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 (C2). 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.