Oligomeric structures of the crystalline dimethylamine adducts Me2(H)N·MH3 (M = Al or Ga) and the dimethylamido derivative [Me2NGaH2]3

Abstract

The adduct Me₂(H)N·AlH₃ (1) has been isolated for the first time at low temperatures from the reaction of [Me₂NH₂]Cl with LiAlH₄ in Et₂O solution at ca. 250 K. Single crystals of this and of the corresponding gallane Me₂(H)N·GaH₃ (2) have been grown, also at low temperature; the structures of these at 150 K determined by X-ray diffraction reveal dimeric units Me₂(H)N·H₂M(μ-H)₂MH₂·N(H)Me₂ with highly unsymmetrical M–H ⋯ M hydrogen bridges, as well as evidence of significant intermolecular N–H ⋯ H–M interactions, particularly for M = Al. The M–N distances are quite similar [1 2.028(1), 2 2.079(3) Å], but the M ⋯ M distance in 2 [3.7519(15) Å] is much longer than in 1 [2.8743(8) Å]. The adducts decompose at ambient temperatures, rapidly in the case of 1, slowly in the case of 2, with the elimination of H₂ and the formation of the corresponding dimethylamido derivative. Unlike the vapour at low pressure in which the dimeric molecule predominates, crystals of the gallane (3) at 150 K are composed of trimers [Me₂NGaH₂]₃ structurally analogous to the corresponding alane, with a chair-like conformation and a mean Ga–N distance of 1.981(3) Å.