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 [Al 2 (N t -Bu) x (NH t -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(N t -Bu)(NH t -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(N t -Bu)(NH t -Bu) 2 ]} 2 ·[LiN(H) t -Bu] ( 7 ) and {Li[Al(N t -Bu)(NH t -Bu) 2 ]} 2 ( 8 ). The gallium analogue of 7, i.e., {Li[Ga(N t -Bu)(NH t -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 [Li 3 Al 2 N 6 E (E = C or N)] clusters of 2, 5, and 7 . Variable-temperature 1 H NMR spectra of 2, 5, and 7 in C 7 D 8 over the range −90 to 23 °C reveal a fluxional process involving Li−N bond breaking and formation.