Molecular Dials: Hindered Rotations in Mono- and Diferrocenyl Anthracenes and Triptycenes

The syntheses, X-ray crystal structures, and molecular dynamics of 9-ferrocenylanthracene, 3, 9,10-diferrocenylanthracene, 4, 9-ferrocenyltriptycene, 7, and 9,10-diferrocenyltriptycene, 8, are reported. At 193 K, 3 exhibits C-s symmetry via oscillation of the ferrocenyl only about the anthracene plane; at higher temperatures, complete rotation about the C(9) ferrocenyl linkage becomes evident with a barrier of 10.6 kcal mol(-1). At 193 K, the ferrocenyls in 4 give rise to syn (C-2v) and anti (C-2h) rotamers that also interconvert at room temperature. In the corresponding triptycyl systems, 7 and 8, these rotational barriers increase to 17 kcal mol(-1); 9,10-diferrocenyltriptycene exists as slowly interconverting meso and racemic rotamers, in which the ferrocenyl moieties are, respectively, eclipsed (C-2v) or staggered (C-2). 2D-EXSY NMR data recorded with different mixing times indicate clearly that these interconversions proceed in a stepwise manner, for example, rac -> meso -> rac, thus behaving as a set of molecular dials.