Aluminum and Gallium Hydrazides as Active Lewis Pairs: Cooperative C-H Bond Activation with H-Cî - C-Ph and Pentafluorobenzene

Uhl W., Willeke M., Hengesbach F., Hepp A., Layh M.

Abstract

Hydroalumination or hydrogallation of a sterically encumbered hydrazone, H10C5N-N=C(C9H14) (NC5H10 = piperidine, C(C9H14) = 2-adamantdiyl), afforded hydrazides that, depending on the steric shielding by the substituents at the metal atoms, had different molecular structures. While both di(tert-butyl)metal derivatives (1a, 1b) are monomeric in the solid state with highly strained MN2 heterocycles (M = Al, Ga), the dimethylmetal compounds (1c, 1d) are dimeric with M2N2 heterocycles and exocyclic N-N bonds. The latter compounds are highly dynamic in solution. 1d crystallized as a mixture of cis- and trans-isomers as detected by crystal structure determinations. These compounds react as active Lewis pairs by their specific donor-acceptor functionality and are able to activate C-H bonds of moderately acidic substrates. Reaction of 1a (M = Al) with H-Cî - C-C6H5 afforded by C-H bond activation and release of H-CMe3 trialkynyl compound 4, in which three alkynyl groups and a neutral hydrazine ligand are bound to Al. 1b (M = Ga) gave only the known dimeric monoalkynyl derivative [(Me3C)2Ga-Cî - C-C6H5]2 (5b). The sterically less shielded dimethyl compounds 1c and 1d similarly yielded trialkynylmetal compounds by methane and hydrogen elimination. In this case a hydrazone ligand is coordinated to the metal atoms. 1d reacted with pentafluorobenzene in an unprecedented reaction to yield a diaryl-methylgallium compound with the metal atom bound to two electron-withdrawing groups and a hydrazone ligand completing the coordination sphere of Ga.