Complexes of organoaluminium compounds. Part VI. Crystal and molecular structures of cis- and trans-cyclotri-µ-methylamido-tris(dimethylaluminium) and of cyclodi-µ-dimethylamido-bis(dimethylaluminium)

Abstract

Two stereoisomers of cyclotri-µ-methylamido-tris(dimethylaluminium), have been characterized by single-crystal X-ray studies from diffractometer data. The rhombohedral cis-isomer (1) crystallizes in space group R3 with a= 9·983 ± 0·004 Å, α= 104° 39′± 3′, and Z= 2. The molecules have crystallographic symmetry C₃ with six-membered (AlN)₃ rings in the chair conformation. All methyl substituents on nitrogen atoms are equatorial. The mean molecular dimensions are: Al–N 1·940 ± 0·005, Al–C, 1·973 ± 0·005, and N–C 1·504 ± 0·010 Å; N–Al–N 102·1 ± 0·4, C–Al–C 116·9 ± 0·4, Al–N–Al 122·3 ± 0·4, C–Al–N 109·4 ± 0·3, and Al–N–C 108·6 ± 0·4°. The monoclinic trans-isomer (2) crystallizes in space group C2/c with a= 11·897 ± 0·008, b= 15·904 ± 0·012, c= 9·778 ± 0·007 Å, β= 107° 50 ± 3′, and Z= 4. The molecules have the same molecular dimensions as those of the rhombohedral isomer, but the configuration at one of the nitrogen atoms is reversed, and the conformation of the (AlN)₃ rings is of the skew-boat type. Crystals of cyclodi-µ-dimethylamido-bis(dimethylaluminium)(3) are monoclinic, space group P2₁/a, with a= 12·770 ± 0·012, b= 8·104 ± 0·007, c= 7·655 ± 0·009 Å, β= 117° 39′± 3′, and Z= 2. The molecules have four-membered (AlN)₂ rings, molecular symmetry which approximates to D_2h, and dimensions: Al–N 1·958 ± 0·005, Al–C 1·950 ± 0·008, and N–C 1·509 ± 0·010 Å; N–Al–N 88·3 ± 0·3, C–Al–C 115·7 ± 0·5, Al–N–Al 91·7 ± 0·2, C–N–C, 107·6 ± 0·6, C–Al–N 112·4 ± 0·3, and Al–N–C 114·3 ± 0·4°. The structures were refined to R values of 5·5 [(1), 996 reflections], 12·1 [(2), 621 reflections], and 6·9%[(3), 525 reflections].