Hans Georg von Schnering

Li21Si5, a Zintl phase as well as a Hume-Rothery phase

Li21Si5 is the most lithium-rich phase in the binaryLiSi system and substitutes the so-called Li22Si5. This was proved by a careful X-ray structure analysis (F4¯3m-T2d; a = 1871.0pm; Z = 16; R = 0.052; 397hkl). The reduced Li content is due to ordered vacancies in the huge 6 × 6 × 6 superstructure of bcc (cF416). This gives rise to the formation ofM26 clusters typical for γ-brass structures. Two different cluster types, namely Li22Si4 and Li20Si6, set up a complex linkage of diamond, zinc blende, and NaCl type partial structures, in which short SiSi distances are avoided. Li21Si5 is on one hand a representative consistent only of main group elements of the Hume-Rothery phases for structural and electronic reasons (VEC=20.5/13= 1.58). On the other hand the twoM26 clusters fulfill electronically an extended Zintl phase formalism, according to [Li22Si44+ and [Li20Si64−. The latter anion follows classical valence rules (Si4− formation), while the former one carries a cage orbital in addition to the filled Si states, which is delocalized over the Li sites. The two quantum mechanically stable units act as donor-acceptor pairs, which is also reflected in the experimental valence electron density. In a new topological description the whole structure can be understood as being built ofM14 cluster nuclei and a continuous periodic curved surface formed by the network of all remaining Li atoms.

First total synthesis of (−)-dioncophylline A (“Triphyophylline”) and of selected stereoisomers: Complete (revised) stereostructure

Abstract The first total synthesis of dioncophylline A (previously “triphyophylline”) and some selected stereoisomers is described, using an intramolecular-type mixed aryl coupling method via ester-type prefixed molecular moieties. Simultaneously, this synthesis establishes the complete (revised) stereostructure of dioncophylline A as 1a, which is thereby the first fully elucidated naphthylisoquinoline alkaloid of Dioncophyllaceae.

β-Nb3Br8 und β-Nb3J8 darstellung, eigenschaften und struktur☆

Zusammenfassung Bei Temperaturen oberhalb 500°C reagiert metallisches Niob mit der stochio-metrischen Menge NbBr5 unter Bildung von β-Nb3Br8. Unter analogen Bedingungen entsteht β-Nb3J8. Die durch chemischen Transport bei 800 760 °C gereinigten und kristallisierten Verbindungen bilden schwarze sechsseitige Blattchen. Die Verbindungen β-Nb3Br8 und β-Nb3J8 sind isotyp. Ihre Struktur wurde aus Einkristalldaten bestimmt. Sie ist nicht mit der des Nb3Cl8 identisch. Bromid und Jodid kristallisieren hexagonal-rhomboedrisch in der Raumgruppe D53d-R−3m: ah = 7.080, c h = 38.975 A , c a = 5.505 fur das Bromid, ah = 7.600, c h = 41.715 A , c a = 5.489 fur das Jodid. Die Nb3X8-Strukturen mit X = Cl, Br, J enthalten trigonale Nb3-Gruppen mit verkurzten Nb-Nb-Abstanden (Me-Me-Bindung). Die Grosse dieser Abstande (2.81; 2.88; 3.00 A) wie auch der Betrag der Verkurzung hangt von der jeweiligen Anionengrosse ab.

The high temperature structure of ß-SnS and ß-SnSe and the B16-to-B33 type λ-transition path

The high temperature modifications ß-SnS and ß-SnSe crystallize in the orthorhombic Til-type structure [Cmcm, No. 63; SnS at 905 Κ : a — 4.148(2), b = 11.480(5), c = 4.177(2)Â; SnSe at 825 K: a = 4.410(6), b = 11.705(7), c = 4.318(4) A]. For the A-type phase transition α-> β from the GeS-type (B16) to the Til-type (B33) the atomic shifts were determined from about 200° below the critical temperature to Tc (878 Κ and 807 K). They show a continuous transition from threefold to fivefold bonded atoms. This change in topology and bond character corresponds to the model of a SN2 chemical reaction. There is an indication of a possible first-order transition near Tc.

