Hydrogallation Reactions Involving the Monoalkynes H5C6‐C≡C‐SiMe3 and H5C6‐C≡C‐CMe3 – cis/trans Isomerisation and Substituent Exchange
Phenyl‐trimethylsilylethyne, H5C6‐C≡C‐SiMe3, reacted with different dialkylgallium hydrides, R2Ga‐H (R = Me, Et, nPr, iPr, tBu), by the addition of one Ga‐H bond to its C≡C triple bond (hydrogallation). The gallium atoms attacked selectively those carbon atoms, which were also attached to trimethylsilyl groups. The cis arrangement of Ga and H across the resulting C=C double bonds resulted only for the sterically most shielded di(tert‐butyl)gallium derivative, while in all other cases spontaneous cis/trans rearrangement occurred with the quantitative formation of the trans addition products. The diethyl compound Et2Ga‐C(SiMe3)=C(H)‐C6H5 (2) gave by substituent exchange the secondary products EtGa[C(SiMe3)=C(H)‐C6H5]2 (7, Z,Z) and Ga[C(SiMe3)=C(H)‐C6H5]3 (8). Interestingly, compound 8 has two alkenyl groups with a Z configuration, while the third C=C double bond has the cis arrangement of Ga and H (E configuration). The reversibility of the cis/trans isomerisation of hydrogallation products was observed for the first time. tert‐Butyl‐phenylethyne gave the simple addition product, R2Ga(C6H5)=C(H)‐CMe3 (9), only with di(n‐propyl)gallium hydride.
Treatment of 1,2-bis(trimethylsilylethynyl)benzene with di-tert-butylaluminum and di-tert-butylgallium hydrides afforded the simple addition products 1,2-[(Me 3 Si)(R 2 E)CdC(H)] 2 C 6 H 4 (R = CMe 3 ; E = Al (1), Ga (2)), which could not be isolated in a pure crystalline form but have been characterized unambiguously by spectroscopic methods. Addition of the Lewis base ethyldimethylamine initiated condensation reactions which gave cage compounds (3 and 4) by the release of the corresponding tri-tert-butyl element derivatives. These cages contain two aluminum or gallium atoms which are bridged by three 1,2-bis(trimethylsilylethenyl)benzene spacers to form molecular capsules. The metal atoms are further coordinated by terminal amino groups. The amino ligands could not be removed from the dialuminum compound 4 without decomposition, but the ligand-free gallium compound 5 was obtained upon heating of 3 (E = Ga) to 80 °C under vacuum. Thermolysis of the aluminum compound 1 in boiling n-hexane gave a unique reaction by the release of tri-tert-butylaluminum and the formal elimination of trimethylsilylethyne (decarbalumination). The product 6 is dimeric in the solid state via Al-C-Al bridges and has a pentacyclic molecular structure.
Reactions of the carbon-bridged bis(dichlorogallium) compounds (Cl3a to 3c), via β-elimination and release of isobutene. 3a to 3c are dimeric in solution and the solid state and contain unprecedented Ga 4 C 2 H 4 heteroadamantane structures in which four metal atoms are bridged by four hydrogen and two carbon atoms. In contrast, n-propyllithium gave the monomeric tetra(n-propyl)digallium compound ( n Pr 2 Ga) 2 C(SiMe 3 )-CH 2 -Ph (2) under similar conditions, which has two coordinatively unsaturated gallium atoms and may be applicable as a chelating Lewis acid.
Terminal and Bridging Coordination of Indium‐Indium Bonds – Remarkable Polymorphism with the Compound In2R2[(OCC6H5)2CH]2 [R = C(SiMe3)3]
Treatment of the dimeric indium(II) subhalide (In2R2Cl2)2 (1) [R = C(SiMe3)3] with four equivalents of lithium dipivaloylmethanide or lithium dibenzoylmethanide afforded by the release of lithium chloride the corresponding diindium diacetylacetonates (2 and 3). The In‐In single bonds of the products were terminally coordinated by chelating acectylacetonato ligands and the bulky alkyl groups. Three different crystal structures were determined for the dibenzoylmethanide derivative 3 which in the solid state adopted trans and gauche conformations across the In‐In bonds. In contrast to the terminally arranged acetylacetonato ligands of compounds 2 and 3 alkylbenzoato ligands R‐COO− (3,5‐dimethylbenzoate and p‐tert‐butylbenzoate) gave the bridging coordination of the In‐In bonds by two chelating carboxylato groups (4 and 5). This particular coordination caused a strong shortening of the In‐In bond length in 4 compared to the unsupported bonds in 2 and 3 (264.6 versus 274.7 to 279.3 pm).
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