Angewandte Chemie International Edition in English

The cover picture shows a section of a high-frequency oven used in a novel synthesis of fullerenes. The reaction zone (at approximately 2800°C, the light is dimmed by the soot beginning to deposit) is depicted at the start of the fullerene production. The inductor, boron nitride insulation tube, quartz tube, and initial formation of smoke can be seen. More about the novel efficient synthetic method is reported by M. Jansen and G. Peters on page 223. The boom in fullerene chemistry continues. The third in a series of highlights on the important recent results (previous updates were published last year in the January and June issues) will appear in the next issue. An overview of the preparations, properties, and reactions of C₆₀ is planned. This issue includes two highlights on silicon clusters including Si₆₀ (p. 173). a communication on He@C₆₀ (p. 183), and, last but not least, an exciting account by H. Kroto on the start of the fullerene story (p. 111).

A “round organic chemistry” has burgeoned this last year following the development of an efficient means of producing fullerenes in 1989/1990. One of the protoganists in the fullerene story describes in a lively personal style how this fascinating new chemistry came about. In this thriller he points out that the discovery of C₆₀ in 1985 did not result from an applications-oriented search but rather from basic research and scientific curiosity.

Almost atomic resolution is achieved today in techniques for studying monomolecular Langmuir films of amphiphilic molecules. Synchrotron X-ray sources providing well-collimated, high-intensity radiation make this possible. In this review the principles and methods used to study thin films are discussed. In these investigations, for instance, a strong influence from solute molecules or ions in the subphase on the crystallinity of the monomolecular layer was revealed: whereas α-glycine increases the crystallinity of a fluorocarbon monolayer, β-alanine decreases it. Possible explanations for the phenomena are also presented.

Millions of years of evolution are reflected in the phenomenon of biomineralization. The large organized regions of macromolecules and crystals produced by organisms are the subjects of intense research. SiO₂ is one of the amorphic minerals which is deposited, for example, by diatoms, in fantastic structures as shown right. The polymeric structure of SiO₂ results in high viscosity and low solubility; the linkage of the coordination polyhedrons through their edges allows the formation of open structures. Both are important factors in the deposition of SiO₂.

Stamped as long since discovered and therefore completely characterized, simple inorganic molecules like the phosphorus nitrides are often neglected. Yet although P₃N₅ was synthesized in 1903, its structure is still a mystery. W. Schnick et al. resurrected the chemistry of these compounds in the synthesis of P-N analogues of known structural types. In this way they succeeded in synthesizing and characterizing the lithium salt of the complex anion P₄N (Angew. Chem.1991, 103, 857; Angew. Chem. Int. Ed. Engl.1991, 30, 830) and Zn₇[P₁₂N₂₄]Cl₂, whose [P₁₂N₂₄] framework is analogous to that of the [Al₆Si₆O₂₄]Cl₂ unit of sodalite (communication on p. 213).

Bearing in mind the further miniaturization in microelectronics and the possibility of discovering new materials, as was the case with the fullerenes, many research groups are at present studying clusters of semiconductor elements like silicon. Reactions of mass-selected Si_n clusters with reactive gases such as ethylene and subsequent analysis of the products provide information on the structures of these complexes. Thus for instance, Si₁₀ does not have the adamantane structure expected from the structure of elemental silicon, but a compact structure (two calculated structures of almost the same energy are depicted for Si₁₀) with fewer free valencies at the Si atoms. There is no evidence for a “round Si₆₀ chemistry” analogous to that of C₆₀, although Si₆₀ represents a stability maximum (calculated structure depicted to the right). A direct structure determination will probably remain elusive for some time, because the absolute stability of Si₆₀ is orders of magnitude smaller than that of C₆₀.

Bearing in mind the further miniaturization in microelectronics and the possibility of discovering new materials, as was the case with the fullerenes, many research groups are at present studying clusters of semiconductor elements like silicon. Reactions of mass-selected Si_n clusters with reactive gases such as ethylene and subsequent analysis of the products provide information on the structures of these complexes. Thus for instance, Si₁₀ does not have the adamantane structure expected from the structure of elemental silicon, but a compact structure (two calculated structures of almost the same energy are depicted for Si₁₀) with fewer free valencies at the Si atoms. There is no evidence for a “round Si₆₀ chemistry” analogous to that of C₆₀, although Si₆₀ represents a stability maximum (calculated structure depicted to the right). A direct structure determination will probably remain elusive for some time, because the absolute stability of Si₆₀ is orders of magnitude smaller than that of C₆₀.

