Molecular “Compasses” and “Gyroscopes.” III. Dynamics of a Phenylene Rotor and Clathrated Benzene in a Slipping-Gear Crystal Lattice

Samples of 1,4-bis(3,3,3-triphenylpropynyl)benzene 3 were prepared by Pd(0)-catalyzed coupling
of 3,3,3-triphenylpropyne (1) and 1,4-diiodobenzene. The structure of compound 3 is such that the central
phenylene can play the role of a gyroscope wheel, while the alkyne bond and trityl groups can act as an
axle and shielding frameworks, respectively. Crystals grown from benzene and dichloromethane were
characterized by X-ray diffraction, variable-temperature 13C CPMAS NMR, quadrupolar echo solid-state
2H NMR, and thermal analyses. The rotational dynamics of benzene molecules and phenylene groups
were characterized in terms of 6-fold rotation and 2-fold flipping models, respectively. The possibility of a
gearing mechanism between adjacent benzene molecules and phenylene groups suggested by the clathrate
structure was investigated. However, it was found that 6-fold rotation of benzene molecules at 300 K occurs
in the gigahertz regime (or higher) and 2-fold flipping of phenylene groups in the kilohertz range in a structure
that can be described as a slipping-gear lattice. The rotational dynamics of the phenylene group in the
solvent-free structure were remarkably similar to those in the clathrate, and both are among the fastest
known for phenylene rotation in solids. The results presented here provide a valuable starting point for the
design and analysis of crystalline solids with correlated molecular motions.