Azide mit zwei verschiedenen Metallatomen

Crystal structures of azides containing two different metal atoms are discussed. Elements with higher electronegativity have the greatest influence on the structures. They form finite or infinite metal azide complexes, the degree of covalent binding betweenM(II) and azide groups raises with increasing ionisation potential of the metal, the stability of complex azides is not influenced seriously thereof. The arrangement ofM(I)-ions is influenced to a large extent by the stereochemistry. The azide groups are essentially asymmetrical with N-N-distances between 1.10 and 1.28 Å, the mean distance is 1.18 (3) Å. Azide groups are slightly bent [N-N-N=177 (5)°] and are always coordinated to several metal atoms. Frequently the central nitrogen atoms of azide groups are located closer toM(I)-ions than terminal nitrogens. Some complex azides contain water which is situated beside the azide groups within the coordination polyhedra.

     

Synthesis of Dioxa-Cages as a New Probe for the Stereochemistry of the Bisadducts of p-Quinone with Cyclopentadiene and Cyclohexadiene

The synthesis of dioxa-cages via iodine-induced cyclization reaction can be used as a new method for the determination of the stereochemistry of the bisadducts of p-quinone with cyclopentadiene and cyclohexadiene. The bisadducts 2 and 8 were converted into the dioxa-cages 13a and 13b via a three-step sequence, respectively. The stereochemistry of 13a and 13b was determined on the basis of NOE experiments.     

Recherches dans la série des cyclitols XXXVII Etude par spectrométrie de masse

Low resolution mass spectra of cyclohexane-triols, -tetrols, -pentols and -hexols and of some of their deuterium labelled derivatives have been measured. The results indicate that for some geometrical isomers quantitative differences between ion intensities are significant, allowing to deduce the stereochemistry of the molecule from its mass spectrum. In all these compounds the electron impact induced elimination of water occurs mainly by interaction between OH groups. There is a correlation between the relative abundance of (M – H₂O)⁺ and (M ?2 H₂O)⁺ ions and the number of axial hydroxyls in the more stable conformation of the molecule. Difficulties encountered in deducing fragmentation patterns, and determination of number and position of deuterium atoms in the molecule of ms-inositol are discussed.     

A DFT-Based Study of the Low-Energy Electronic Structures and Properties of Small Gold Clusters

Gold clusters Au_n of size n = 2–12 atoms were studied by the density-functional theory with an ab-initio pseudopotential and a generalized gradient approximation. Geometry optimizations starting from a number of initial candidate geometries were performed for each cluster size, so as to determine a number of possible low-energy isomers for each size. Along with the lowest-energy structures, metastable structures were obtained for many cluster sizes. Interestingly, a metastable planar zigzag arrangement of Au atoms was obtained for every cluster size n ≥ 5. The stable electronic structure, binding energy, relative stability and HOMO–LUMO gap for the lowest-energy isomer were calculated for each cluster size. Variation of the electronic properties with size is investigated in this paper and compared with experimental results and other calculations.     

Über die Stereochemie der Bisoxirane aus den 4-N-Phenyl-triazolin-3,5-dion-addukten von Vitamin D3: Röntgenstrukturanalyse eines der Benzoylderivate

Summary Upon complete epoxidation of the vitamin D₃ 4-phenyltriazoline-3,5-dione adducts2 and3 withMCPBA, four diastereomeric bisoxiranes7, 8, 9, and10 were generated. The stereochemistry of all the chiral centers in these compounds has been established by chemical correlation with compounds of known stereochemistry and by single crystal X-ray structure determination of the benzoyl derivative10 b.

     

Iridium(III) Complexes with Sulfonyl and Fluorine Substituents: Synthesis,Stereochemistry and Effect of Functionalisation on their Photophysical Properties

The synthesis and photophysical and electrochemical characterisation of new heteroleptic iridium complexes with electron‐withdrawing sulfonyl groups and fluorine atoms bound to phenylpyridine ligands are reported. The emission energy of these materials strongly depends on the position of the sulfonyl groups and on the number of fluorine substituents. A 90 nm wide tuning range of photoluminescence from the blue‐green (λ_em=468 nm) of iridium(III)bis[2‐(4′‐benzylsulfonyl)phenylpyridinato‐N,C2′][3‐(pentafluorophenyl)‐pyridin‐2‐yl‐1,2,4‐triazolate] to the orange (λ_em=558 nm) of iridium(III)bis[2‐(3′‐benzylsulfonyl)phenylpyridinato‐N,C2′](2,4‐decanedionate) has been achieved. Emission quantum yields ranging from 47 to 71 % have also been found for degassed solutions of the complexes, and a surprisingly high value of 16 % was recorded for iridium(III)bis[2‐(5′‐benzylsulfonyl‐3′,6′‐difluoro)phenylpyridinato‐N,C2′](2,4‐decanedionate) in air‐equilibrated dichloromethane. A unusual stereochemistry of the benzylsulfonyl‐substituted dimer and heteroleptic complexes has been detected by ¹H NMR spectroscopy, and is characterised by the mutual cis disposition of the pyridyl nitrogen atoms of the phenylpyridine ligands, which differs from the most common trans arrangement reported in the literature.     

