Chiral Phosphino‐ and (Phosphinooxy)‐Substituted N‐Heterocyclic Carbene Ligands and Their Application in Iridium‐Catalyzed Asymmetric Hydrogenation

Enantiomerically pure iridium complexes with phosphino‐ and (phosphinooxy)‐substituted N‐heterocyclic carbene (NHC) ligands were synthesized. Investigation of their electronic properties showed a similar trans influence of the phosphino (or phosphinooxy) and the NHC units. The complexes were tested in iridium‐catalyzed hydrogenation. While low conversions were observed with unfunctionalized olefins, the catalysts proved to be suitable for hydrogenation of the α,β‐unsaturated ester 20 , allylic alcohol 21 , and imine 22 . The enantioselectivities were, however, moderate.     

COSMO-RS: A novel view to physiological solvation and partition questions

Both, dielectric continuum solvation models as well as surface or group based methods using polarity and lipophilicity parameters have been proven to be useful tools for the analysis of solvation and partition questions. For the first time, COSMO-RS provides an integrated theory, which combines the aspects of continuum solvation and surface interactions, and which ends up with chemical potentials of molecules in almost arbitrary solvents and mixtures. Due to its sound theoretical basis, COSMO-RS does not only provide a new quantitative access to solvation and partition properties in well defined solvents, but it also opens a novel view and gives a better understanding of the general problem of solvation. Finally, this allows for a generalisation of COSMO-RS to sophisticatedphysiological partition problems involving as complex phases as blood, brain, or cell membranes. The use of COSMO-RS for drug discovery and design is demonstrated by applications to blood-brain partition coefficients, and water solubility.     

The crystal structure of tetraethylammonium aquatetracyanooxorhenate(V)

Summary The crystal structure of the tetraethylammonium salt of [ReO(H₂O)(CN)₄]^– has been determined from threedimensional x-ray diffraction data. The light blue crystals are monoclinic, space group P2₁/m witha=8.760(1),b=9.518(5),c=11.718(1) Å, =102.63(1)^o with two molecules per unit cell. The final R value using 2009 observed reflections and anisotropic thermal parameters for all the non-hydrogen atoms was 0.038. The [ReO(H₂O)(CN)₄]^– ion has a distorted octahedral geometry with the rhenium atom displaced by 0.30 Å out of the plane formed by the four carbon atoms of the cyano ligands towards the oxo ligand. Bond distances: Re=O=1.667(8), Re–OH₂=2.142(7) and Re–C (average)=2.11(1) Å.     

Translational-energy spectroscopy of NO+ and NO2+

High-resolution translational-energy spectroscopy (up to 0.1 eV) has been carried out on 8

V beams of NO^? and NO^2?. Several features in the spectrum of NO⁴ are assigned to the transitions within the triplet manifold of this ion. The observed transitions originate in several excited electronic states, indicating long lifetimes (> 10 μs) for the states involved. Two electronic transitions in NO²' have been observed at low energies which are attributed to excitations to the first two excited states of this ion.     

1.2-Bis(pentamethylcyclopentadienyl)tetrachlorodisilane and its reduction to decamethylsilicocene

Reaction of pentamethylcyclopentadienyl(pentachloro)disilane (2), prepared from hexachlorodisilane and potassium pentamethylcyclopentadienide (Cp^*K), with a further equivalent of Cp^*K leads selectively to the title compound Cp^* ₂ Si ₂ Cl ₄ (3) which was characterized by NMR and X-ray structural data. Dehalogenation of 3 with four equivalents of sodium naphthalenide offers an alternative route for the synthesis of decamethylsilicocene (1). Dedicated to Professor Mitsuo Kira on the occasion of being honoured with the Wacker Silicon Award 2005.     

Coherent anti-Stokes Raman Scattering Microscopy.

