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

Abstract

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.