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

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–C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–C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.