Darstellung,Kristallstrukturen und Schwingungsspektren neuer ternärer Verbindungen mit dem Anion [N–B–N]3–

Ternary Nitridoborates. 2. Synthesis, Crystal Structure, and Vibrational Spectra of New Ternary Compounds with the [N–B–N]^3– Anion The isotypic compounds LiM₄[BN₂]₃ (M = Ca, Sr, Ba, Eu) and NaM₄[BN₂]₃ (M = Sr, Ba) are formed as colorless to pale yellow prismatic crystals (black with Eu) by reaction of the binary components Li₃N, M₃N₂, EuN and Na, NaN₃, Ba₃N₂ and BN in sealed niobium ampoules at 1375 and 1275 K, respectively. The linear anions [N–B–N]^3– have bond lengths d(B–N) between 132.6 and 136.6 pm. Vibrational frequencies and force constants f(B–N) = 7.25–7.89 Ncm^–1 reveal significant drifts related to bond length and effective anionic charge. The cubic crystal structures (Im3m (No. 229), Z = 2; LiM₄[BN₂]₃: a(Ca) = 711.5 pm; a(Sr) = 745.6 pm; a(Eu) = 742.5 pm, a(Ba) = 788.0 pm and NaM₄[BN₂] ${}_{\text{3}\text{: a(Sr) = 756.8 pm; a(Ba) = 791.7 pm)) are stuffed derivatives of the β‐PtHg4}}$ structure type, and the range of existence of this cubic structure is derived from the molar volume and the ionic radii. The cations form a partial structure of centered cubes E1(E2)₈ which are condensed to a [E1(E2)_8/2] network (E1 = Li, Na; E2 = Ca, Sr, Ba, Eu). The remaining open cubes are filled by the [BN₂]^3– anions yielding two interpenetrating [E1(BN₂)_6/2] networks. Periodic Nodal Surfaces (PNS) of Im3m symmetry show the regions of different interactions.     

Crystallization and decomposition of amorphous silicon-aluminium films

The crystallization and decomposition of vacuum-deposited amorphous silicon-aluminium films have been examined by means of transmission electron microscopy. Depending on the aluminium concentration, the transformation of the metastable amorphous phase into the stable phases of aluminium and silicon proceeds by different reactions such as pre-crystallization of aluminium, polymorphous transformation into supersaturated crystalline solid solutions or eutectic decomposition. The temperature dependence of the eutectic crystallization was measured. The results are discussed in terms of the thermodynamics of amorphous-to-crystalline transformation.     

Microwave‐Assisted Dibromoolefination of Aromatic and Heteroaromatic Aldehydes and Ketones

Microwave (MW) irradiation was successfully employed to convert aromatic and heteroaromatic aldehydes and ketones efficiently to the corresponding dibromoolefins. Exemplified by the successful dibromoolefination of traditionally inert pyridyl‐flanked carbonyls, MW activation significantly broadens the scope of this valuable transformation, although some limitations especially with electron‐rich aromatic ketone derivatives remain.     

The Palladium Way to N‐Heteroacenes

Novel synthetic methodologies allow increasingly efficient access to known organic materials, as well as the preparation of otherwise inaccessible species. Pd‐catalyzed coupling of aromatic dihalides to ortho‐diaminoarenes furnishes embedded stable N,N′‐dihydropyrazines expediently and in often excellent yields. The embedded N,N′‐dihydropyrazines can then be oxidized by MnO₂ to give substituted azatetracenes, azapentacenes, azahexacenes, and azaheptacenes, which are soluble, processable, and stable. This powerful Pd‐catalyzed methodology allows the preparation of azaacenes, including diaza‐, tetraaza‐ and hexaazaacenes. In combination with a suitable Pd precursor, Buchwald‐type biarylphosphines have been shown to give excellent results. Activated dihalides such as 2,3‐dihaloquinoxalines are coupled easily under simplified conditions, whereas 2,3‐dibromoacenes require more stringent conditions and advanced catalyst precursors. Pd catalysts effect the assembly of azaacenes with otherwise difficult to obtain substitution patterns. High yields and flexibility make this method most attractive.