Preparation and characterization of lithium imidoalanate complexes Li(2)[(RN)(4)(AlH(2))(6)] (R = Me or (t)()Bu): first examples of aluminum-rich imidoalanes with an adamantoid structure

Whereas methylammonium chloride, MeNH(3)]Cl, reacts with LiGaH(4) in an ether solution to give, according to the conditions, either the adduct MeH(2)N x GaH(3) or the cationic derivative (MeH(2)N)(2)GaH(2)](+)Cl(-), the corresponding reaction of MeNH(3)]Cl or (t)BuNH(3)]Cl with LiAlH(4) proceeds mainly, with H(2) elimination, to the imidoalane Li(2)(RN)(4)(AlH(2))(6)] (R = Me, 1, or (t)Bu, 2). The crystal structure of 1 x 2Et(2)O reveals, for the first time, anionic units with an adamantane-like Al(6)N(4) skeleton. The Li cations exist at two distinct sites, each linked via Li(mu-H)Al bridges to two (MeN)(4)(AlH(2))(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 (i)PrNH(3)]Cl and LiAlH(4) under similar conditions is the known neutral, hexameric imidoalane (i)PrNAlH](6), 3, the crystal structure of which has been redetermined.