Lewis base complexes of AlH3: structural determination of monomeric and polymeric adducts by X-ray crystallography and DFT calculations

Abstract

The AlH₃ adducts of TMEDA (Me₂NCH₂CH₂NMe₂), DIOX (O(CH₂CH₂)₂O), TEA (Et₃N), BDMA (PhNMe₂), and TMPDA (Me₂NCH₂CH₂CH₂NMe₂) have each been characterised by single-crystal X-ray diffraction at low temperature, by ¹H, ¹⁴N and ²⁷Al NMR and FT-Raman and FT-IR spectroscopy, and by DFT calculations and elemental analysis. Hence, AlH₃·TMEDA and AlH₃·DIOX are both shown to adopt a polymeric structure, with the bidentate ligand bridging two Al centres, each of which adopts a trigonal bipyramidal (TBP) arrangement with equatorial hydride moieties. The 1 : 2 adduct AlH₃·2BDMA is monomeric but the geometry at the Al centre resembles closely that of the polymeric TMEDA and DIOX complexes. AlH₃·TEA alone adopts a monomeric structure in which the Al centre is tetrahedrally coordinated by three hydride and one amine ligand. The Al–L bond distance of 2.0240(17) Å for AlH₃·TEA is the shortest of all the complexes in this study, and AlH₃·TEA also possesses the shortest Al–H bonds. AlH₃·DIOX has the shortest Al–L bond distance of the polymeric species (2.107(14) Å) on account of the higher electronegativity of the oxygen donor. The structure of AlH₃·TMEDA was determined at low temperature (monoclinic space group P2₁/c), and salient features are compared to the previous room temperature study, for which a highly disordered orthorhombic space group (P2₁2₁2₁) was reported. The polymeric structures appear to be stabilised by a number of intermolecular interactions and unconventional hydrogen bonds; these are most pronounced for AlH₃·DIOX, whose chains are connected by highly directional C–H⋯H–Al bonding with an H⋯H distance of 2.32(6) Å.