Coherent anti-Stokes Raman scattering (CARS) microscopy is presented as a new nonlinear optical technique. The combination of vibrational spectroscopy and microscopy allows highly sensitive investigations of unlabelled samples. CARS is an ideal tool for studying a broad variety of samples. The main drawback of the technique is its non-zero-background nature, which implies that the signal has to be detected against a nonresonant background. The need to solve this problem is reflected in the rapid technological developments that have been observed during the last decade. Recent results show that CARS microscopy has the potential to become an important complementary technique that can be used with other well-established microscopic methods. Although it has some limitations, it offers unique access to many problems that cannot be tackled with conventional techniques. For this reason, it can be expected that the impressive growth of the field will continue.     

Strukturen und Molekül-Eigenschaften ladungsgestörter Moleküle. 52. Mitteilung. 2, 3-Diphenylchinoxalin-Radikalanionen in Lösung und im Kristall

Structures and Molecular Properties of Charge-Pertubed Molecules. 2, 3-Diphenylquinoxaline Radical Anions in Solution and in Crystals The Na⊕ and K⊕ radical-ion salts of 2, 3-diphenylquinoxaline seem to be (according to a structural database search) among the first ones of N-heterocyclic radical anions in crystals. The one-electron reduction in aprotic 1, 2-dimethoxyethan (DME) solution at metal mirrors and the crystallization under Ar have been preceded by cyclovoltammetric (CV) and ESR/ENDOR measurement. The first electron insertion at ?1.63 V proves to be reversible, whereas the irreversible second step, which is accompanied by an overcrossing of the CV line, can be rationalized by an ‘ECE-DISP’ mechanism via a dianion redox disproportionation. The ENDOR spectrum resolves four ¹H couplings and allows to simulate the ESR spectrum including the ¹⁴N hyperfine splittings. Both dark-blue single crystals of the radical ion salts $[2,3{\rm - diphenylquinoxaline}^{{.} \ominus} {\rm Met}^ \oplus ({\rm DME})]^.$ show unexpected similarities for Met⊕ = Na⊕, K⊕ despite the 36-pm difference in their ionic radii. The largest structural changes inflicted by the one-electron reduction of the N-heterocyclic molecule are observed in the vicinity of the N-centers bearing the highest effective nuclear charge. The DME-chelated metal cations coordinate at the N electron pairs and form Met⊕(DME)-bridged polymer chains of the radical anion, which are differently ondulated in the Na⊕ and K⊕ radical anion salts. The take-home lesson suggests that many more N-heterocyclic molecules might be analogously reduced under optimized conditions and isolated as single crystals.     

Is laser-ablation-ICP-MS an alternative to solution analysis of solid samples?

The basic obstacles for the general use of laser-ablation(LA)-ICP-MS in analytical laboratories are connected with its reproducibility and calibration. A mathematical relation deduced from the correlation function of the analytical signals allows the estimation of the number of craters needed for representative analyses. The procedure was applied to different samples such as manganese crusts and soils. The ion intensities of the major elements in the manganese crusts and nodules were used as internal standards, improving relative standard deviations by factors between 2 and 3. Selected samples of wood and manganese crusts were analyzed by LA-ICP-MS and the results compared with those obtained by solution ICP-MS. The agreement of the values is within the 95% confidence limits. Powdered reference materials and, in the case of wood analysis, cellulose doped with standard solutions were used for the calibration.     

Laser desorption mass spectrometry on thin liquid jets

A recently developed soft desorption method for mass spectrometry is presented, which is called Laser Induced Liquid Beam Ionization/Desorption (LILBID). Analyte ions are desorbed from a thin jet of analyte solution directly into vacuum by means of an IR laser pulse, which has been tuned to a vibrational resonance of the solvent. A comparative experiment with ammonium chloride and aniline hydrochloride shows that ion formation via proton transfer takes place in the solution. Thermally unstable compounds, as well as supra- and biomolecular complexes, can be detected intact and mass analyzed in a reflectron time-of-flight (Re-TOF) mass spectrometer. During the desorption process, noncovalent interactions and some solvation characteristics are preserved. Three examples for the capacity of LILBID are given in this short overview: (a) ion-solvent interactions with the formation of a clathrate structure Cs⁺(H₂O)₂₀, (b) host-guest interactions with the K⁺ selectivity of valinomycin, and (c) noncovalent interactions with the dimerization of gramicidin. Received: 29 July 1997 / Revised: 4 September 1997 / Accepted: 12 September 1997