bluuebunny/crossref_metadata_2025 huggingface dataset:
Stereospecific Reductions with Hydrogen Gas, Modified Metal Catalysts, Methyl Viologen and Enzymes or Microorganisms
Authors: Thanos Iordanes , Simon Helmut
Published: December 2003

The coupling of reductases and microorganisms for bioreductions on hydrogen gas has been accomplished with the system mentioned in the title. Decisive for this is the modification of palladium, platinum, and Raney nickel with a commercially available fluorine‐containing surfactant. 2‐Enoates and 2‐oxo‐carboxylates are reduced highly enantioselectively. The reduction systems are especially simple, and the greater expense involved in electrochemical reductions is circumvented.
Orthophosphates in the Ternary System Al2O3P2O5H2O
Authors: Kniep Rüdiger
Published: December 2003

Orthophosphates of aluminum have been the subject of numerous studies since the middle of the nineteenth century. They have acquired importance in such diverse fields as the earth sciences, chemical engineering, and medicine. The chemical problems associated with this Al2O3P2O5H2O system mainly arise from the marked tendency to form polymeric aluminum orthophosphate complexes in solution; equilibrium is reached only with difficulty, and crystallization of single phases is the exception rather than the rule. However, the crystal structures of several aluminum orthophosphates (including minerals) have been elucidated, and it is now possible to give a systematic description of this class of compounds and the conditions necessary for their formation.
Reduced Cobalt‐meso‐Tetraphenylporphyrin Complexes: Synthesis and Structure of [Na(thf)3]2[Co(TPP)]
Authors: Ciurli Stefano , Gambarotta Sandro , Floriani Carlo , Chiesi‐Villa Angiola , Guastini Carlo
Published: December 2003

Not the monoanion but ratehr the dianion, [Co(TPP)]2⊖, of cobalt(II) meso‐tetraphenylporphyrin [Co(TPP)] is obtained upon reduction of the latter with two equivalents of sodium metal. It is of particular importance for the exact determination of the experimental conditions under which the monoanion, [Co(TPP)]⊖, which is frequently employed as a metal nucleophile, is formed. The structure of the title complex is characterized by a symmetrically arranged threefold contact ion.
t‐(ET)2Au(CN)2 and t‐(ET)2AuBr2, Organic Radical Cation Salts with Twist Stucture
Authors: Amberger Eberhard , Fuchs Helmut , Polborn Kurt
Published: December 2003

Not only the size of the anions but also their “Pearson softness” is decisive for determining which structures (ET)2X radical‐cation salts form. Exchange of the anion in superconducting, two‐dimensional β‐(ET)2AuI2 for Au(CN) or AuBr leads to quasi one‐dimensional twist structures with ET double bonds and two‐dimensional η and ζ structures, none of which is a superconductor.
Aminoacetylene and Its Mono‐ and Dication—Identification of Potentially Interstellar Molecules
Authors: van Baar Ben , Koch Wolfram , Lebrilla Carlito , Terlouw Johan K. , Weiske Thomas , Schwarz Helmut
Published: December 2003

Interpreted as indirect evidence of aminoacetylene in the gas phase, are the identical mass spectra of the radical cation 1, which is formed by decarbonylation of 2, and of the radical cation that was obtained after neutralization of 1 by re ionization. The structure 1 is proven by a peak at m/z 16, which does not appear in the spectra of other radical cations of empirical formula C2H3N. Ab‐initio calculations show high activation energies for fragmentations or isomerizations, both for 1 as well as for aminoacetylene.
equation image
The Isolobal Analogy “”
Authors: Hofmann Peter , Schmidt Helmut R.
Published: December 2003

A useful guideline for planning syntheses, in particular of new di‐ or polynuclear complexes containing d6‐CpM(CO)3 groups (MCr, Mo, W), could be the isolobal realtionship between these groups and the ubiquitous C5H ligand. This relationship is demonstrated on the basis of qualitative considerations, MO calculations and MO contour diagrams, and on the basis of the known structures of complex molecules containing CpM(CO) or Cp⊖ groups.
NaNb3O3F–a Niobium‐Niobium Triple Bond with “Side‐On” Coordination by Nb Atoms
Authors: Köhler Jürgen , Simon Arndt
Published: December 2003

The remarkable new compound NaNb3O5F is formed on annealing NaF, Nb powder, and Nb2O5. According to an X‐ray structure analysis, Nb is present in the +4 and +2 oxidation states. The Nb2⊕ ions form pairs and a structural group (with eight surrounding O‐atoms) as is observed in [Re2Cl8]2⊕. The short d(NbNb) distance of 261.4 pm (observed for the first time in the case of Nb) is explained in terms of a triple bond. This Nb2 dumbbell is coordinated fourfold “side‐on” by the Nb4⊕ ions.
Linear Reactivity‐Selectivity Correlations in Additions of Diarylcarbenium Ions to Alkenes; a Rebuttal of the Reactivity‐Selectivity Principle
Authors: Mayr Herbert , Schneider Reinhard , Grabis Ute
Published: December 2003

Does selectivity increase with decreasing reactivity? No! In a system consisting of diarylcarbenium ions (see previous communication) and alkenes all conceivable types of relationships between reactivity and selectivity were detectable. Moreover, observations within a set of similar reactions show that a certain reactivity‐selectivity behavior is no proof of a certain interaction mechanism.
Photochemistry of Transition‐Metal Atoms: Reactions with Molecular Hydrogen and Methane in Low‐Temperature Matrices
Authors: Ozin Geoffrey A. , McCaffrey John G. , Parnis J. Mark
Published: December 2003

