Hydroalumination or hydrogallation
of a sterically encumbered hydrazone,
H10C5N–NC(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–CC–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–CC–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.