Crystal and molecular structure of 1,3,6,8-tetra-azatricyclo[4.4.1.13,8]-dodecane, the 2:1 condensation product of formaldehyde and 1,2-diaminoethane, and the conformation of this system

Abstract

X-Ray structure analysis of the 2 : 1 condensation product of 1,2-diaminoethane and formaldehyde has shown it to be (II; X = N) 1,3,6,8-tetra-azatricyclo[4.4.1.1^3,8]dodecane, which contains only seven-membered rings. The unit cell is tetragonal, space group I2m, Z= 2 with a=b= 767(2), c= 743(2) pm. The structure was solved by Patterson and Fourier methods and refined by full-matrix least-squares to R 0·079 for 178 photographic data. The structure consists of a close-packed lattice of ellipsoidal molecules with unexpectedly high symmetry (D_2d). Bond lengths are as expected (C–N 146, C–C 154 pm) but the molecule has a large degree of angle strain as bond angles are distorted outwards by 10°. The conformation of the molecule is shown to be mainly governed by bond torsion energies and intramolecular non-bonded repulsions, but vibrational motion in the solid suggests that less-symmetrical conformations, with S₄ symmetry, may also be possible.