Reactions of metal atoms with molecular hydrogen and alkanes are prototypical of more complex systems involving metal‐mediated reactions in solution, on supports, and on surfaces. The simplicity of metal‐atom reactions makes them particularly attractive for fundamental experimental and theoretical studies. Following the first reports of H2 and CH4 activation by photoexcited metal atoms in cryogenic matrices, the behavior of a number of transition‐metal atoms in their ground and selected electronically excited states has been examined with molecular hydrogen, methane, and ethane. A wealth of structural, electronic, reactivity, and selectivity information is emerging from these studies and it is now becoming possible to recognize the controlling factors at work in these fundamental chemical reactions. In this article, some experimental results for a number of metal‐atom systems, in particular, those involving silver, copper, manganese, and iron atoms, are presented along with a discussion of the factors that are believed to be important in the understanding of the observed physical and chemical behavior.
Coordination‐Polymeric Paracyclophane Complexes of Copper and Silver
Authors: Schmidbaur Hubert , Bublak Wolfgang , Huber Brigitte , Reber Gabriele , Müller Gerhard
Published: December 2003

Instead of η6 coordination to the outer side of cyclophanes, η2 or η2/η3 coordination has been found for the complexes 1 and 2 of the coingage metals Cu1 and Ag1, respectively. In their crystal structures, each metal center bridges two cyclophane molecules. However, on account of the differing coordination of the counterion, screwlike chains are formed for 1, whereas layers are formed for 2. Upon exposure to air, compound 1 discolors and becomes electrically conducting.
equation image
Novel Ni‐Clusters with Se and PR3 (RPh, Et) as Ligands
Authors: Fenske Dieter , Ohmer Johannes
Published: December 2003

Parallel planes formed by three Ni and three Se atoms are common features of the complexes 2–4 in the crystal. Complex 1 contains a trigonal Ni6 prism with Se atoms lying above each face. Complexes 2–4 consist of face‐sharing Ni6 octahedra with the Se atoms as μ3 and μ4 ligands.
equation image
New Reductive Transformations of Unsaturated Cyclic Hydrocarbons [New Synthetic Methods (66)]
Authors: Müllen Klaus
Published: December 2003

The successive reduction of fully conjugated cyclic hydrocarbons leads to singly and multiply charged ions with unusual bonding. The charge distribution in these ions can be determined spectroscopically, and the information so obtained is then used in kinetically controlled trapping reactions for the regioselective introduction of electrophilic groups. When non‐benzenoid substrates are used, syntheses become possible which can either not be carried out or can only be carried out with great difficulty in other ways. Examples of new preparative applications are cycloannelation and bridging reactions as well as polymerization reactions. The ion pair structure of the intermediate and the type of electrophile used are of paramount importance in controlling the mechanism of these reductive transformations.
Allylamine Derivatives—a New Class of Active Substances in Antifungal Chemotherapy
Authors: Stütz Anton
Published: December 2003

Fungal infections (mycoses) are found throughout the world. Only a few structural classes of compounds currently satisfy the demands of modern chemotherapy in their treatment; hence, the quest for new types of active substances is of major scientific and therapeutic importance. The first representative of a new class of substances–the allylamine derivatives–namely naftifine ((E)‐N‐methyl‐N‐(1‐naphthylmethyl)‐3‐phenylallylamine) was discovered by accident and has recently become commercially available as a topical antimycotic. The exploration of structure‐activity relationships on the basis of naftifine and new synthetic strategies led to the discovery of the currently most active compound of this type, terbinafine, the first pharmaceutical agent to contain a (E)‐1, 3‐enyne structural element. Terbinafine exhibits considerably higher activity than the original “lead” structure naftifine both in vitro and in vivo; it is also up to one order of magnitude more effective than standard preparations in various chemotherapeutic animal tests after topical or oral administration. According to clinical experience gained so far, terbinafine is well tolerated and shows promising activity against various types of mycoses. Allylamines act as potent, selective inhibitors of fungal squalene epoxidase via a novel mechanism, which, unlike in the case of other inhibitors of steroid biosynthesis, does not depend on cytochrome P450.
“Two‐Electron Reduction” of Phosphane‐substituted Tricobalt Clusters—Evidence for a 17‐Electron Intermediate in the Dissociative ETC Ligand Substitution
Authors: Hinkelmann Klaus , Mahlendorf Falko , Heinze Jürgen , Schacht Hans‐Thomas , Field John S. , Vahrenkamp Heinrich
Published: December 2003

Reduction and ligand elimination result in reversible transformation of the μ3‐alkylidyne complex 1 into the 17‐electron species 2⊙⊖. This was confirmed by cyclic voltammetry and multisweep experiments with 1 as well as by trapping of the released phosphane ligand. The further reduction to 22⊖ is also reversible. The radical anion 2⊙⊖, which is an essential 17‐electron intermediate for the ETC ligand exchange, is the first such species to be unequivocally detected.
equation image
The Pyrolysis of Azides in the Gas Phase
Authors: Bock Hans , Dammel Ralph
Published: December 2003

