Conformational study and structure of bis-N,N′-p-bromo-salicylideneamine-1,2-diaminobenzene

Bis-N,N'-p-bromo-salicylideneamine-1,2-diaminobenzene (C20H14Br2N2 O-2) has been investigated by X-ray analysis and AM1 semi-empirical quantum mechanical method. Thc crystal is in the orthorhombic space group Pbca with a = 18.805(3), b = 25,394(4), c = 7.549(2) Angstrom, V = 3604.9(1) Angstrom (3), Z = 8, D-c = 1.747 g cm(-3) and mu (MoKalpha) = 4.515 mm(-1). The title structure was solved by direct methods and refined to R = 0.0477 for 1262 reflections [l > 2 sigma (1)]. The title compound is photochromic and the molecule is not planar. Intramolecular hydrogen bonds occur between O(1) and N(1) (2.605(7) Angstrom) and between O(2) and N(2) (2.601(8) Angstrom) atoms, the hydrogen atom essentially being bonded to the oxygen atom. Minimum energy conformations from AM 1 were calculated as a function of two torsion angles theta (1) (C8-N1-C7-C6) and theta (2) (C15-C14-N2-C13), varied every 5 degrees. The optimized geometry of the crystal structure corresponding to the non-planar conformation is the most stable conformation in all calculations. The results strongly indicate that the minimum energy conformation is primarily determined by nonbonded hydrogen-hydrogen repulsions between the ortho-hydrogens on the aldehyde rings. (C) 2000 Elsevier Science B.V. All rights reserved.