MOLECULAR-CONFORMATIONS OF 9,9'-BIANTHRYL, DI-9-ANTHRYLMETHANE, AND SOME RELATED TWISTED ANTHRACENE-DERIVATIVES

The molecular conformations of 9,9'-bianthryl (1), its 10,10'-diphenyl derivative (2), 9-p-tolylanthracene (3), and 9,10-diphenylanthracene (4) have been investigated by X-ray diffraction. The dihedral angle between the two aromatic ring systems in crystalline 1, in which the asymmetric unit consists of two discrete molecules, was found to be 74.65 (4) and 81.76 (5)-degrees, respectively. In 2, the two anthracene moieties are at an angle of 87.42 (4)-degrees, and the phenyl rings are twisted out of the plane of the adjacent anthracene by 73.2 (1) and 78.1 (1)-degrees. The corresponding torsion angle between the two aromatic ring systems in 3 is 79.59 (8)-degrees. In centrosymmetric 9,10-diphenylanthracene (4), the phenyl rings deviate from coplanarity with the anthracene ring system by 67.63 (5)-degrees. Separating the two anthracene pi-systems of 9,9'-bianthryl by a methylene group increases steric congestion, as was established by X-ray diffraction analysis of di-9-anthrylmethane (5). Its molecular geometry is characterized by a 2-fold axis of symmetry, and the two anthracene pi-systems are in an orthogonal arrangement. Intramolecular hydrogen-hydrogen distances in 5 are as short as 1.93 (3) angstrom, and pi-orbital interaction was found to be topologically facilitated by a contact distance of 2.601 (2) angstrom between the 9- and 9'-carbon atoms. By contrast, in 9-(9-anthrylmethylidene)-9,10-dihydroanthracene (6), which formally derives from 5 by a 1,5-hydrogen shift, the closest intramolecular hydrogen-hydrogen contact is 2.28 (6) angstrom. Relief of steric strain derives from folding of the dihydroanthracene moiety by 39.0 (2)-degrees.