The chemistry of the non‐metallic elements has in recent years passed through a period of rapid development, often referred to as its “renaissance”. To emphasize just one of the key facets: numerous short‐lived molecules containing multiple bonds to elements of the third and higher periods have been discovered, often accompanied by the planned synthesis of derivatives which are sterically shielded by bulky groups and thus kinetically stabilized. Thus today molecules such as silabenzenes H6C6−nSin and silaethenes H2SiCH2 or R2SiCR2, disilenes R2SiSiR2 and diphosphenes RPPR, silaphenylisonitrile H5C6NSi, or methylidyne‐phosphanes RCP, are all well‐known species. Sandwich compounds with P6 rings or silicon centers demonstrate that there are now hardly any barriers to impede the imagination of the non‐metal chemist. In sharp contrast is our lack of knowledge regarding the “microscopic” pathways of chemical reactions: thus apart from information provided for example by molecular beam experiments, or from exact numerical calculations involving species consisting of only a few atoms, it remains largely unknown from which directions medium‐sized molecules must approach each other to successfully collide and form a “reaction complex”, in which way their structures are changed in such a process or which role is played by molecular dynamics in the energy transfer.–The pyrolysis of azides XN3, i.e. compounds which tend to explode violently when ignited in the condensed phase but can be heated in low‐pressure gas flow systems without much risk, illustrates that studies of reactive intermediates are of interest not only because novel molecules may be discovered and isolated, and thereby possibilities for synthesis expanded. Moreover, some aspects of the “microscopic” pathways of these azide pyrolyses can be described satisfactorily on the basis of calculated energy hypersurfaces, and the influence of molecular dynamics becomes experimentally visible in the “chemical activation” of intermediates which leads to their “thermal explosion”.
Crown Thioether Complexes of AgI and CuI: The Crystal Structures of [{Ag3L3}AgL{AgL2}](ClO4)4 and [LCuI] (L = 1, 4, 7‐Trithiacyclononane)
Authors: Küppers Heinz‐Josef , Wieghardt Karl , Tsay Yi‐Hung , Krüger Carl , Nuber Bernhard , Weiss Johannes
Published: December 2003

A crown thioether molecule as bridging ligand and two types of cations in the molecule are the most striking features of the silver complex [Ag4L5](ClO4)4 (L = 1,4,7‐trithiacyclononane). In the cation [AgL2]⊕, Ag⊕ is surrounded by six S atoms, in the cation [Ag3L3]3⊕ by four. One S atom of each of the three ligands in [AgL,]3⊕ is bound to two Ag⊕. In contrast, the soft Lewis base I⊕ in [LCuI] competes with the S atoms for coordination positions. In the resulting monomeric units, Cu⊕ is coordinated in a distorted tetrahedral fashion by three S atoms and one I⊕.
Fcu[AuF4], an Unusual Copper(II) Fluoroaurate(III)
Authors: Müller Bernd G.
Published: December 2003

The title compound, a ternary fluoride of two coinage metals, has unusual composition, structure, and magnetic properties. The kind and extent of Jahn‐Teller distortion for Cu2+ are unexpected; the coordination number for Cu2+ is 2 + 2 + 2 = 6, while gold, on the other hand, shows the usual squareplanar coordination for Au3+‐compounds. The compound is antiferromagnetic; the one‐dimensional, magnetic interaction takes place along the linear Cu‐F chains parallel to [001].
The Mechanism of Substitution Reactions at [LRe(NO)(CO)(CH3)]⊕ in Acid Solution and the Structure of [{LRe(NO)(CO)}2(μ‐CH2OCH2)]I2 (L = 1,4,7‐Triazacyclononane)
Authors: Pomp Christa , Duddeck Helmut , Wieghardt Karl , Nuber Bernhard , Weiss Johannes
Published: December 2003

The ligand CH in 1 can be replaced by Cl⊖ by acid catalysis. The reaction, which proceeds with retention of the configuration at Re, may be initiated by protonation of an Namine atom and cleavage of the ReNamine bond. In contrast, on attempting to replace CH by I⊖ the dinuclear complex 2 with the previously unknown 2‐oxapropane‐1,3‐diide bridge, which is stabilized by two O…HN bridges, is formed if the reaction is not carried out in the absence of light.
equation image
Natural Fecapentaene‐14 and One Fecapentaene‐12 Component are all‐trans Stereoisomers
Authors: Baptista José , Krepinsky Jiri J. , Pfaendler Hans Rudolf
Published: December 2003

Fecapentaenes, which are extremely sensitive chemically, are mutagens and may also be colon carcinogens. The naturally occurring compounds have now been compared with two synthetically prepared samples having a known arrangement of double bonds. HPLC and 1H‐NMR and UV spectroscopy revealed that the natural components are all‐trans‐fecapentaene‐12 1 and all‐trans‐fecapentaene‐14 2.
equation image
Hexacarbonylzirconate(2–), [Zr(CO)6]2–: The First Binary Carbonyl Complex of Zirconium
Authors: Chi Kai Ming , Frerichs Scott R. , Philson Stephen B. , Ellis John E.
Published: December 2003

A “pure” carbonylmetalate–containing only metal and CO – has now been obtained for zirconium as the salt 1. The synthesis of 1 could only be achieved in the presence of a complexant for alkali metals, e.g., a crown ether such as [15]crown‐5 (15‐C‐5). The 91Zr‐NMR spectrum of[Zr(13CO)6]2‐ (99% 13C) reveals that the anion exhibits octahedral geometry.
equation image
The Use of Light in the Study of Interfacial Electrochemical Reactions
Authors: Gerischer Heinz
Published: December 2003

