Influence of Rotator Design on the Speed of Self-Assembled Four-Component Nanorotors: Coordinative Versus Dispersive Interactions

Four-component nanorotors are prepared by the self-assembly of stator [Cu₄(<b>4</b>)]⁴⁺ with its four copper­(I)-loaded phenanthroline stations and various rotators carrying one, two, or three pyridine terminals. The fourth component, 1,4-diazabicyclo[2.2.2]­octane, serves as a connecting axle between rotator and stator. Capitalizing on the heteroleptic pyridyl and phenanthroline metal complexes concept, the rotator’s pyridine terminals are connected to the copper­(I)-loaded phenanthroline stations (N_py → [Cu­(phen)]⁺) in the STOP state and disconnected in the transition state of rotation. As the barrier of the thermally activated rotation, measured by variable-temperature ¹H NMR, is mainly governed by attractive forces between stator stations and rotator terminals, it increases along the series <i>E</i>_a (monopyridine rotator) < <i>E</i>_a (dipyridine rotator) < <i>E</i>_a (tripyridine rotator). However, there are even distinct differences in rate between rotors with equal number of rotator terminals. The change from the 5,10-dipyridyl (<i>cis</i>) to 5,15-dipyridyl (<i>trans</i>) zinc porphyrin rotator enhances the rotational frequency by almost 1000-fold. Density functional theory computational results suggest that not only coordinative N_py → [Cu­(phen)]⁺ interactions but also dispersive attraction influence the barrier of rotation.