Role of Central Arene Rotators and Ag⁺, I⁺, and PdCl₂ Centers in Hexagonal Macrocycles as Molecular Rotors

Both trapezoidal and hexagonal macrocycles with central arene rotators have previously been reported to be synthesized and have the potential for use as molecular rotors. Hexagonal macrocycles with four different central arene rotators and three different coordinating centers (Ag⁺, PdCl₂, I⁺) were examined using M06-2X/def2-SVP density functional theory and basis set. Minima were determined along with the rotational potential energy surface of the central arene to investigate the feasibility of the central arene rotating fully around the alkynyl axles and to explore what interactions with the surrounding stator might slow or block the movement of the rotator. Only the smallest central arene, 1, a phenyl group, was found to be able to rotate fully through the macrocycle. In the gas phase, this rotation occurs with reorganization of the surrounding stator to minimize steric clashes between the endohedral hydrogens of the stator and the hydrogens of the rotator. For the benzothiadiazole rotator in 3 and the extended alkyne rotator of 4, the sizes of the rotating units prevent full rotation through the macrocycle frame, and coordinating and aryl-aryl interactions with the hexagonal macrocycle, respectively, stabilize the rotator at 40°/140° angles with respect to the plane of the stator. Methoxy substituents on the rotator of 2 block full rotation and provide O···H-C attractions that stabilize the rotator near 40°/140° angles with respect to the plane of the stator. Systems utilizing a PdCl₂ coordinating center generally exhibit the highest barriers to rotation, as movement of the Cl-Pd-Cl rotators in concert with the central arene comes with an energetic cost.