Reaktionen mit cyclischen Oxalylverbindungen, XXIV. Zur Reaktion von 4-Benzoyl-5-phenyl-furan-2,3-dion mit Phenylhydrazonen bzw. Phenylhydrazin

Abstract4-Benzoyl-5-phenyl-furan-2,3-dione (1) reacts with various phenylhydrazones2 at 60–80°C to the pyrazole carboxylic acid3 a, which then can be decarboxylated to 4-benzoyl-1,5-diphenyl-pyrazole (5).1 and phenylhydrazine combine again yielding3 a as the main product and the isomeric pyridazinone6 as by-product. At higher temperatures (120–140°C) the reaction of1 with2 a leads to the formation of dibenzoyl acetic acid hydrazide derivatives8.The structures of all products were confirmed by IR, MS,15N- and13C-NMR spectroscopic measurements, in the case of the pyridazinone6 also by an X-ray study.6 crystallizes with one moleDMSO monoclinically in space group P 21/n (Nr. 14) with 4 molecules6 andDMSO per cell.The reaction pathways leading to the compounds3, 6 and8 are discussed.

Contributions to the chemistry of silicon-sulphur compounds—LX. Synthesis, molecular structure and properties of the dimeric cadmium(II) bis(tri-tert-butoxysilanethiolate) (t-C4H9O)3SiS2Cd2

Abstract The preparation and X-ray crystal structure of cadmium(II) bis(tri-tert-butoxysilanethiolate) from tri-tert-butoxysilanethiol and cadmium acetate are described. The compound forms a dimeric molecule (t-C4H9O)3SiS2Cd2 with a central four-membered Cd2S2 ring. According to the mass spectra the dimeric molecule is not preserved in the vapour phase. The compound is also characterized by 1H, 13C, 29Si NMR and UV spectra. Reactions with 1,10-phenanthroline and 2,2′-bipyridine afforded adducts with formal (RS)2,Cd(NN) stoichiometry. The central Cd2S2 kernel is folded with an angle of 150.6° at the CdCd diagonal. The cadmium atoms are three-fold bonded to thiolate sulphur atoms and the plane coordination is Cd(μ-SR)2(SR) with the mean distances of d(Cdμ-S) 256.0 pm and d(CdS) 240.8 pm. Additionally, there are weaker interactions with two oxygen atoms of two different tri-tert-butoxysilanethiolate ligands [d(CdO) 268 and 289 pm]. The comparison with the corresponding lead compound clearly shows the influence of the stereochemically active lone electron pair at lead(II).

Hexa-t-butyldigerman und Hexa-t-butylcyclotrigerman: Moleküle mit den derzeit längsten GeGe- und GeC-Bindungen☆

Reductive halogen elimination from di-t-butyldichlorogermane gives hexa-t-butylcyclotrigermane (2) in low yield. The X-ray structure analysis of 2 reveals an equilateral triangle of D3 symmetry with very long GeGe (256.3(1) pm) and GeC (205.6(3) pm) bonds. Hexa-t-butyldigermane (6) is formed analogously by treatment of tri-t-butylchlorogermane with lithium naphthalenide. The molecular structure of 6 has the longest GeGe (d|− 271.0 pm) and GeC (d|− 207.6 pm) bonds found in any molecule up to now. Despite the extreme bond lengths, 6 is thermally and solvolytically very stable.

Synthesis and structure of new anion radical salts from DCNQIs

Abstract Anion radical salts [2-R 1 ,5-R 3 -DCNQI] m M n (M=metal or organic cation) are described, which are derived from the newly introduced class of acceptors, N,N′-dicyanoquinonediimines. Single crystals were easily prepared from DCNQIs and metal wires. The salts are discussed in terms of structural properties and (temperature dependent) conductivities together with the behaviour of alloys.

Siliciumverbindungen mit starken intramolekularen sterischen wechselwirkungen: XXVI. Neuartige sechsringe durch photolyse von hexa-t-butylcyclotrisilan in gegenwart von nitrilen

Abstract Photolysis of hexa-t-butylcyclotrisilane (1) in the presence of acetonitrile or benzonitrile yields the respective 3,6-disila-3,6-dihydropyrazines (2, 3) whereas co-photolysis of 1 and pivalonitrile gives 2,2,4,5,5,6-hexa-t-butyl-2,5-disila-2,5-dihydropyrimidine (5). The X-ray structure analysis of 5 reveals a nearly planar six-membered ring with somewhat shortened SiN, but very large SiC bond distances within the ring. The endocyclic bond angles CNSi and CSiC display the largest deviations from 120° upwards and downwards, respectively. 2 rearranges in solution by a twofold hydrogen transfer from the methylgroups to the nitrogen atoms, to give a heterocycle with two exocyclic CC double bonds.

The Cluster Anion Si94

Solely on the basis of Raman spectra and quantum chemical calculations, the previously unknown cluster anion Si94- (structure shown) was characterized and its structure determined. The anion is formed as a component of solid phases by the thermal decomposition of alkali metal monosilicides.