Spectroscopic analysis techniques have become particularly important in the study of interfacial electrochemical processes since it has become possible to increase their sensitivity to the extent where changes occurring in the absorption of light by fractions of monolayers can be detected. At the same time the development of new techniques in surface physics, though most of these can only be used in ultra‐high vacuum, has also had a considerable influence on electrochemistry since it has enabled studies to be carried out on electrodes after transfer to UHV. The availability of well‐defined semiconductors in which photocurrents can be generated by the absorption of light has led to the development of photoelectrochemical methods that can be used for the study of electrode reactions at semiconductors or at metal surfaces which are covered with a semiconducting film. Finally, it has been observed during the study of semiconductor electrodes that redox reactions at an electrode can result in the generation of luminescence. This, in turn, can lead to important information on the mechanism of the charge transfer process. These recent developments are reviewed in the present article.
Enzymic Baeyer–Villiger Oxidations by Flavin‐Dependent Monooxygenases
Authors: Walsh Christopher T. , Chen Y.‐C. Jack
Published: December 2003

There has recently been a revival of interest in microbial biochemistry. Part of this resurgence is due to the increased presence of man‐made organic compounds in the biosphere and the resultant interest in microorganisms which can degrade these xenobiotic molecules. Also, advances in genetic engineering have raised the possibility of utilizing the chemical machinery of bacteria for commercial profit and social benefit. The Baeyer‐Villiger reaction is a useful transformation in organic synthesis which provides chemists with a gentle means of converting ketones into esters or lactones. The reaction, however, suffers from the problems of low yield and the need, in some cases, to utilize harsh conditions. There exist bacteria, capable of growth on aliphatic molecules, that contain enzymes which can catalyze Baeyer‐Villiger reactions. These enzymes, known as monooxygenases, are involved in the breakdown of acyclic and alicyclic ketones to provide simpler carbon units for further catabolism. The gamut of reactions catalyzed by some of these enzymes is remarkable. This diversity, plus their availability in pure form in quantity by genetic engineering raises the possibility that these biocatalysts can be useful as reagents in organic synthesis.
Oriented Domains and Twins in the “Tetragonal” Superconductor LaBa2Cu3O6.7
Authors: Hervieu M. , Domengès B. , Provost J. , Deslandes F. , Raveau B. , Chevalier B. , Etourneau J.
Published: December 2003

Electron microscopy and electron diffraction studies on the 70K‐superconductor LaBa2Cu3O6.7 have shown that two sorts of crystals are present; those with tetragonal symmetry and those in which the tetragonal unit cell is distorted orthorhombically. The crystals contain, on the one hand, oriented domains (formation of superstructures) and, on the other, microdomains; these stem from the distortions of the unit cell. There is possibly a connection between the structural distortions and the superconductivity, an assumption which is supported by comparative consideration of the superconductors LaBa2Cu3O6.7 and YBa2Cu3O6.9 and the non‐superconductor LaBa2Cu3O7‐X.
Pheromones in Nanogram Quantities: Structure Determination by Combined Microchemical and Gas Chromatographic Methods [New Analytical Methods (35)]
Authors: Attygalle Athula B. , Morgan E. David
Published: December 2003

What can a chemist do with a few nanograms of valuable organic substance, pure or impure? The answer may be, a great deal. Its components may be quantified, separated, isolated, degraded, derivatized, extracted by solvent partition, as well as the more obvious recording of their mass spectra. Provided a substance is not contaminated by large quantities of solvent, manipulations are possible without much loss or dilution. Necessity is the mother of invention. The examples given in this review show what can be accomplished today in nanochemistry and should act as a stimulus to direct the scientist to discover other applications appropriate to his needs and his circumstances. The examples are largely taken from the field of insect chemistry, but their general applicability is emphasized. This review illustrates how gas chromatography and reaction gas chromatography, alone or combined with mass spectrometry are potent tools in the hands of the chemist to deduce the unique structure of volatile organic substances available only in nanogram quantities.
The Cyclopropyl Group in Studies of Enzyme Mechanism and Inhibition
Authors: Suckling Colin J.
Published: December 2003

The cyclopropyl group—particularly in compounds such as cyclopropylmethanol, phenylcyclopropylamine, and cyclopropanone hydrate—has proved to be very useful in the study of enzyme reactions. Whether the cyclopropane ring is opened to form a radical or remains intact can provide much information about reactions occurring at the active site.
The Crystal Structures of the Superconducting Oxides Bi2(Sr1−xCax)2CuO8−δ and Bi2(Sr1−yCay)3Cu2O10−δ with 0≤ x≤0.3 and 0.16≤y≤0.33
Authors: von Schnering H. G. , Walz L. , Schwarz M. , Becker W. , Hartweg M. , Popp T. , Hettich B. , Müller P. , Kämpf G.
Published: December 2003

Contrary to reports (see T. Forgan, C. Greaves, Nature 332 (1988) 14), the BiSrCaCu oxide superconductor was not first synthesized in December 1987 in Japan but rather two months earlier in Stuttgart. A patent application was submitted on November 25, 1987. X‐ray and neutron diffraction revealed that the two phases given in the title are present. The positions of the O atoms, which, on the basis of the X‐ray intensities, could only be determined with great difficulty, were found by neutron diffraction. The unique feature of the two structures is that in the (Bi2O4−δ) part they accommodate all oxidation states of Biq⊕ for 3<q<5. This might play an important role in the mechanism of superconductivity.
Base‐Induced Intramolecular Formation of a MoMo Bond in [L2Moμ‐OH(μ‐CH3CO2)2]3⊕(L = N,N′,N″‐Trimethyl‐1,4,7‐triazacyclononane)
Authors: Neves Ademir , Bossek Ursula , Wieghardt Karl , Nuber Bernd , Weiss Johannes
Published: December 2003

