Three Bromides of Lanthanum: LaBr2, La2Br5, and LaBr3

Colourless LaBr₃ was obtained via the ammonium-bromide route and in singlecrystalline form by chemical vapour-phase transport with aluminium tribromide. Black single crystals of LaBr₂ and La₂Br₅ have been grown from the melts that are obtained by lithium reduction of lanthanum tribromide in sealed tantalum ampoules at 850°C. The crystal structures of the three bromides of lanthanum were refined: LaBr ₂. 2H₂–MoS₂ type, hexagonal (P6₃/mmc), a = 409.88(4) pm, c = 1390.0(1) pm, R = R_w = 0.092; La ₂ Br ₅. Pr₂I₅ type, monoclinic (P2₁/m), a = 789.09(6) pm, b = 424.89(4) pm, c = 1342.3(1) pm, β = 91.60(1)º, R = 0.067, R_w = 0.055; LaBr₃ . UCl₃ type, hexagonal (P6₃/m), a = 797.13(4) pm, c = 452.16(4) pm, R = 0.036, R_w = 0.032.     

Breakdown of Chlorophyll: Partial synthesis of a putative intermediary catabolite. Preliminary communication

The partial synthesis of 10,22-dihydro-4,5-dioxo-4,5-secopheophorbide a ( 1 ) from pheophorbide a methyl ester (2) is described. A regioselective, photooxygenolytic reaction of (pheophorbidato a methyl ester)cadmium(II)( 3 ) provides the entry to the crucial 4,5-secoporphinoid structure in form of the (10,22-dihydro-4,5-dioxo-4,5-seco-pheophorbidato a methyl ester)cadmium(II) ( 4 ). The hydride reduction of this 4,5-dioxo-4,5-secophytoporphyrin ester occurs selectively at the ‘eastern’ meso-position to lead (after demetallation) to 10,22-dihydro-4,5-dioxo-4,5-secopheophorbide a methyl ester ( 5 ). This oxobilin-carbaldehyde has the structure assigned earlier to an ester of an isolation form of the red pigment(s) from Chlorella protothecoides. Hydrolysis of the propanoate ester function of 5, selectively catalyzed by pig liver esterase, then yields the title compound 1 . The red tetrapyrrole 1 may represent an intermediary chlorophyll catabolite in degreening plants.     

Darstellung von Dithiatetrazocinen und Folgereaktionen

Preparation of Dithiatetrazocine and Secondary Reactions Li[PhCN₂(SiMe₃)₂] ( 1 ) or PhCN₂(SiMe₃)₃ ( 3 ) react with SCl₂ to give in good yields the dithiatetrazocine PhC(NSN)₂CPh ( 2 ). By analogy, p-MeC₆H₄C(NSN)₂CC₆H₄Me-p ( 7 ), p-NO₂C₆H₄C(NSN)₂-CC₆H₄NO₂-p ( 8 ), and p-CF₃C₆H₄C(NSN)₂CC₆H₄CF₃-p ( 9 ) are obtained from the reaction of p-MeC₆H₄CN₂(SiMe₃)₃ ( 4 ), Li[p-NO₂-C₆H₄CN₂(SiMe₃)₂] ( 5 ), und Li[p-CF₃C₆H₄CN₂(SiMe₃)₂] ( 6 ) with SCl₂. Reaction of 2 /LiCl with AgAsF₆ in liquid SO₂ leads to [PhCN₂S₂]⁺[AsF₆]⁻ ( 10 ) and 3[PhCN₂S₂]⁺2[AsF₆]⁻Cl⁻ ( 11 ). The structures of 10 and 11 are confirmed by X-ray analyses.     

Darstellung und Struktur von Tetrafluoro(η5-pentamethylcyclopentadienyl)niob und Tetrafluoro(η5-cyclopentadienyl)niob

Preparation and Structure of Tetrafluoro(η⁵-pentamethylcyclopentadienyl) Niobium and Tetrafluoro(η⁵-cyclopentadienyl) Niobium A facile preparation method for (η⁵-C₅Me₅)NbF₄ 3 and (η⁵-C₅H₅)NbF₄ 4 is reported by using AsF₃ as a fluorinating agent. Single crystals obtained from AsF₃ contain the solvent molecule as well as HF. The composition of the crystal is [(η⁵-C₅Me₅)NbF₄(AsF₃)₂]₂ · [(η⁵-C₅Me₅)NbF₄(HF)AsF₃]₂ 5 . The X-ray crystal structure of 5 will be reported. 5 crystallizes triclinic with one furmula in the space group P1 and lattice constants a = 843.1(4), b = 1154.9(6), c = 1910.2(10) pm, α = 91.68(3)°, β = 99.30(3)°, γ = 104.44(2)°.     

