An aromatic hydrocarbon π-system with a 42° dihedral twist: X-ray crystal structure of 1,3-bis(trichloroacetyl)homoazulene

Red crystals of 1,3-bis(trichloroacetyl)homoazulene (2), prepared by Friedel–Crafts acylation of homoazulene (1), have been examined by X-ray diffraction. The 10 carbon–carbon bonds that make up the perimeter π-system of 2 average 1.402 Å in length, with an average deviation (absolute value) from this length of only 0.016 Å. Thus, like benzene, this nonbenzenoid π-system exhibits bond length convergence and thereby qualifies as ‘aromatic’ by the geometric criterion. The aromaticity of homoazulene (1) had previously been based on its magnetic, thermodynamic and chemical properties. The crystal structure of 2 also reveals that the 1,5-bridge severely distorts the [10]annulene π-system away from planarity and induces torsional angles as large as 42.2° at a bridgehead position. To the best of our knowledge, dihedral angles this large have never before been observed in any other π-system that is still capable of supporting cyclic delocalization of electrons. The ‘homoconjugation’ gap between the two bridgehead positions was found to be 2.254(5) Å.