The MoMo distance shrinks from 3.555 to 2.885 Å on deprotonation of the OH‐bridge in 1. This, as also the drastic changes in the UV/VIS spectrum and in the magnetic properties, clearly indicates a transition from a classical Werner complex to a complex with metal–metal bonding. MO considerations suggest that the Mo‐‐Mo bond in deprotonated 1 is an intramolecularly formed triple bond. The deprotonation of 1 is a completely reversible reaction.
equation image
Tellurium‐Rich Tellurides
Authors: Böttcher Peter
Published: December 2003

The tellurides represent a class of compounds which exhibit a wide range of physical and chemical properties. In addition to the salt‐like compounds, some tellurides show metallic properties while others have strong covalent bonds. Like tellurium itself, many of these are semiconductors, e.g., the II‐VI compounds CdTe and MnTe, and in this respect are of special interest. Other tellurides, e.g., NbTe4 and ZrTe5, have low‐dimensional electronic transport properties. Some ternary transition metal‐main group metal tellurides appear in an amorphous state with spin glass properties. The tellurium‐rich tellurides are characterized without exception by directed Te‐Te bonds. It turns out that general structural chemical relationships exist between them which allow these different compounds to be treated as a single comprehensive class of substances. They represent a link between the Zintl phases on the one hand, and molecular compounds on the other.
Distorted Triple‐Diamond Structure of 3,3‐Bis(carboxymethyl)glutaric Acid (“Methanetetraacetic Acid”)
Authors: Ermer Otto , Eling Andreas
Published: December 2003

The construction of interpenetrating networks is the fascinating possibility of‐fered by tetracarboxylic acids containing tetrahedrally directed carboxyl groups. In the crystal they can form concatenated, diamondoid networks held together by pairs of hydrogen bonds. The cavities are filled owing to the interpenetration of several such networks. In the title compound C(CH2COOH)4 three translationally equivalent, tetragonally elongated diamondoid networks intermesh in a unique fashion. This kind of molecular architecture is a candidate for host structures in solid‐state inclusion compounds. In addition, these structures are models for novel substances exhibiting unusual properties (diamondoid polymers, ferroelectric compounds).
Surfactants, Micelles and Fascinating Phenomena
Authors: Hoffmann Heinz , Ebert Gerlinde
Published: December 2003

In this article we shall describe processes and phenomena which may appear startling and unusual for someone who is familiar with the behavior of solutions of low molecular weight solutes. Some of these phenomena might even be surprising for someone who works with surfactants. The authors still remember their astonishment when they first became aware of a viscoelastic surfactant solution. This was many years after our group had started investigations on surfactants. But all of the solutions we had encountered until that time had behaved normally. Some of the phenomena which will be discussed are also encountered in solutions of polymers. Due to their high molecular weight one expects to find properties which are different from the properties of low molecular weight solutions and does not find them so surprising. Surfactant solutions, however, are aqueous solutions of small molecules with typical molecular weights of 200 to 400. For some of our experiments even dilute solutions were used which contained only up to one percent of surfactant.
Reaction of [NiCp(PPh3)Cl] with E(SiMe3)2; the Structures of [Ni4Se2Cp4] and [Ni4E2Cp4(PPh3)2] (E = Se, Te)
Authors: Fenske Dieter , Hollnagel Achim , Merzweiler Kurt
Published: December 2003

X‐ray structure analyses of 1a and 2 indicate the presence of NiNi bonds in 1 and the absence of such bonds in 2. The complexes 1 and 2 are formed together in the reaction of [NiCp(PPh3)Cl] with E(SiMe3)2 (a, E = Se; b, E = Te). But 2 is also accessible from 1 and PPh3. In this reaction, all bonds of the square Ni4‐cluster of 1 are broken, and the μ4‐E bridges become μ3‐E bridges.
equation image
Modified Cyclodextrins—Novel, Highly Enantioselective Stationary Phases for Gas Chromatography
Authors: König Wilfried A. , Lutz Sabine , Wenz Gerhard
Published: December 2003

Enantiomer separation by gas chromatography in a few minutes and at temperatures between 45 and 130°C is now possible for compounds which cannot form H‐bonds. For peralkylated or partially alkylated and acylated cyclodextrins separation factors of between 1.021 and 1.279 have been achieved. The enantioselectivity of the stationary phase depends—at least partially—on inclusion effects.
Mechanism of Ethylene Hydrogenation by the Molecular Hydrogen Complex [Ir(H)2(H2)(PMe2Ph)3]⊕—Characterization of Intermediates
Authors: Lundquist Eric G. , Huffman John C. , Folting Kirsten , Caulton Kenneth G.
Published: December 2003

The use of a coordinated H2 molecule as leaving group in order to generate an unsaturated reactive complex has proven to be an excellent entry to a new route for ethylene hydrogenation. 1 initially affords the 16e‐complex 2, which immediately coordinates C2H4 with formation of 3, the first species of a catalytic cycle. With an excess of C2H4, the bisethylene complex 4 (as BF salt) can be isolated as stable end product.
equation image
Synthesis and Structure of [NaTi(2,6‐Me2C6H3O)4(py)]2: a Dimeric Material Containing a Novel Phenoxide Bridge
Authors: Durfee Loren D. , Fanwick Phillip E. , Rothwell Ian P.
Published: December 2003

Two η6‐arenesodium fragments constitute a special feature of the structure of the deep‐violet title complex 1. Furthermore the sodium atoms are each part of an NaO2Ti four‐membered ring. The TiIII centers are trigonal‐bipyramidally coordinated, whereby one apical position is occupied by the pyridine N atom. 1 is formed upon reduction of Ti(OR)4, R = 2,6‐Me2C6H3, with sodium amalgam and subsequent treatment with pyridine. The findings of this study can be of importance for explaining the poorly defined products which have previously been obtained in similar reactions.
equation image
Two‐Dimensional Solid‐State NMR Spectroscopy: New Possibilities for the Investigation of the Structure and Dynamics of Solid Polymers [New Analytical Methods (38)]
Authors: Blümich Bernhard , Spiess Hans Wolfgang
Published: December 2003

