Dialkylaluminum hydrides R2Al–H (R = Me, Et, iBu, CH2tBu, tBu), reacted with tert‐butylethyne, H–C≡C–CMe3, by hydroalumination and formation of the corresponding vinylic compounds, R2Al–C(H)=C(H)–CMe3. The products are dimeric in the solid state with the negatively charged vinylic carbon atoms in the bridging positions and adopt usually a centrosymmetric structure with the C=C double bonds on different sides of the central Al2C2 heterocycle. Only with R = Me the C2v structure with the cis arrangement of the alkenyl groups was found. Complicated mixtures of isomeric substances were detected in solution. Quantum‐chemical calculations revealed a considerable charge separation in the C=C double bonds of the dimeric formula units. The α‐carbon atoms coordinated by both aluminum atoms show high negative NBO charges of about –1.0, while the β‐carbon atoms are almost uncharged (0.0 to –0.1). The rotational barriers decreased upon dimerization. Hence, the formation of such dimers may be the key step in the well‐known cis/trans isomerization process of hydroalumination and hydrogallation products. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

In recent investigations we found an easy access to dialkyl(alkynyl)aluminum compounds by the deprotonation of phenylethyne with di(tert-butyl)-and dimethylaluminum hydride with evolution of molecular hydrogen. In contrast, the reactions of the bulky hydride R 2 Al-H [R ) CH(SiMe 3 ) 2 ] with different alkynes R-CtC-H (R ) CMe 3 , SiMe 3 , C 6 H 5 , H) yielded vinylic aluminum compounds, R 2 Al-CHdCH-R′, by hydroalumination. Due to the effective steric shielding of the bulky alkyl substituents, the products were monomeric even in the solid state, with the alkenyl groups bonded to coordinatively unsaturated, tricoordinated aluminum atoms. Quantum-chemical calculations verified that there was no interaction between the π-electrons of the CdC bond and the vacant p-orbital at the coordinatively unsaturated aluminum atoms.

Monomeric versus Dimeric Dialkylaluminum Alkynides R2Al‐C≡C‐R′ – Tricoordinated Aluminum Atoms Attached to C≡C Triple Bonds Addition of lithium ethynides, Li‐C≡C‐R (R = C6H5, CMe3), to cooled solutions of the chloroalane Cl‐Al[CH(SiMe3)2]2 (1) afforded the corresponding dialkylaluminum alkynides. Caused by the effective steric shielding the products (2 and 3) are monomeric in solution and in the solid state. They possess coordinatively unsaturated, tricoordinated aluminum atoms attached to C≡C triple bonds and relatively short bonds between aluminum and the alkinyl carbon atoms (<191.4 pm). Owing to quantum‐chemical calculations an interaction between the empty p‐orbital at aluminum with the π‐system of the triple bond can clearly be excluded. The sterically less shielded di(tert‐butyl)aluminum chloride yielded the dimeric aluminumalkynide 4 upon treatment with lithium tert‐butylalkynide. Lithium‐trimethylsilylethynide, Li‐C≡C‐SiMe3, and 1 gave always the alanate Li[(Me3Si‐C≡C)2Al{CH(SiMe3)2}2] (5) independently of the stoichiometric ratio of the starting compounds.

The four Ga-Ga bonds of the title compound 2 are bridged by two 4,4′-biphenyldicarboxylato and four hydroxo ligands to yield a molecular box with an edge length of 1.5 nm. 2 was obtained by treatment of the digallium compound R 2 Ga-GaR 2 (1; R ) CH(SiMe 3 ) 2 ) with 4,4′-biphenyldicarboxylic acid in the presence of water.

Synthesis of a Functional Aluminium Alkynide, Me3C‐C≡C‐AlBr2, and its Reactions with the Bulky Lithium Compound LiCH(SiMe3)2Treatment of aluminium tribromide with the lithium alkynide (Li)C≡C‐CMe3 afforded the aluminium alkynide Me3C‐C≡C‐AlBr2 (1) in an almost quantitative yield. 1 crystallizes with trimeric formula units possessing Al3C3 heterocycles and the anionic carbon atoms of the alkynido groups in the bridging positions. A dynamic equilibrium was determined in solution which probably comprises trimeric and dimeric formula units. Reaction of 1 with one equivalent of LiCH(SiMe3)2 yielded the compound [Me3C‐C≡C‐Al(Br)‐CH(SiMe3)2]2 (2), which is a dimer via Al‐C‐Al bridges. Two equivalents of the lithium compound gave a mixture of four main‐products, which could be identified as 2, Li[Me3C‐C≡C‐Al{CH(SiMe3)2}3] (3), Me3C‐C≡C‐Al[CH(SiMe3)2]2 (4), and Al[CH(SiMe3)2]3. The lithium atom of 3 is coordinated by the C≡C triple bond and an inner carbon atom of one bis(trimethylsilyl)methyl group. Further interactions were observed to C‐H bonds of methyl groups.