Complexes of organoaluminium compounds. Part 10. Crystal and molecular structures of the cage compounds bis-µ-methylamido-hexa-µ3-methylimido-bis(dimethylaluminium)-hexakis(methylaluminium) and bis-µ-methylamido-hexa-µ3-methylimido-bis(dimethylgallium)-hexakis-(methylgallium)

Abstract

The adduct AlMe₃·NH₂Me decomposes at 215 °C to give the cage compound (MeAlNMe)₈ besides the well established (MeAlNMe)₇. The intermediate (Me₂AlNHMe)₂(MeAlNMe)₆(1), isolated from reactions at lower temperatures, has been characterised by spectroscopic and X-ray methods. Crystals are orthorhombic (space group Abam) with a= 15.389(2), b= 19.747(4), c= 11.613(1)Å, and Z= 4. The structure has been solved by sharpened Patterson and Fourier methods and least-squares refinement converged at R= 0.087 (927 reflections measured by diffractometer). The molecules have cage structures, apparently with C_2h symmetry, with each nitrogen and each aluminium four-co-ordinate and methyl groups pointing outwards. Mean molecular parameters are: Al–N 1.910(6), Al–C 1.97(2), N–C 1.52(2)Å; N–Al–N 91.3(3) or 106.7(10), Al–N–Al 88.4(3) or 115.6(10), N–Al–C 113.5(6), Al–N–C 111.4(11), C–Al–C 109(1)°. The gallium analogue (Me₂GaNHMe)₂(MeGaNMe)₆(2), obtained similarly, crystallises in space group P1 with a= 9.670(2), b= 9.905(2), c= 11.156(3)Å, α= 98.24(2), β= 108.23(2), γ= 115.57(1)°, and Z= 1. The structure has been solved by direct methods and refined to R= 0.05 for 2 143 reflections collected by diffractometer. The molecular structure is almost the same as that of the aluminium compound and mean bond lengths and angles are: Ga–N 1.970(4), Ga–C 1.986(4), N–C 1.49(2)Å; N–Ga–N 89.8(2) or 106.8(10), Ga–N–Ga 89.8(3) or 115(1), Ga–N–C 111.4(9), N–Ga–C 113.1(7), C–Ga–C 113.2(10)°. The Me₂ENHMe (E = Al or Ga) fragments of both structures are disordered and two isomers, one with C₂(cis) and the other with C_i symmetry (trans), apparently crystallise together. The cis and trans isomers may be detected in n.m.r. spectra of solutions of the compounds in hydrocarbons.