MH4P6N12 (M=Mg,Ca): New Imidonitridophosphates with an Unprecedented Layered Network Structure Type

Isotypic imidonitridophosphates MH₄P₆N₁₂ (M=Mg, Ca) have been synthesized by high‐pressure/high‐temperature reactions at 8 GPa and 1000 °C starting from stoichiometric amounts of the respective alkaline‐earth metal nitrides, P₃N₅, and amorphous HPN₂. Both compounds form colorless transparent platelet crystals. The crystal structures have been solved and refined from single‐crystal X‐ray diffraction data. Rietveld refinement confirmed the accuracy of the structure determination. In order to quantify the amounts of H atoms in the respective compounds, quantitative solid‐state ¹H NMR measurements were carried out. EDX spectroscopy confirmed the chemical compositions. FTIR spectra confirmed the presence of NH groups in both structures. The crystal structures reveal an unprecedented layered tetrahedral arrangement, built up from all‐side vertex‐sharing PN₄ tetrahedra with condensed dreier and sechser rings. The resulting layers are separated by metal atoms.     

rac‐2,3‐Di­bromo­succinic acid

rac‐2,3‐Di­bromo­succinic acid, C₄H₄Br₂O₄, is the product of the electrophilic addition of bromine to maleic acid. Whereas the carboxyl groups and the bromo ligands are in a gauche arrangement with respect to each other, the tertiary H atoms attached to the central C atoms are in a trans arrangement. The hydroxyl groups form hydrogen bonds with the carbonyl O atoms of neighbouring mol­ecules.     

Synthesis,Spectral and Magnetic Properties,and Crystal Structures of Copper(II) Pyridinecarboxylate Adducts with N‐Heterocyclic Ligands

Synthesis and characterization of seven new complexes [Cu(2‐MeSnic)₂(CH₃OH)]₂ (where 2‐MeSnic is 2‐methylthionicotinate), [Cu(2‐MeSnic)₂L₂]₂ (where L is pyridine — py, ethylnicotinate — Etnic and butylnicotinate — Bunic), [Cu(2‐MeSnic)₂L₂(H₂O)₂] (where L is py and nicotinamide — nia) and [Cu(2‐MeSnic)₂(N‐Menia)₂(H₂O)₂]·2H₂O (where N‐Menia is N‐methylnicotinamide) are reported. The characterization were based on elemental analysis, infrared, electronic and EPR spectra, and magnetic susceptibility measurements over a temperature range of 1.8 — 300 K or 70 — 300 K. Three complexes of different type were studied by X‐ray analysis. The molecule of [Cu(2‐MeSnic)₂(CH₃OH)]₂ has dimeric paddle‐wheel cage structure with a tetragonal pyramidal arrangement around Cu^II. The dimer results from the fact that carboxyl groups of four 2‐MeSnic anions function as bridging in a syn‐syn arrangement. On the other hand [Cu(2‐MeSnic)₂(py)₂]₂ forms dimers with hexacoordinated Cu^II atoms in highly distorted coordination octahedra, each with two oxygen atoms of bridging carboxyl groups in an anti‐anti arrangement of two 2‐MeSnic anions, with two oxygen atoms of one asymmetrically chelating 2‐MeSnic anion and with two nitrogen atoms of two pyridine ligands. The temperature independent EPR spectrum for this complex exhibits an axial signal which corresponds to almost isolated S = ¹/₂ magnetic ions. Magnetic data for the dimer show a weak antiferromagnetic interaction between the two metal ions with J = —0.65 cm^—1. The Cu^II atom in complex [Cu(2‐MeSnic)₂(py)₂(H₂O)₂] is hexacoordinated in an elongated centrosymmetrical tetragonal‐bipyramidal arrangement (4 + 2). Based on the molecular structure the electronic, infrared, electron paramagnetic resonance spectra and magnetic properties are discussed and stereochemistry as well as the mode of ligand coordination in new solid complexes under study have been determined.     