NMR spectroscopy is an effective method not only for examining liquid samples but also for characterizing molecular sturcture, order and dynamics in amorphous and ordered solids. Recent developments in the area of solid‐state NMR spectroscopy span from model‐dependent studies of conventional one‐dimensional spectra to the more definitive two‐dimensional (2D) spectra which provide more specific information. For example, with 2D‐NMR spectroscopy it is possible to determine the orientational distribution functions of molecular segments in drawn polymers and to distinguish different mechanisms of complex molecular motions. Following an introduction to basic NMR spectroscopy, an overview of the current state‐of‐the‐art of 2D methods in solid‐state NMR spectroscopy is presented and demonstrated with selected examples.
Metal‐Ion‐Selective Exchange Resins by Matrix Imprint with Methacrylates
Authors: Kuchen Wilhelm , Schram Jürgen
Published: December 2003

Cu2⊕ ions could be removed form waste water selectively and down to levels below the limit of detection with ion‐exchange resins in whcih copper(II) methacrylates had been used for their synthesis. This shows that the metal complexes present during the polymerization imprint the cavity structure of the resin such that ions of the same element are preferably taken up in the cavities after the elution of these metal ions.
29Si‐29Si Long Range Couplings in Strained Cyclic Polysilanes
Authors: Kuroda Mutsumi , Kabe Yoshio , Hashimoto Masayuki , Masamune Satoru
Published: December 2003

Unusually large coupling constants nJ(29Si,29Si) can occur in polycyclosilanes highly strained cyclic polysilanes, e.g. in the title compounds of the above communication. 2J can even be larger than 1J! On reducing the ring size of monocyclosilanes (from six to three) 1J becomes smaller and 2J larger, as is also the case with the 13C‐13C coupling constants of analogous carbocycles; in the case of cyclic polysilanes, however, 1J and 3J or 4J can have the same order of magnitude.
Main‐Chain Chirality and Optical Activity in Polymers Consisting of CC Chains
Authors: Wulff Günter
Published: December 2003

Main‐chain chirality is the optical activity resulting from the configurational or conformational arrangement in the main chain of a polymer. The chirality of the most important types of structures has been investigated on the basis of systematic considerations of symmetry. This has led to the surprising result that even in polymers derived from 1‐substituted or nonsymmetric 1,1‐disubstituted olefins (the technologically most important polymers) several types of chiral structures exist, which are expected to result in optical activity if a particular enantiomer is favorably formed. By carrying out an asymmetric cyclopolymerization, it has been possible to obtain certain structural types in the form of optically active copolymers or homopolymers (e.g., copolymers of styrene with methyl methacrylate, or even the homopolymer of styrene). Another new group of optically active polymers consists of the atropisomeric helical polyisocyanides, poly(trityl methacrylates), and polychlorals. Optically active polymers are already used as adsorbents for the chromatographic separation of racemic mixtures. Further applications are likely to emerge.
Investigation of the Tautomerism of 15N‐Labeled Hydroporphyrins by Dynamic NMR Spectroscopy
Authors: Schlabach Martin , Rumpel Helmut , Limbach Hans‐Heinrich
Published: December 2003

The barrier height for tautomerism of the inner protons of porphyrin is significantly influenced by specific reduction of the pyrrole rings. In the tetraphenyl derivatives of chlorin and bacteriochlorin an aromatic ring current was established 1H NMR spectroscopically; in contrast, in the tetraphenyl derivative of isobacteriochlorin it is virtually absent. The activation barrier in the hydrogenation of a pyrrole ring is ca. 20 kJ mol −1 higher. In the hydrogenation of two adjacent rings a system is formed in which two symmetric double minimum potentials of differing barrier height have been demonstrated for the first time for an intramolecular proton transfer.
A Silicogermanate with a Si : Ge Ratio ≥ 2—an MFI Zeolite of Novel Composition
Authors: Gabelica Zelimir , Guth Jean‐Louis
Published: December 2003

A pure Ge‐MFI zeolite is expected to be difficult to synthesize. This is the conclusion drawn from a study on the possibilities and effects of Si/Ge exchange in Si‐MFI zeolites. When for example, the mixture GeCl4SiO2HF0.5 Pr4NBr8 CH3NH235 H2O is heated to 180°C for 15 h there results a solid with MFI structure, which, after calcination, has the composition Si63Ge33O192. The unusually high Ge content surpasses all previously observed degrees of substitution in MFI zeolites.
Biosynthetic Studies on Antibiotics
Authors: Floss Heinz G. , Beale John M.
Published: December 2003

The intricacies of the synthetic chemistry executed by living cells in the assembly of complex organic natural products can be probed in great detail by stable‐isotope labeling in conjunction with sophisticated 2D and multiple‐quantum NMR methods of analysis. Mechanistic details are revealed by unraveling the “cryptic” stereochemistry of biosynthetic reactions with the aid of stereospecific isotope labeling. Analysis of blocked mutants and cloning of biosynthetic genes provides new tools for the study of biosynthetic pathways and for the exploitation of the chemical capabilities of living organisms.
(tBuSiP)4–The First Silaphosphacubane
Authors: Baudler Marianne , Scholz Guido , Tebbea Karl‐Friedrich , Fehér Magda
Published: December 2003

