Entrapment of tert-Butyllithium and Lithium tert-Butylamide Monomers by Partially Lithiated Derivatives of Aluminum and Gallium Primary Amide Complexes

The lithiation of the dimeric trisamido group 13 derivatives {M[N(H)t-Bu]3}2 (1a, M = Al; 1b, M = Ga) by alkyllithium reagents RLi (R = t-Bu or Me), at different stoichiometries, was investigated. On the basis of NMR data, the initial lithiation of 1a occurs at the bridging N(H)t-Bu groups to give the mono- and dilithiated derivatives Li x [Al2(Nt-Bu) x (NHt-Bu)6 - x ] (x = 1, 2). The reaction of 1a with t-BuLi in a 1:3 molar ratio in n-pentane at 23 °C produces {Li[Al(Nt-Bu)(NHt-Bu)2]}2·(t-BuLi) (2) in 67% yield, whereas the treatment of 1a with MeLi in diethyl ether at 23 °C yields {Li[Al(Nt-Bu)(NHt-Bu)2]}2·[LiN(H)t-Bu] (7) and {Li[Al(Nt-Bu)(NHt-Bu)2]}2 (8). The gallium analogue of 7, i.e., {Li[Ga(Nt-Bu)(NHt-Bu)2]}2·[LiN(H)t-Bu] (5), was obtained in 47% yield by the reaction of 1b with t-BuLi in a 1:3 molar ratio in n-pentane at 23 °C. The structures of 2, 5, and 7 have been determined by X-ray crystallography and were shown to involve the entrapment of a monomeric t-BuLi or LiN(H)t-Bu fragment by a dilithiated derivative of 1a or 1b. The six fused four-membered rings give rise to a concave, lounge-chair arrangement for the 12-atom [Li3Al2N6E (E = C or N)] clusters of 2, 5, and 7. Variable-temperature 1H NMR spectra of 2, 5, and 7 in C7D8 over the range −90 to 23 °C reveal a fluxional process involving Li−N bond breaking and formation.