Structure and Characterization of the [Rbx (NH4)1-x]2TeCl6 Tellurate Family at Room Temperature

The crystal structure of rubidium-ammonium hexachlorotellurate [Rb_0.94(NH₄)_0.06]₂TeCl₆ was determined by X-ray single crystal analysis at room temperature. The space group is Fm3m with the lattice parameter a = 10.2503(5) ? and Z = 4. The refinement converged to R(F) = 0.015 and wR(F ²) = 0.032. As in the studied [Rb0.94(NH₄)₀.06]2TeCl6 family, this compound has an antifluorite-type arrangement. Tellurium atoms are surrounded by an octahedron of chlorine atoms. The Rb or N atoms are located between TeCl₂⁶ octahedra ensuring the stabilization of the structure by ionic and hydrogen bonding contacts N-HCl. The substitution of rubidium by ammonium groups does not affect the structural arrangement, but it leads to a decrease in the a lattice cell dimension. IR and Raman spectroscopic studies at room temperature were performed to confirm the X-ray crystallographic results.     

Control of the regiospecific synthesis of dialkylpyrrolizidinones and their stereochemical behaviour using gas chromatography/mass spectrometry with chemical ionization

Pyrrolizidine alkaloids were synthesized via the cyclization of an epoxide prepared from a lactam. The regiospecific control of this reaction is demonstrated using gas chromatography/mass spectrometry. Ammonia chemical ionization mass spectrometry permits the determination of the cis–trans stereochemistry of the final products.     

Polymerization of surface-active monomers. VIII. Tacticity of polymers prepared by radical polymerization of quaternary salts of dimethylaminoethyl methacrylate in micellar and isotropic media

The effect of monomer micellization on the polymerization was studied from the standpoint of stereochemistry in the polymerization. Quaternary salts (C_nBr) of dimethylaminoethyl methacrylate with n-alkyl bromide having N (=4, 8 and 12) carbon atoms were polymerized with radical initiators in isotropic and anisotropic media and the resulting polymers were converted to poly (methyl methacrylate) (PMMA) to determine their tacticity. Tacticities of poly (C₁₂Br)s were little affected by initiators and solvents used for their preparations. There was little dependence of the tacticities on alkyl chain length (N) for poly (C_nBr)s prepared in water and dimethylformamide (DMF). Most of polymers produced here conformed to Bernoullian propagation statistics and a definite difference was not found in the tacticities between the polymers prepared in isotropic and anisotropic media. From the results obtained here it was deduced that the micellar aggregation has little influence upon the stereochemistry in the polymerization of the quaternary monomers. © 1994 John Wiley & Sons, Inc.     

CdBiO2Cl: synthesis and powder structure solution

The title compound, cadmium bismuth dioxide chloride, CdBiO₂Cl, was obtained as a white powder by reaction of solid BiOCl with CdO at 973 K. Ab initio crystal structure determination was carried out using X-ray powder diffraction techniques, including direct methods for atom location and Rietveld fitting for the final refinement. Being monoclinic, the crystal structure can be related to tetragonal Sillen layered phases. The main structural elements present are CdBiO₂⁺ pleated metal–oxy­gen layers alternating with Cl layers along the c axis, whereas along the b axis, all atoms are on mirror planes. The formation of a strong Cd—Cl bond draws the layers together, causing layer deformation and a monoclinic distortion in the layer arrangement.     

exo-Stereoselectivity in Diels-Alder Addition of Halogenocyclopropenes to Butadienes and Furans

The stereochemistry of the Diels-Alder reaction of 1,2-dichloro-3,3-difluorocyclopropene ( 5a ) to 1,4-diphenyl-1,3-butadiene ( 6 ) and 1,3-diphenylisobenzofuran ( 7 ) was unambiguously established by X-ray structure determination. In all cases known so far, tetrahalogenocyclopropenes add exo to open-chain dienes and furans. The previously reported exo-addition product ( 2a ) of l-bromo-2-chlorocyclopropene ( 5b ) to 7 allows assignments of the stereochemistry of other additions of 5b to furans. exo-Addition usually predominates, but in some cases endo- adducts are also formed. This contrasts with reports in the literature that 5b adds preferentially endo to open-chain dienes.     

