of the surface bond. The effect can also be used as a probe for the investigation of complex adsorption layers. Transfer of much smaller energies to the adsorbate can lead to vibrational excitation. This process may find application in the vibrational spectroscopic study of adsorbates. If the energy transferred in the inelastic collision is high enough to ionize low-lying levels of the surface atoms, radiationless re-ordering of the electron configuration (Auger-effect) can occur subsequently. The secondary electrons of characteristic energy emitted in these processes can be used for qualitative analysis of the surface.     

Reduction of chemical reaction networks using quasi-integrals

We present a numerical method to identify possible candidates for quasi-stationary manifolds in complex reaction networks governed by systems of ordinary differential equations. Inspired by singular perturbation theory, we examine the ratios of certain components of the reaction rate vector. Those ratios that rapidly approach a nearly constant value define a slow manifold for the original flow in terms of quasi-integrals, that is, functions that are nearly constant along the trajectories. The dimensionality of the original system is thus effectively reduced without reliance on a priori knowledge of the different time scales in the system. We also demonstrate the relation of our approach to singular perturbation theory which, in its simplest form, is just the well-known quasi-steady-state approximation. In two case studies, we apply our method to oscillatory chemical systems: the 6-dimensional hemin-hydrogen peroxide-sulfite pH oscillator and a 10-dimensional mechanistic model for the peroxidase-oxidase (PO) reaction system. We conjecture that the presented method is especially suited for a straightforward reduction of higher dimensional dynamical systems where analytical methods fail to identify the different time scales associated with the slow invariant manifolds present in the system.     

Oxo-Phosphaniminato-Komplexe von Molybdän und Wolfram. Die Kristallstrukturen von [Mo(O)2(NPPh3)2] und [WO(NPPh3)3]2[W6O19]

Oxo-phosphoraneiminato Complexes of Molybdenum and Tungsten. Crystal Structures of [Mo(O)₂(NPPh₃)₂] and [WO(NPPh₃)₃]₂[W₆O₁₉] The dioxo-phosphoraneiminato complexes [Mo(O)₂(NPPh₃)₂] ( 1 ) and [W(O)₂(NPPh₃)₂] ( 2 ) originate from hydrolysis of the nitrido complexes [MN(NPPh₃)₃] (M = Mo, W). They form colourless crystals, which are characterized by IR and NMR spectroscopy as well as by mass spectrometry. According to the crystal structure analysis of 1 (space group Fdd2, Z = 8; lattice dimensions at –83 °C: a = 1953.3(1), b = 3275.8(3), c = 953.4(1) pm) there are monomeric molecules with tetrahedrally coordinated molybdenum atoms. The distances MoO of 171.2 pm and MoN of 185.9 pm correspond to double bonds. In dichloromethane solution 2 undergoes further hydrolysis with colourless crystals of [WO(NPPh₃)₃]₂[W₆O₁₉] ( 3 ) originating, which are characterized crystallographically (space group Pbcn, Z = 4; lattice dimensions at –50 °C: a = 3225.1(6), b = 1803.6(3), c = 1811.9(3) pm). 3 consists of cations [WO(NPPh₃)₃]⁺ with tetrahedrally coordinated tungsten atoms and of the known [W₆O₁₉]^2– anions. The tungsten atoms of the cations show distances WO of 171.8 pm and WN of 182 pm which correspond to double bonds as in 1 .