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)
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 C2h 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 Ci 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.