Die Kristallstruktur von [YbCl2(THF)5]+[WOCl4(THF)]- / Crystal Structure of [YbCl2(THF)5]+[WOCl4(THF)]-

Blue moisture sensitive single crystals of [YbCl2(THF)5]+[WOCl4(THF)]- were obtained as a by-product from the reaction of the ytterbium trisamide Yb[N(SiMe3)2]3 with tungsten hexachloride in THF solution. The crystals were suitable for an X-ray crystal structure determination. Space group P2/c, Z = 2, lattice dimensions at -80 °C: a = 1192.5(1), b = 1117.1(1), c = 1349.9(1) pm, ß = 104.115(7)°, R = 0.0424. The structure consists of cations [YbCl2(THF)5]+ in which the ytterbium atom is coordinated in a pentagonalbipyramidal way by the chlorine atoms in the axial positions and by the oxygen atoms of the THF molecules in equatorial positions. Counterions are the well-known [WOCl4(THF)]- ions.

Magnetic Studies on the 1-D Antiferromagnetic Chains of enH2Mn(Fe)F5

Measurements of the 57Fe Mössbauer effect and of magnetic susceptibilities have been performed on the 57Fe doped or pure quasi-1-d antiferromagnetic chain compound enH2MnF5 as a function of temperature. Particular attention was paid to the regions very near the Néel point at TN = 14.5(5) K. The Mössbauer spectra fitted by the Blume-Tjon model show definite relaxation effects, which are attributed to short-range order with temperature-dependent relaxation times. The soliton model of non-linear excitations was applied. Experimental data confirm the predicted exponential temperature dependence of the thermal excitation of moving domain walls. From the activation energy EA/k = 184(5) K and the 1-d exchange energy J/k = -13.6(2) K a local anisotropy energy D/k of -2.4 (3) K was obtained which is smaller than D/k = -3.5 K derived from our single crystal measurements of magnetic susceptibilities. Within the 3-d magnetic ordering the interchain interaction amounts to J′/k ≈ 0.22(4) K (J′/J = 15.8 10-3). A zero-spin reduction ⊿S/S equal to 17% was obtained. 1-d correlations are observed for TN<T< |J|S (S + 1)/k ≈ 78 K, and a crystallographic phase transition at 216 K was indicated in the magnetic properties as well as by DSC and X-ray powder diffraction measurements.

Structure and phase transitions in Ca2CoSi2O7–Ca2ZnSi2O7 solid-solution crystals

While the incommensurability in melilites is well documented, the underlying atomic configurations and the composition-dependent phase behavior are not yet clear. We have studied the transition from the incommensurate phase to the high-temperature normal phase (IC-N), and to the low-temperature commensurate phase (IC-C) of selected members of the Ca2Co1 − x Zn x Si2O7 system using X-ray and single-crystal electron diffraction, as well as calorimetric measurements. The space group of the unmodulated normal phase and of the basic structure of the incommensurate phase is P\bar 4 2_1m; the commensurate lock-in superstructure was refined as a pseudomerohedral twin in the orthorhombic space group P21212. We found that the commensurate modulation is mainly connected with a sawtooth-like periodicity of rotations of the T 1 tetrahedra in the 3 × 3 superstructure. In this structure, the clustering of the low-coordinated Ca2+ ions is not complete so that only imperfect octagons were detected. Generally, the effect of increasing substitution of Co by Zn was a continuous reduction of the IC-N and IC-C transition temperatures.

Photooxygenation of the helimers of (-)-isocolchicine: regio- and facial selectivity of the [4 + 2] cycloaddition with singlet oxygen and surprising endoperoxide transformations

Photooxygenation of the helimeric mixture of (-)-(M,7S)/(P,7S)-isocolchicine (6) with the superdienophile singlet oxygen has been studied. Cycloaddition occurred with high regioselectivity at the 7a,11-positions of the alkaloid and predominantly at the diene face anti to the amidic substituent at the stereogenic center C-7, leading to two endoperoxides 7 (syn) and 8 (anti) with an 1:7 ratio. The structure of the minor product 7 was established by X-ray analysis. Investigation of the triethylamine induced transformation of the predominant endoperoxide 8 furnished a mixture of two isomers (M,7S)-10a/(M,7S)-10b in a 2:1 ratio possibly with constitutional interconversion and with (M,7S)-9 as plausible intermediate. Treatment of this mixture with silica gel/ethyl acetate at ambient temperature surprisingly led to an atropenantiomerically pure colchicinoid (M,7S)-12 characterized by an eightmembered oxocine B-ring, the structure and absolute configuration of which could be determined by X-ray analysis. For the unprecedented formation of the novel colchicinoid (M,7S)-12 a plausible reaction pathway is suggested, involving a complete transfer of the (M) helical asymmetry of the intermediate (M)-11 into (S) asymmetry of the newly formed carbon center of (M,7S)-12. Prerequisite for such a scenario is the configurational stability of the intermediate pseudobiaryl (M)-11, under the conditions employed, allowing to transmit the axial chirality onto the chiral center of the product (M,7S)-12.

Bis(2,2'-bipyridine)(dicyanamido)copper(II) tetrafluoroborate

In the title compound, [Cu(C(2)N(3))(C(10)H(8)N(2))(2)]BF(4), the Cu(II) atom shows distorted trigonal-bipyramidal geometry, with the dicyanamido ligand in the equatorial plane. The two out-of-plane Cu-N bond lengths to bipyridine are 2.006 (3) and 1.998 (3) A, whereas the in-plane Cu-N distances are 2.142 (3) and 2.043 (3) A to the bipyridine, and 2.015 (3) A to the dicyanamide.

A Five-Membered Ring with Three Negative Charges and Solvent-Free Lithium Counterions

Remarkably short distances to the ring plane are shown by the eta(5)-bound lithium ions in the first compound with a triply negatively charged five-membered ring, 1, which was obtained by reduction of 2 with lithium. R=CH(SiMe(3))(2), Dur=2,3,5,6-tetramethylphenyl.