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

Red crystals of 1,3-bis(trichloroacetyl)homoazulene (2), prepared by Friede l-Crafts acylation of homoazulene (1), have been examined by X-ray diffract ion. The 10 carbon-carbon bonds that make up the perimeter a-system of 2 av erage 1.402 Angstrom in length, with an average deviation (absolute value) from this length of only 0.016 Angstrom. Thus, like benzene, this nonbenzen oid pi -system exhibits bond length convergence and thereby qualifies as 'a romatic' by the geometric criterion. The aromaticity of homoazulene (1) had previously been based on its magnetic, thermodynamic and chemical properti es. The crystal structure of 2 also reveals that tile 1,5-bridge severely d istorts the [10]annulene sr-system away from planarity and induces torsiona l angles as large as 42.2 degrees at a bridgehead position. To the best of our knowledge, dihedral angles this large have never before been observed i n any other pi -system that is still capable of supporting cyclic delocaliz ation of electrons. The 'humoconjugation' gap between the two bridgehead po sitions was found to be 2.254(5) Angstrom. (C) 2001 Elsevier Science Ltd. A ll rights reserved.