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 C-13 CPMAS NMR, quadrupolar echo solid-state H-2 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.