For corrigendum see DOI:10.1002/anie.198905022
Flexible Syntheses of Optically Active Isocarbacyclins
Authors: Hemmerle Horst , Gais Hans‐Joachim
Published: December 2003

For corrigendum see DOI:10.1002/anie.198912821
Metallocene Carbene Complexes and Related Compounds of Titanium, Zirconium, and Hafnium
Authors: Erker Gerhard
Published: December 2003

The structure and creativity spectrum of transition‐metal carbene complexes has been widened significantly by the bent metallocenes of titanium, zirconium, and hafnium. Proceeding from metal carbonyls and reactive (butadiene)‐, (aryne)‐, or (olefin) MCp2 complexes, many new Fischer‐type metaloxycarbene complexes of Zr, Hf, V, Nb, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Ni, and Rh have been synthesized. The incorporation of titanocene, zirconocene, or hafnocene fragments allows new types of carbene complexes to be prepared. For the (alkylidenamido)metallocene complexes
equation image
, metallocene ylides
equation image
, and binuclear (μ‐alkynyl)metallocene compounds, some metalligand π bonding is indicated. Metallocene complexes with metallaoxirane units,
equation image
, show similar chemical behavior to that of binuclear (μ‐methylene)complexes
equation image
. The methylene groups of zirconaoxirane complexes, which are derived from carbon monoxide, may be expelled as ethylene by thermally induced CC coupling. With metal hydrides, CH2 transfer with insertion into the metal–hydride bond occurs. In one case, methylene insertion into a metal–carbon bond can even be observed. These reactions of Ti‐group metallaoxiranes could be models for postulated intermediates in the Fischer–Tropsch synthesis.
Alkoxide Clusters of Molybdenum and Tungsten as Templates for Organometallic Chemistry: Comparison with Carbonyl Clusters of the Later Transition Elements
Authors: Chisholm Malcolm H. , Clark David L. , Hampden‐Smith Mark J. , Hoffman David H.
Published: December 2003

Alkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant dπ orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled dπ orbitals. Both ligands readily form bridges that span MM bonds. In solution fluxional processes that involve bridge–terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)3], [CpW(CO)2], [Co(CO)3], and CH are related by the isolobal analogy. Thus the compounds [(RO)3W  W(OR)3], [Cp(CO)2WW(CO)2Cp], hypothetical [(CO)3CoCo(CO)3], and HCCH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding—e.g., [W3(μ3‐CR)(OR′)9] versus [Co3(μ3‐CR)(CO)9] and [W4(C)(NMe)(OiPr)12] versus [Fe4(C)(CO)13]—but differ with respect to MM bonding. The carbonyl clusters use eg‐type orbitals for MM bonding whereas the alkoxide clusters employ t2g‐type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18‐electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or MM bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand‐centered and the LUMO metal‐centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M2(OR)6] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry.
Discovery of a D‐Stereospecific Aminopeptidase and Its Use as a Catalyst in Organic Synthesis
Authors: Asano Yasuhisa , Nakazawa Akiko , Kato Yasuo , Kondo Kiyosi
Published: December 2003

The specific hydrolysis of D‐amino acid amides and the stereospecifc synthesis of peptides with D‐alanine‐N‐alkylamide residues is made possible by a new aminopeptidase isolated in pure form from cell extracts of Achromobacter sp. SCRC C1‐38. This enzyme, called D‐alanine aminopeptidase, has a molecular weight of about 122 000 and consists of two identical subunits.
The Search for Highly Colored Organic Compounds
Authors: Fabian Jürgen , Zahradník Rudolf
Published: December 2003

Until now the study of organic compounds in which the π‐electron system is excited by absorbed light has been mainly concentrated on the ultraviolet and visible regions of the electromagnetic spectrum. Various new applications, such as the use of conjugated organic compounds as dye lasers or as materitals for storing information with the help of diode lasers, led to the synthesis of new compounds which absorb light in the near in infrared (NIR). It is possible to use structure‐color relationships to predict the properties of such new compounds when they belong to dyestuff classes which have already been studied in detail; in this case the approach involves decreasing the energy difference between the ground state and the first excited state. A less conventional starting point is provided by molecular structures in which from the outset there is only a very small energy difference between the lowest‐energy electronic states; such diradicaloid molecules occupy a special position among the various types of organic compounds. It is possible by means of suitable structural modification to stabilize such molecules in a singlet from which absorbs light at very long wavelengths (i.e. at small wave numbers).
Gene Synthesis [New Synthetic Methods (77)]
Authors: Engels Joachim W. , Uhlmann Eugen
Published: December 2003

Oligonucleotide synthesis, until a few years ago the rather exotic preserve of a few experts, has become an integral part of the arsenal of molecular‐biological techniques. The last decade, in particular, has seen unbelievably rapid development in this area. DNA synthesis has been automated and can now produce genes greater than 1000 base pairs in length. Tailor‐made synthetic genes also permit the synthesis of altered or even novel proteins (de novo protein design) by gene‐technological methods. Together with modern methods of gene isolation, sequencing, and expression, gene synthesis has played a major part in the enormous advances achieved in gene technology.
Gas‐Chromatographic Enantiomer Separation of Unfunctionalized Cycloalkanes on Permethylated β‐Cyclodextrin
Authors: Schurig Volker , Nowotny Hans‐Peter , Schmalzing Dieter
Published: December 2003