Controlled ripening of tomatoes—the application of antisense-RNA technology by an American research group made this possible. The biosynthesis of ethylene, the signal compound which induces ripening, can be suppressed by these molecular biological methods. The storage and transportation of the green fruits is much easier. Their subsequent exposure to ethylene then triggers ripening. Antisense-RNA technology can be applied in other areas, for example in medical research.

For compounds with poor thermal conductivity and samples of only a few milligrams the heat capacity may be determined as a function of temperature with great accuracy with the microcalorimeter presented here. This was demonstrated on two Fe^II spin-transition systems. Comparison with the results of measurements conducted with a Nernst calorimeter on a sample almost 1000 times larger was possible in one case; the agreement of the two data sets was quite good. Simultaneously the change in the thermal conductivity at the spin transition was determined for the first time.

In contrast to m-xylylene and related diradicals, bisnitroxide 1 has a singlet ground state according to ESR spectroscopic investigations. Diradical 1 can be isolated as two stable atropoisomers. In both, the free electron is possibly localized on the oxygen atom, because conjugation is prevented by the steric hindrance of the methyl groups. The parent compound with an unsubstituted benzene ring has a triplet ground state.

The catalysis of the same reaction through different mechanisms was attributed to the diversity of the immune response. The chorismate–prephenate conversion 1 → 3 can be catalyzed by antibodies raised against an analogue of the transition state 2. These antibodies give rise to a variety of activation parameters for the catalysis. An extremely low activation entropy was found for the antibody 11F1-2E11, which supports the notion that conformational restriction may play an important role in antibody catalysis.

Neutralization-reionization mass spectrometry was used to show that for both neutral He@C₆₀ and He@C, the noble gas atom is incorporated within the fullerene cavity. The C₆₀H^˙+ ions were generated by high-energy collisions of C with quasi-stationary ⁴He target atoms and cleaved in the mass spectrometer by loss of C₂ or multiples thereof, as is usual for endohedral MC₆₀ complexes.

No direct SiSi bond is present in 1 in spite of the short interatomic distance; however, the long CC distance does not prevent weak bond formation in 2. These results are predicted by an analysis of the compounds with the electron localization function. The explanation for the short SiSi bond in 1 lies in the different sizes of C and Si.

To determine solid-state structures, the electron localization function (ELF) must be interpreted somewhat differently than for molecules. The example of the diamond structures of C (right, top), Si, Ge, and α- and β-Sn (right, bottom) show clearly that ELF depicts the electronic changes in regional space as the covalent bond gives way to metallic bonding.

The structure and dynamics of monomolecular adsorbed layers of long-chain 1-alkanols can be made visible by electron scanning tunneling microscopy. The packing in single domains and the pattern at the domain boundaries (shown here for 1-octadecanol) confirm the crucial role of the hydroxy groups in the packing of molecules; the common structures at the domain boundaries suggest that all potential hydrogen bonds are indeed formed. The reorganization of regions containing many defects and the incorporation into the domain structure of impurities are also observed.

A shell of iron atoms and tetradentate amino ligands surrounds the close-packed cluster core [Fe₇(μ₃-OH)₆(μ₂-OH)₄{(μ₃-O)Fe}₂]¹³⁺ common to 1 and 2. These types of compounds could help in understanding naturally occurring hydroxo-(oxo)iron clusters.

From simple achiral starting materials the chiral organoiron(II) complex 1 can be prepared as either the R or S enantiomer. A Michael addition and subsequent stereospecific alkylation followed by oxidative cleavage of the Fe complex fragment and hydrogenolysis of the O-benzyl function yields the α,α-dialkyl-α-hydroxycarbonyl compounds with the desired chirality.

