Reversible Interconversion of a Static Metallosupramolecular Cage Assembly into a High-Speed Rotor: Stepless Adjustment of Rotational Exchange by Nucleophile Addition

The self-assembled cage ROT-1was prepared from the pyridine-terminated rotator 1, the phenanthroline-appended stator 2, DABCO, and copper(I) ions in a ratio of 1:1:1:4. This four-component assembly is held together by two pyridine→[Cu(phenAr2)]+as well as two DABCO→zinc porphyrin interactions (phenAr2= 2,9-diarylphenanthroline) and does not show any motion on the NMR time scale (k< 0.1 s–1, 298 K). However, it is converted to the fast nanorotor ROT-1xCD3CNby addition of CD3CN [x= (v/v)% of acetonitrile in dichloromethane] due to acceleration of both pyridine→copper(I) dissociation steps. Now the rotator is able to visit all four copper(I)-loaded phenanthroline stations of the stator. Depending on the amount of CD3CN, the exchange frequency of the nanorotor varies from 0.7 s–1(CD3CN:CD2Cl2= 1:29) to 8000 s–1(CD3CN:CD2Cl2= 1:5) at 25 °C. When iodide (I–) is added to the static assembly ROT-1, the rotational speed increases even more drastically (k= 20 000 s–1), again due to accelerating the rate-determining pyridine→copper(I) dissociation step. In both cases, a sigmoidal relationship is established between exchange frequency and the concentration of added nucleophile (CD3CN or iodide) that suggests the presence of a cooperative effect. Reversible switching between the static assembly and fast rotor was performed several times without any decomposition of the system. In contrast, addition of the common nucleophile PPh3to ROT-1does not increase the rotational speed, a finding that is explained on thermodynamic grounds.