In contrast to established procedures for the gas‐chromatographic separation of enantiomers (use of amide phases, complexation gas chromatography), which require the presence of polar functional groups in the compounds being separated, a new procedure now makes it possible to discriminate between the enantiomers of unfunctionalized hydrocarbons. This communication describes the enantiomer separation of cis‐l‐ethyl‐2‐methylcyclohexane, trans‐1,1,3,5‐tetramethylcyclohexane, and other alkyl‐substituted cyclohexanes on glass capillary columns containing permethylated β‐cyclodextrin in polysiloxane solution (inclusion mechanism).
The Complexation of Gold Colloids
Authors: Schmid Günter , Lehnert Andreas
Published: December 2003

Are colloids large clusters? Both can be stabilized by simple low‐molecularweight ligands, as was demonstrated by complexation of gold colloids with the water‐soluble phosphane Ph2PC6H4SO3Na. This approach allows not only the study of gold colloids in aqueous solution at any desired concentration, but also the investigation of noncoagulated colloids. These findings open up totally new perspectives in colloid chemistry.
Generation of Complex Metal Oxides by Aerosol Processes: Superconducting Ceramic Particles and Films
Authors: Kodas Toivo T.
Published: December 2003

Complex metal oxides, including ferroelectric, ferrimagnetic and superconducting ceramics, have a variety of technologically useful properties that can be exploited for a number of applications. Fabrication of complex metal oxides ceramics with specific properties reqires high‐purity powders with controlled chemical compositions, size distributions, and morphologies. Powders with these characteristics can be produced by aerosol processes in which fine particles are generated in gaseous flow systems. The particles can also be deposited from the gas phase onto surfaces to form films. This paper will discuss the use of aerosol processes for the generation of complex metal oxide powders. A review of aerosol processes will be presented first, followed by a discussion of the methods used for the generation of superconducting ceramic powders. Examples include the production of YBa2Cu3O7, La1.85Sr0.15CuO4, and Tl‐Ca‐Ba‐Cu‐O powders and films. Emphasis will be placed on defining the conditions required for the generation of chemically homogeneous particles with controlled morphologies.
A Structural Basis of Light Energy and Electron Transfer in Biology (Nobel Lecture)
Authors: Huber Robert
Published: December 2003

Aspects of intramolecular light energy and electron transfer are discussed for three protein cofactor complexes whose three‐dimensional structures have been elucidated by X‐ray crystallography: the light harvesting phycobilisomes of cyanobacteria, the reaction center of purple bacteria, and the blue multi‐copper oxidases. A wealth of functional data is available for these systems which allows specific correlations to be made between structure and function and general conclusions to be drawn about light energy and electron transfer in biological materials.
The Active Site of Glutathione Reductase: An Example of Near Transition‐State Structures
Authors: Sustmann Refiner , Sicking Willi , Schulz Georg E.
Published: December 2003

Interactions in the active site of glutathione reductase were calculated at the MNDO level (PM3 parametrization). The reaction partners were found to be arranged in an ideal geometry for hydride transfer from NADPH to FAD and for nucleophilic opening of the protein disulfide bridge by FADH⊖. The X‐ray structure analysis alone would have been insufficient for such an interpretation.
Are Lithiosulfones Configurationally Stable?
Authors: Gais Hans‐Joachim , Hellmann Gunther , Günther Harald , Lopez Fernando , Lindner Hans J. , Braun Sigmar
Published: December 2003

For corrigendum see DOI:10.1002/anie.198912821
Lipophilic Tartaric Acid Esters as Enantioselective Ionophores
Authors: Prelog Vladimir , Kovačević Miće , Egli Martin
Published: December 2003

Tartaric acid esters with lipophilic alcohol moieties can discriminate between enantiomeric ammonium ions such as norephedrinium ions. Quantitative results were obtained by partitioning the components between water and 1, 2‐dichloroethane. The stereoselectivity was characterized by the free energy difference of the partition process (ΔΔG). Diamond‐lattice sectors were used to construct models of the (unstable) lipophilic ester/ammonium salt complexes from X‐ray structures of the individual components (esters and ammonium salts). These models can be used to interpret the effect of structure and configuration of the alcohol moiety on the stereoselectivity and enantioselectivity towards 1‐phenyl‐2‐amino‐1‐propanol (norephedrine) salts.
Aurophilicity as Concerted Effect: Relativistic MO Calculations on Carbon‐Centered Gold Clusters
Authors: Rösch Notker , Görling Andreas , Ellis Donald E. , Schmidbaur Hubert
Published: December 2003

Three features must be taken into consideration in discussing the bonding in carbon‐centered gold clusters: the tangential AuAu bonds, the radial bonds between Au and the central atom as well as between Au and the outer ligands, and the preference for 6s‐5d over 6s‐6p hybridization. Nonrelativistic and relativistic MO calculations revealed that the formally closed d10 shell of the Au⊕ ions in the cluster [(LAu)6C]2⊕ is increasingly disrupted by interaction with the other Au centers, the central atom, and the ligands L, thereby increasing the probability of AuAu interactions.
Liquid Crystalline Elastomers
Authors: Zentel Rudolf
Published: December 2003

Today, material science is directed towards the development of multifunctional and oriented structures. One example of such supramolecular systems are liquid crystalline (LC) elastomers which combine the properties of LC phases (the combination order and mobility) with rubber elasticity, one of the most typical polymer properties. Their most outstanding characteristic is their mechanical orientability; strains as small as 20% are enough to obtain a perfectly oriented LC monodomain. This orientability, if LC elastomers with chiral phases are used, leads, for example, to elastomers with chirals smectic C*phases which are likely to show piezo‐electric behavior.