The isolation of enantiomerically pure L-amino acids is now possible with a bacterial acylase from N-acylated racemic precursors 1 (R₁ Me, Et, nPr; R₂ Me, Et); chemical preparation of these enantiomerically pure compounds is difficult. The viability of the reaction depends on the substitution pattern at the α carbon atom, but similar aminoacylases with a still wider substrate spectrum are expected to be discovered by microbiological screening.

Another form of molecular self-assembly—the threading and unthreading of ring-shaped molecules on a polymer chain (represented schematically here) was studied by NMR spectroscopy and followed qualitatively for the formation of polymeric inclusion compounds made up of cyclodextrins and poly-(iminooligomethylene)s. The structures of the polymeric inclusion compounds are proven by their conversion to polyrotaxanes.

The opposing effects of activation enthalpy and entropy most likely cause the transition from isotactic via atactic to syndiotactic polypropylene in polymerizations with the soluble metallocene catalyst 1 (X Ph) and methylalumoxane when the reaction temperature is increased from −50 to +10°C. This proves how important temperature considerations are for structure–reactivity discussions.

Through coaxial mixing of a pulsed jet of CH₃NC/Ar with a continuous stream of HF/Ar, the complex CH₃NCċHF is formed. Since the two components are separate until their expansion in the Fourier-transform microwave spectrometer, their high reactivity can be suppressed. The rotation spectrum of the complex is characteristic of that of a symmetric top, that is, the C_3v symmetric complex contains a linear CNCċHF arrangement.

Applications in information storage and in thermally activated recording systems are anticipated for the novel allyl aryl ether 1. At 180°C 1 rearranges to phenol lactone 2, which in turn undergoes intramolecular proton migration to provide colored 3.

The coordinatively flexible and redox-active complex fragment [Mo(NO)(′S4′)] in 1 makes the stepwise, and in principle catalytic, reduction of NO to NH₂OH possible. The two single steps can be clearly distinguished as 2e/2H₊ and 1e/1H₊ reductions. This reaction may be considered a model for the molecular processes in the reduction of NO to NH₂OH within the conversion of NO to NH₃ in the nitrogen cycle in nature.

Proximity effects and electrophilic activation combined account for the accelerated ring-opening in the transesterification of 1 with amidinium alcohol 2. The spatial proximity effects arise in a contact ion-pair of 1 and 2. Electrophilic activation causes the 9000-fold rate enhancement of the reaction of 1 with 2 relative to the reaction with phenylalcohol; the addition of 2 to the transesterification of 1 with phenyl alcohol to form 3 caused a 25-fold rate increase.

Imidazolophanes 3 bridged by short chains (n 5, 6) are readily accessible by [3 + 2] cycloadditions of imines with 2, formed from the bicyclic azirines 1 under electron-transfer conditions. Other heterocyclophanes can also be prepared by this method. DCN naphthalene dicarbonitrile.

The γ turns not found experimentally in cyclic hexapeptides are also not found in molecular dynamics (MD) simulations if the calculations take the surrounding solvent into consideration. In this report the structures obtained from simulations conducted for the peptide in vacuum were used in subsequent simulations with dimethyl sulfoxide as the solvent. The two γ turns originally obtained disappeared, and only the experimentally proven β turns remained. These results show that the solvent must be explicitly included in MD simulations of peptide conformations in solutions.

The first step in the transition from two- to three-dimensional systems in reduced oxoniobates was achieved in the title compound, which was prepared from NbO₂, Ba₅Nb₄O₁₅, and Nb at temperatures above 1800 K. The structure (shown right) can be considered as “intergrowths” of BaO and NbO or as the perovskite BaNbO₃ and NbO.

A three-dimensional network of corner-sharing PN₄ tetrahedrons is found in the sodalite-like [P₁₂N₂₄] framework of the title compound, synthesized simply from NH₄C1, (PNC1₂)₃, and ZnCl₂. A Cl⁻ ion surrounded tetrahedrally by Zn²⁺ ions occupies the center of the β cages, as is shown on the right. The modification of the sodalite framework by exchanging O atoms for N atoms opens new horizons in zeolite research. • P; o N.

Only after the addition of the substrate, methylmalonyl-CoA, to the binary complex of highly purified methylmalonyl-CoA mutase and coenzyme B₁₂ is a clear ESR signal observed. This proves that radical intermediates that have only been postulated in the past are indeed involved in the enzymatic rearrangement of the carbon skeleton.

