Preparation and Characterization of Lithium Imidoalanate Complexes Li₂[(RN)₄(AlH₂)₆] (R = Me or <i>^t</i>Bu): First Examples of Aluminum-Rich Imidoalanes with an Adamantoid Structure

Whereas methylammonium chloride, [MeNH3]Cl, reacts with LiGaH4 in an ether solution to give, according to the conditions, either the adduct MeH2N·GaH3 or the cationic derivative [(MeH2N)2GaH2]+Cl-, the corresponding reaction of [MeNH3]Cl or [tBuNH3]Cl with LiAlH4 proceeds mainly, with H2 elimination, to the imidoalane Li2[(RN)4(AlH2)6] (R = Me, 1, or tBu, 2). The crystal structure of 1·2Et2O reveals, for the first time, anionic units with an adamantane-like Al6N4 skeleton. The Li cations exist at two distinct sites, each linked via Li(μ-H)Al bridges to two [(MeN)4(AlH2)6]2- cages. Despite disordering of the tBu groups, the crystal structure of 2 evidently includes analogous anionic units. By contrast, the main product of the reaction between [iPrNH3]Cl and LiAlH4 under similar conditions is the known neutral, hexameric imidoalane [iPrNAlH]6, 3, the crystal structure of which has been redetermined.