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

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–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.

A Dimeric Gallium Hydrazide as an Active Lewis Pair – Complexation and Activation of Me2GaH and Various Heterocumulenes

“…Monomeric dialkylaluminium or dialkylgallium hydrazides are promising candidates in this field. They have strained AlN 2 heterocycles with a long donor‐acceptor and a relatively short polar covalent Al–N bond and are mostly generated by the reaction of hydrazones with dialkylaluminium or ‐gallium hydrides . They were shown to be highly efficient active Lewis pairs for the coordination and insertion of small molecules, which are both favored by the cooperative functionality of the metal and N atoms.…”

“…The bulky substituents are advantageous for the growing of single crystals and the purification of the hydrazides and their activation products by recrystallization. Recent publications detailed the influence of the steric bulk of the substituents R on Al on the structures and reactivity of the hydrazides R 2 Al–N(2‐Ad)–NC 5 H 10 ( R = t Bu, CH 2 t Bu, i Bu, Me; 2‐Ad = 2‐adamantyl) . Monomeric compounds featuring strained AlN 2 heterocycles were obtained for R = t Bu, CH 2 t Bu, and i Bu, whereas a dimer with a four‐membered Al 2 N 2 heterocycle and exocyclic N–N bonds was formed in the case of R = Me .…”

“…180 and 200 pm) surprisingly both Al–N bonds were reactive and allowed the insertion of suitable substrates such as CO 2 , CS 2 , R NCO, or R NCS ( 1 to 3 ) (Scheme ) , . The reaction of t Bu 2 Al–N(2‐Ad)–NC 5 H 10 with Ph–C≡C–H afforded by cooperative C–H bond activation followed by unexpected alkane elimination a trisalkynylaluminium compound, (PhC≡C) 3 Al · (H 10 C 5 N‐N(H)(2‐Ad) ( 4 ) (Scheme ) …”

“…The dimeric dimethylaluminium derivative [Me 2 Al–N(2‐Ad)–NC 5 H 10 ] 2 acted similarly as an active Lewis pair and yielded comparable products . Much less is known about the reactivity of the corresponding Ga hydrazides ( R = Me, CMe 3 ).…”

“…Much less is known about the reactivity of the corresponding Ga hydrazides ( R = Me, CMe 3 ). The dimeric hydrazide, [Me 2 Ga–N(2‐Ad)–NC 5 H 10 ] 2 ( 5 ) was obtained from Me 2 GaH and H 10 C 5 N–N=C(C 9 H 14 ) and gave a mixture of the trans and the cis isomers in the solid state (Scheme ) . It was recently found to show unique reactivity towards the moderately C–H acidic substrates Ph–C≡C–H and H–C 6 F 5 .…”