BaLn2Se4 (Ln = Er,Tm and Yb)

The compounds BaLn₂Se₄ (Ln = rare‐earth metal = lanthanide = Er, Tm and Yb), namely barium di(erbium/thulium/ytterbium) tetraselenide, crystallize in the orthorhombic space group Pnma in the CaFe₂O₄ structure type. In this structure type, all atoms possess m symmetry. The Ln atoms are octahedrally coordinated by six Se atoms. A three‐dimensional channel structure is formed by the corner‐ and edge‐sharing of these LnSe₆ octahedra. The Ba atoms are coordinated to eight Se atoms in a bicapped trigonal–prismatic arrangement, and they occupy the channels of the three‐dimensional framework.     

The PtX n Coordination Polyhedra (X = S,Se, Te) in Crystal Structures

Using the Voronoi–Dirichlet partition procedure and the method of intersecting spheres, it is demonstrated that in the crystal structures of chalcogen-containing compounds, Pt(IV) atoms form only PtX₆ octahedra (X = S, Se, Te), whereas in the case of Pt(III) and Pt(II), square coordination by X atoms is typical. The Pt(II) atoms can also form PtX₅ square pyramids (X = S, Se), PtS₆ octahedra, and PtTe₃Pt₃ quasi-octahedra in which a platinum atom is located in the trans-position to each coordinated tellurium atom. It was found that Pt(II) atoms in the PtX₄ squares (X = S, Se), unlike square-coordinated Pt(III) atoms, can form one or two Pt–M bonds (M is a d metal) and 1 to 4 secondary Pt–Q bonds, where Q is an s metal or hydrogen. The main features of platinum stereochemistry depending on the metal valence state and coordination number (CN) and on the nature of the chalcogen atom were quantitatively characterized in terms of the Voronoi–Dirichlet polyhedra.     

Study of active carbons by X-ray methods

The X-ray scattering properties of carbon black and charcoals, activated chemically and by steam have been studied in a transmission arrangement. The radial distribution functions calculated from the experimental data indicate that the majority of carbon atoms in chemically activated samples have sp³ configurations, characteristic of strained ring systems. Activation by steam produces a small amount of crystalline graphite. Most of the carbon atoms in carbon black are in an sp²-hybrid state, apparently because of the presence of contaminating atoms in appreciable amount, although no crystalline graphite can be detected.     

Circular dichroism studies on absolute configuration assignment of peptidomimetics with a terminal chiral 3-arylpropionic acid unit

The relationship between the molecular structure of peptidomimetics 1 and their chiroptical properties has been studied using circular dichroism spectroscopy. It was demonstrated that the sign of the Cotton effects occurring around 230 and 270 nm enables the direct assignment of stereochemistry at C-1 and C-3 carbon atoms. The TD-DFT calculations of the circular dichroism (CD) spectrum of one of the model compounds confirm the absolute stereochemistry assignment made experimentally. Thus, CD spectroscopy proved to be a simple, reliable and general tool for the determination of the absolute configuration of the stereogenic centers of peptidomimetics with a terminal 3-arylpropionic group.     

Quantum chemical prediction of the 13C NMR shifts in alkyl and chlorocorannulenes: correction of chlorine effects

Prediction of the ¹³C NMR shifts of sym-pentachlorocorannulene and decachlorocorannulene provided impetus for the development of a correction scheme based on a regression of experimental and quantum chemical data. A training set of 15 compounds (18 carbon signals) comprising carbons atoms bearing 1–4 chlorine atoms leads to an estimated error per chlorine atom of about 10–12 ppm. Specifically, linear regression of the data obtained at B3LYP/cc-pVDZ leads to y = −3.77 + 13.11x, with R = 0.982. Ultimately, experiment and theory converge for sym-pentachlorocorannulene and decachlorocorannulene, the former by correction of the theory, the latter by collecting the proper experimental data. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Stereochemical Studies on Ovatoxin‐a

Ovatoxin‐a is the main toxin produced by Ostreopsis ovata, a benthic dinoflagellate that has bloomed massively across the Mediterranean basin over the past years, inflicting both human and environmental suffering. Ovatoxin‐a has recently been isolated from cultures of O. ovata and structurally identified as an analogue of palytoxin: in comparison with palytoxin, ovatoxin‐a lacks three hydroxy groups at the 17‐, 44‐ and 64‐positions, but features an extra hydroxy functionality at the 42‐position. Herein we report on the NMR‐based elucidation of the stereochemistry of ovatoxin‐a, which includes 7 stereogenic double bonds and 62 asymmetric carbon atoms. Understanding the full stereochemistry of ovatoxin‐a is a step towards the elucidation of its mechanism of action on a molecular level.