The substitution of two chlorine atoms by dimethylaminopyridino groups in PCl₃ (1), MeOPCl₂ (2), and MeOP(O)Cl₂ (3) yields phosphorus derivatives that are dicationic (1 and 2) or monocationic (3). These molecules are highly activated toward nucleophilic attack. For the first time, in nucleophilic reactions a cationic group is the leaving group. In fact, the nucleofuge derived from 3 is a better leaving group than the trifluoromethane sulfonate ion.

Interpenetrating diamond-shaped Ta₂Ni₂ clusters in two-dimensional layers are found in the novel metal-rich layer compound TaNi₂Te₂, which is synthesized at 950 °C from the elements (structure right: large dotted spheres Te, medium-sized partially shaded spheres Ta, small spheres Ni). Band-structure calculations predict that TaNi₂Te₂ should have metallic properties.

Extremely fast and at low temperatures, alkoxido complex 1 reacts with CS₂ to the corresponding Re–O insertion product by direct attack of the electrophile on the O atom of the alkoxido ligand. In contrast, the reaction of aryloxido complex 2 with CS₂ requires higher temperatures (100 °C) and seven days' reaction time; here the loss of a phosphine ligand probably constitutes the first step. The importance of such studies in the design of catalyst systems is obvious.

Larger amounts of fullerenes at the remarkably low temperature of formation of 2700 °C (helium pressure 150 kPa) are provided by a new synthetic method. Cylindrical carbon bodies are inductively heated and evaporated, the resulting fullerenes are extracted from the soot formed, and the product distribution is determined by mass spectrometry.

Complexes of propargyl carbonates 1 react with organocopper(I) compounds stereo selectively to yield the α-(allenylbutadiene)tricarbonyliron complexes 2. Decomplexation with eerie ammonium nitrate furnishes the α-[(lE,3E)-dienyl]allenes 3 rapidly and in high yields. R¹ Ph, tBu, SiMe₃; R² Et; E Co₂Me.

Its speed and accuracy and the extremely simple sample preparation required recommends ion-spray mass spectrometry as a technique for studying highly lipophilic and amphiphilic molecules of high molecular mass and, in particular, for biomolecules (which have not previously been able to be characterized by mass spectrometry). Routine analyses detect molecular masses up to 6000 u with an accuracy of ± 1 u.

For Pd-catalyzed allylic alkylations a series of novel C₂-symmetric bis-(diphenylphosphino) ligands have proved to be extremely efficient. For example, the model reaction 1 → 2 with diamide 3 as ligand proceeds in 94% yield and 88% ee. Ts para-toluolsulfonyl, dba dibenzylidenacetoacetonate.

A new type of potentially helical ligand is represented by 1. The twist of the molecule's chain required to form double helical complexes by self-organization occurs at the CC bonds between the arene rings. Ligand 1 forms double helical complexes with Cu⁺, Ag⁺, and Ni²⁺ salts. A crystal structure analysis of the complex [Ni₂(OAc)₂(lb)₂](PF₆)₂ revealed neither intermetallic interactions nor stacking interactions between almost coplanar rings.

Under mild reaction conditions and catalyzed by Rh⁺, the (E)/(Z)-γ-hydroxyvinylstannanes 1 and 3 and -silanes 2 and 4 were successfully hydrogenated in an autoclave at about 10⁷ Pa. Till now, hydrogenations were possible only for olefins with alkyl, aryl, and acyl substituents. The diastereoselectivity of the new reaction depends on the size of the substituents R and R′ (R C₆H₁₁, Me, H; R′ H, alkyl, SnBu₃; X SnBu₃, SiPhMe₂).

Distinctly higher yields of functionalized cyclopropanes such as 1 can be achieved when 1,2-bis(dimethylamino)ethane, followed by CO, are added to the reaction of π-allylpalladium complexes with lithium enolates of esters. The usual, energetically favored allylic alkylation is efficiently suppressed in this way. The role of the amine ligands is crucial: addition of 1,2-bis(dimethylamino)methane produces no cyclopropanation product; with sparteine only low yields are obtained.