Austauschreaktionen an komplexgebundenen liganden von elementen der IV. hauptgruppe : I. Darstellung und eigenschaften von cyclopentadienyldicarbonyl-alkoxysilyl-eisen-komplexen

The reaction of CpFe(CO)₂SiCl₃ with alcohols yields a partial Cl-OR exchange. By completing the reaction with stoichiometric quantities of the corresponding alcoholate the pure compounds CpFe(CO)₂Si(OR)₃ (R = CH₃, C₂H₅, n-C₃H₇, i-C₃H₇, t-C₄H₉) can be prepared. Excess of alcoholate splits the FeSi bond yielding the [CpFe(CO)₂]^-ion. The compounds are characterised by IR and mass spectra.     

Mononuclear aluminum hydroxide for the design of well-defined homogeneous catalysts

An unprecedented aluminum hydroxide LAlMe(OH) (5; L = HC[(CMe)(2,6-iPr2C6H3N)]2) has been prepared by the hydrolysis of LAlMeCl (4). For the preparation of 5, the reagents of KOH, water, and KH, as well as the two-phase ammonia/toluene system, were used. Further reactions of 5 with Cp2ZrMe2 (8) and Cp2ZrHCl in toluene lead to the intermolecular elimination of CH4 and H2 and the formation of mu-O-bridged dinuclear aluminum and zirconium complexes [LAlMe(mu-O)ZrMeCp2] (6) and [LAlMe(mu-O)ZrClCp2] (7), respectively, in high yields. The crystal structure reveals that 5 is a monomer with terminal OH and Me groups. The X-ray structure analysis shows that 6 and 7 contain a bent Al-(mu-O)-Zr core with terminal Al-Me and Zr-Me or Zr-Cl arrangements. The methylalumoxane (MAO)-activated compounds 6 and 7 exhibit high catalytic activity for the polymerization of ethylene. Under comparable polymerization conditions, the MAO/6 and MAO/7 catalyst systems show considerably higher activity and much lower MAO:catalyst ratios than that of MAO/8.     

Characterization of micros-ms dynamics of proteins using a combined analysis of 15N NMR relaxation and chemical shift: conformational exchange in plastocyanin induced by histidine protonations

An approach is presented that allows a detailed, quantitative characterization of conformational exchange processes in proteins on the micros-ms time scale. The approach relies on a combined analysis of NMR relaxation rates and chemical shift changes and requires that the chemical shift of the exchanging species can be determined independently of the relaxation rates. The applicability of the approach is demonstrated by a detailed analysis of the conformational exchange processes previously observed in the reduced form of the blue copper protein, plastocyanin from the cyanobacteria Anabaena variabilis (A.v. PCu) (Ma, L.; Hass, M. A. S.; Vierick, N.; Kristensen, S. M.; Ulstrup, J.; Led, J. J. Biochemistry 2003, 42, 320-330). The R1 and R2 relaxation rates of the backbone 15N nuclei were measured at a series of pH and temperatures on an 15N labeled sample of A.v. PCu, and the 15N chemical shifts were obtained from a series of HSQC spectra recorded in the pH range from 4 to 8. From the R1 and R2 relaxation rates, the contribution, Rex, to the transverse relaxation caused by the exchanges between the different allo-states of the protein were determined. Specifically, it is demonstrated that accurate Rex terms can be obtained from the R1 and R2 rates alone in the case of relatively rigid proteins with a small rotational anisotropy. The Rex terms belonging to the same exchange process were identified on the basis of their pH dependences. Subsequently the identifications were confirmed quantitatively by the correlation between the Rex terms and the corresponding chemical shift differences of the exchanging species. By this approach, the Rex terms of 15N nuclei belonging to contiguous regions in the protein could be assigned to the same exchange process. Furthermore, the analysis of the exchange terms shows that the observed micros-ms dynamics in A.v. PCu are caused primarily by the protonation/deprotonation of two histidine residues, His92 and His61, His92 being ligated to the Cu(I) ion. Also the exchange rate of the protonation/deprotonation process of His92 and its pH and temperature dependences were determined, revealing a reaction pathway that is more complex than a simple specific-acid/base catalysis. Finally, the approach allows a differentiation between two-site and multiple-site exchange processes, thus revealing that the protonation/deprotonation of His61 is at least a three-site exchange process. Overall, the approach makes it feasible to obtain exchange rates that are sufficiently accurate and versatile for studies of the kinetics and the mechanisms of local protein dynamics on the sub-millisecond time scale.     

Film growth and interface reaction of ultra thin 3d-transition metal oxide/metal layer structures

The interface reactions between transition metal oxide substrates and ultra thin metal layers of different thickness (<15 nm) have been investigated by using x-ray photoelectron spectroscopy (XPS). To a different extent oxidation of deposited metal and reduction of the oxidic substrate is visible for complementary layer systems. For the interface nickel oxide/manganese an enhanced reaction is found compared to the system manganeseoxide/nickel. Based on the experimental data a model of the evolving interface is proposed.