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

Abstract

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

Crystalline molecular machines: Encoding supramolecular dynamics into molecular structure

“…4, rates of rotation in the megahertz (10 6 s Ϫ1 ) range may be observed at a relatively low temperature of Ϸ65°C. The rotary dynamics of the benzene molecules in the solvent clathrate of 2 was also investigated by 2 H NMR with crystals grown from C 6 D 6 (34). In agreement with previous studies (36,37) and as expected for a volume-conserving process, benzene molecules were found to undergo in-plane whole-body rotations described by 60°jumps with rate constants exceeding 10 8 s Ϫ1 , even at 200 K.…”

“…An energy barrier of 12-14 kcal͞mol for a motion involving angular displacements of 180°for the central phenylene was unprecedented for a crystalline solid and validated our original hypothesis (11,34,35). As indicated in Fig.…”

“Giant” Gyroscope‐Like Molecules Consisting of Dipolar Cl‐Rh‐CO Rotators Encased in Three‐Spoke Stators That Define 25–27‐Membered Macrocycles

“…Two prominent classes of molecular rotors are compasses and gyroscopes. [3,4] When rotators feature dipole moments, they can be aligned in static electric fields, similar to compasses, or forced to turn unidirectionally in rotating electric fields, [1] similar to gyroscopes. In all cases, it is desirable that rotation be as "frictionless" as possible, without interference from nearby molecules or ions.…”

Molecular Gyroscopes: {Fe(CO)3} and {Fe(CO)2(NO)}+ Rotators Encased in Three‐Spoke Stators; Facile Assembly by Alkene Metatheses

“…The symmetries of 3 a and 4 a-c are too high to probe the rate of {Fe(CO) 3 (2:1), indicating a slow-exchange limit with respect to any process (e.g., {HFe(CO) 3 } rotation) that can render the bridges equivalent.…”